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Cell evolution and the problem of
membrane topology
Gareth Griffiths
Abstract | Cells somehow evolved from primordial chemistry and their emergence
depended on the co-evolution of the cytoplasm, a genetic system and the cell
membrane. It is widely believed that the cytoplasm evolved inside a primordial lipid
vesicle, but here I argue that the earliest cytoplasm could have co-evolved to high
complexity outside a vesicle on the membrane surface. An invagination of the
membrane, aided by an early cytoskeletal system, may have formed the first cells
— initially within primordial vesicles.
Genomic analysis leaves no doubt that the
three main kingdoms of life — eubacteria,
archaea and eukaryotes — evolved from
a common ancestor1. The idea that all
life evolved from one source is perhaps
the most profound consequence of the
Darwin–Wallace model of evolution. With
increasing amounts of DNA sequence
information available, the search is on to
find the minimum set of genes that existed
in the last common ancestor (LCA) — the
precursor cell(s) from which all living
organisms subsequently evolved. The LCA
must have been a rather sophisticated cell
because it contained all the cellular machinery that is common to all present living
forms, including a DNA-based information
system2,3. From sequence analysis, the LCA
is suggested to contain 250–600 genes4,5.
Mycoplasma genitalium, the simplest known
prokaryote cell (which is nevertheless
dependent on host-cell parasitism), requires
270 of its 380 genes for normal function, as
revealed when all genes were individually
knocked out6. So, the minimum conceivable
cell needs a lot of genetic information.
The Earth is ~4.56 billion years (Gyr)
old, and the best current estimates argue
that the first cells appeared by 3.0–3.3 Gyr
at the latest7. How these first cells emerged
is one of the biggest unsolved problems in
biology. It seems undeniable that the LCA
must have been preceded by a spontaneous
generation of cells from abiotic precursor
molecules. Even Darwin in his theory of
evolution was obliged to accept the necessity
of some kind of spontaneous generation.
He brilliantly speculated (in a letter to
Hooker) that in a “warm little pond” a
particular chemistry involving “all sorts of
Acidic
Polar uncharged
Glu
Asp
Asn
Gln
Ser
Pro
Val
Ala
Gly
Cys
Thr
1
2
4
5
6
3
RNA
Mineral
surface
chemistry
ammonia and phosphoric salts, light, heat,
electricity etc. was present so that a protein
compound was chemically formed ready
to undergo more complex changes on the
path towards life”8. It seems likely that his
(unpublished) hypothesis was not far from
the mark. Modern theories were initiated
by Oparin in 1924 (Ref. 9) and by Haldane10,
who first discussed the origin of membranes
and speculated that an ‘oily film’ on the surface of sea water evolved into the lipid-rich
cell membrane.
Here, I attempt to build up a plausible
scenario for how cells may have evolved
— with an emphasis on the more difficult
question of the origin of the cell membrane.
A brief discussion of the (more actively pursued) question of the evolution of cytoplasm,
including the nucleic acids, sets the stage for
the main hypothesis.
The evolution of the cytoplasm
Cells are made up of nucleic acids, proteins,
carbohydrates, lipids and many small
molecules suspended in a particular ionic
Basic
Non-polar
hydrophobic
Ile
Met
Leu
7
Lys
Arg
8
Evolutionary time
Vesicles
9
10
Aromatic
Phe
Tyr
11
RNA-DNA
Phospholipids
12
His
Trp
13
14
DNA
Self-synthesizing
membrane
Figure 1 | Biosynthetic pathways for amino acids, phospholipids and central metabolism.
Reviewspathways
| Molecular
Cell Biology
It has been proposed by Davis22,26 and many others that primitiveNature
biochemical
existed
before
a genetic-based system. These include the reductive carbon cycle (the equivalent of the tricarboxylic
acid (TCA) cycle working in reverse), the reductive pentose pathway and the central trunk (proposed
to be a remnant of the formose cycle), which together make up the central biochemical pathway (CBP).
Davis proposes that the emergence of genetically encoded amino acids correlates with the number
of chemical reactions from the CBP that are required to generate each amino acid (evolutionary steps
1–14). Acidic amino acids (Glu and Asp) are close to the CBP (1–2 reactions), whereas aromatic residues
(such as Trp) require up to 14 steps to be synthesized and may have appeared later. The hydrophobic
amino acids that could associate with membranes required four steps. The synthesis of phospholipids
requires 10 steps and, therefore, self-synthesizing membranes might only have arisen after this point.
Davis provides detailed analysis to argue that the evolutionary appearance of the different amino acids
correlates well with the emergence of their corresponding triplet codes; this implies co-evolution of
biochemistry and the genetic code, an idea extensively championed by Wong60. Davis also identified
an 11-amino acid sequence in the FtsZ–tubulin family that he mapped to his evolutionary stage 7.5,
which could support a role for this protein in cellularization. Gly, Cys and Pro cannot easily be placed
into any of the five categories shown.
1018 | December 2007 | volume 8
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Perspectives
a
b Cytoplasm inside
Cytoplasm outside
vesicle
vesicle
Protocytoplasm
d
Gel
–P
+
+
c
Lipid vesicle
P
–
+
P
–
+
P
P
–
+– +
+
P
–
+
Obcell
P
–
Protocell
Mineral
+ surface
Unstable permeable
bilayer
Mineral surface
Less permeable bilayer
Transient breaks
in the membrane
Liposome
RNA
‘Stable’ bilayer
Autonomous
cell divides
Ribosome
Protein
Pore/channel
Single acyl chain lipids
• Monoglycerides
• Fatty acids
• Carboxylic acids
Figure 2 | Surface interactions and the membrane problem. a | A pure
lipid vesicle in contact with a protocytoplasmic milieu. The system provides
two catalytic surfaces, the positively charged mineral surface and the
negatively charged, hydrophobic lipid vesicle. b | Two proposed membrane
scenarios for cellularization during evolution. In the ‘inside-out’ or ‘leaky
liposome’ model (left) the cytoplasm evolved within the vesicle and the
topology of the membrane (outer leaflet outside) remains in place during
the evolutionary process. In the ‘cytoplasm outside’ model, the cytoplasm
co-evolves with the membrane by associating with the outer membrane
surface (which will later become the cytoplasmic surface). A gel-like filamentous material may prevent the diffusion of protocytoplasmic components36. c | The ‘cytoplasm inside the vesicle’ hypothesis requires that the
molecular precursors of life must have found a way to pass selectively
through the liposome barrier. Deamer proposed that the earliest vesicles
environment. Before cells emerged, how
were sufficient amounts of the necessary
starting materials generated? Following the
pioneering experiments of Miller9, extensive
studies described many chemical reactions
that plausibly occurred under the presumed
conditions of the early Earth. These reactions could synthesize many (but not all)
amino acids and other key precursors, such
as purines, and precursors of liposomes
such as carboxylic acids11–13.
The key precursors of life could have
been made on Earth or in outer space and
carried to Earth via meteorites, such as
the Murchison meteorite that landed in
Australia in 1969 (Ref. 14). Meteorites, as
well as interplanetary dust particles, bring
an extraordinary selection and amount
of chemicals and biochemical precursors of
life from space, including >90 different
amino acids (of which 19 are found in
living organisms) and, as discussed below,
bilayer-forming lipids15,16.
Two acyl chain
phospholipids
Impermeable membrane
• Electric potential
• Proton gradient
contained bilayer membranes made up of single acyl chain lipids that are
Reviews
Molecular
Cell evolved
Biology
more permeable to many molecules,Nature
including
ions.| As
the system
to use the more complex two acyl chain phospholipids, the membrane
became more impermeable. Additional proposed mechanisms that overcome the permeability barrier include osmotic forces, transient breaks in
the membrane caused by polymerized amino acids such as polyleucine,
and transient openings in the bilayer caused by temperature changes or
freeze-thaw cycles20. d | The obcell model of the ‘outside-in’ hypothesis of
cellularization, as proposed by Blobel38 and Cavalier-Smith39. The early
liposome system is postulated to induce cisternae to fuse with themselves
to form double-membrane ‘obcells’ (inside-out cells)39. The model necessi­
tates the loss of the outer of the two membranes to release a protocell that
has the correct topology (with the luminal domains of the membrane
proteins facing outwards).
The cytoplasms of most modern cells
have a similar chemical composition with a
reducing environment, neutral pH and an
ionic composition that is rich in K+, Cl– and
Mg2+ but low in Ca2+ and Na+. Perhaps this
universal composition reflects the environment where the first cells evolved; if so, the
environment where the earliest chemistry
that preceded life occurred is likely to
have been in fresh water17,18 rather than
the high-salt ocean environment proposed
by others10,19. The latter is also difficult to
reconcile with the need for primordial lipid
vesicles, which are unstable at high salt
concentrations20.
For a protocytoplasm to emerge, many
schemes have been postulated that involve
the occurrence of chemical catalysis and
autocatalytic reactions on the surface of
positively charged minerals, such as clay
or iron pyrites21–25. A useful ‘yardstick’
for timing the main events leading to
cellularization is a scheme proposed by
nature reviews | molecular cell biology
Davis22,26 (FIG. 1; see below). The evolution
of the cytoplasm is often proposed to have
occurred in parallel with the emergence of a
liposome system, which eventually became
the delimiting membrane of the cell.
The emergence of membranes
In a water-based system, many lipids can
self-assemble into bilayer-containing
vesicles. Phospholipids are the main bilayerforming lipids in bacteria and eukaryotes
(assuming that the ether-lipid-based
membranes in archaea are a later adaptation to extreme environments22). However,
the simplest bilayer-forming lipids are
long-chain (>C9) fatty acids, carboxylic
acids and monoglycerides18,20. When such
lipids were extracted with solvents from
the Murchison meteorite, they formed
bilayered vesicles in aqueous solution16.
Deamer and colleagues proposed that
vesicles containing lipids made up of single
acyl chains formed the first template for the
volume 8 | December 2007 | 1019
© 2007 Nature Publishing Group
Perspectives
Box 1 | Double membrane compartments in modern cells
a Vaccinia virus
Golgi complex
TGN
(Microtubule)
IMV
b Budding yeast
Plasma membrane
(Actin)
EEV
Golgi
Spore
Spindle
pole body
Nucleus
c Autophagy
Mitochondrion
Fusion
with
lysosome
There are at least three examples in modern cells that are topologically similar to the ‘life outside the
vesicle’ model (see figure). a | During the cellular release of vaccinia
the|trans
GolgiCell
network
Naturevirus,
Reviews
Molecular
Biology
(TGN) cisternal domain engulfs the intracellular mature virus (IMV)56. This intracellular enveloped
virus is transported along microtubules and fuses with the plasma membrane. During fusion with the
plasma membrane, or shortly thereafter, the local polymerization of actin around the TGN-derived
membrane facilitates virus release into the extracellular space (extracellular enveloped virus (EEV))57.
As proposed in the evolutionary schemes presented in FIG. 3, actin and microtubules also interact
here with membranes. b | During sporulation in the budding yeast Saccharomyces cerevisiae, a
double-membrane prospore cisterna is formed that originates from post-Golgi vesicles that fuse
around the forming (haploid) spore (only one spore is shown but up to four may be made). The Golgi
vesicles aggregate on the surface of the nucleus (pink) at the spindle pole body to enclose the spore
within two membranes58. c | The formation of autophagic vacuoles is proposed to occur by the
formation of a double-membrane structure59. A cytoplasmic cisternal structure (of unknown origin
but suspected to originate from the endoplasmic reticulum) wraps around cytoplasmic components
such as a mitochondrion to form a double-membrane vesicle. The outer membrane fuses with a late
endocytic compartment, whereas the inner membrane is expected to be lysed by the hydrolytic
conditions of the lysosomal lumen. In parts a and c, the purple ball and stick structure represents a
membrane-spanning protein, with the ball representing the luminal domain.
cell membrane and that more complex (two
acyl chain) phospholipids emerged later20
(FIG. 2). Phospholipids can be synthesized
without enzymes under plausible abiotic
conditions12,13,27–29, but it seems likely that
these lipids became more important when
RNA ribozymes or ribosomes and protein
enzymes emerged that could synthesize
them (FIG. 1).
The evolution of cells
If we assume that an increasingly complex
cytoplasm with protein-synthesizing ribo­
somes, nucleotides, RNA and even DNA
emerged in contact with a system of liposomes, we face a crucial unresolved issue in
understanding the origin of cells and the cell
membrane: on which side of the liposomes
did the first key reactions occur?
The scenario of life within the vesicle. Most
specialists think that such a system could
only have evolved within the primordial
vesicle20,30–34 (FIG. 2b). How then could the
precursors of the key molecules of life have
crossed a bilayer that presents a significant
barrier to charged molecules? Although
vesicles comprised of single acyl chain
1020 | December 2007 | volume 8
lipid membranes are more permeable to
uncharged and charged molecules, their
overall permeability is several orders of
magnitude lower than phospholipid bilayers.
Significantly, all bilayers are highly impermeable to ions such as K+ (Refs 18,20).
As membrane complexity increases, the
bilayer becomes more impermeable. In this
‘cytoplasm within the vesicle’ scenario, the
emerging ribosomes inside the vesicle would
eventually evolve the capacity to insert
membrane proteins from the inside with
their extracellular domains facing outwards,
as in modern cells (FIG. 2b,c).
Many elegant experiments have been conducted to try and reconstitute some aspects
of this hypothesis. For example, investigators
trapped enzymes and whole transcription–
translation systems within vesicles and found
that these became functionally active 20,33–35
(FIG. 2c). Such analyses are still a long way
from reconstituting life in such vesicles, as
conceded even by some of the strongest
proponents of this hypothesis18.
The ‘outside the vesicle’ scenario. Given
the difficulties with the above hypothesis,
let us consider another scenario in which
the cytoplasm evolved to a high degree of
complexity outside the vesicles. A mineral
surface could provide an environment
on which lipid vesicles become attached,
and these surfaces could provide the
environment where some rudimentary
components of a complex protocytoplasm
— for example, perhaps ATP, GTP, proteins,
RNA and maybe even DNA — could evolve.
Given the problem of these key precursors
diffusing away from the site of action, it is
attractive to consider the emergence of a
polymer in the space around the vesicles
that can form a hydrophilic gel (Ref. 36). This
could initially be a simple polymer such as
polysugars or polyglutamic acid that might
later be replaced by a cytoskeletal protein
polymer such as actin (see below). Such
systems of filaments could serve to attach
the vesicles to the mineral surface, capture
precursor molecules within their gel-like
matrix or provide a scaffold for protection
as well as an additional catalytic surface for
emerging biochemistry. Under these conditions, there would be three catalytic surfaces
— the positively charged mineral surface24,25,
the surface of the gel36 and the negatively
charged lipid vesicle surface — that could
provide a rich surface for many reactions37.
The mineral surface could itself catalyse the
assembly of lipid vesicles25. In this microenvironment, sandwiched between these
surfaces, a sophisticated protocytoplasm
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Box 2 | Interactions of actin and microtubules with membranes in modern cells
Lamellipodia
Cell motility
Filopodia
–
Myosin
Exocytosis
+
–
Comet
+
Endocytosis
of an ‘obcell’ or ‘inside-out cell’ (FIG. 2d).
Whereas the ‘life within the vesicle’ model
(FIG. 2c) has no obvious mechanistic prece­
dents in modern cells, there are examples
from present-day cells in which a cisternal
wrapping process occurs that is topologically similar to the ‘cytoplasm outside the
vesicle’ model (BOX 1). Below, I argue
that this (admittedly complex) inversion
occurred by numerous steps and depended
on the earliest cytoskeletal filaments.
Functions of the cytoskeleton
Not so long ago, textbooks dogmatically
stated that only eukaryotes contain actin filaFusion
+
ments and microtubules. Recently, this idea
Microtubule
has been overturned with the realization that
Phagocytosis
the actin- and microtubule-family proteins
Macropinocytosis
are universally expressed and interact with
+ Motor
+ Motor
membranes. The prokaryotic GTPase FtsZ,
MreB (actin homologue)
a homologue of tubulin, was shown to form
filaments and to interact with the membrane
during bacterial cell division40.
FtsZ (tubulin homologue)
Actin homologues have been discovered
in eubacteria and archaea. It had been long
appreciated that hexokinase and chaperones
of the heat shock protein-70 (HSP70) family
were homologous to actin41. Recently, new
FtsA (actin homologue)
Cytokinesis
members of this family, the MreB and
In eukaryotes, the actin family is involved in many membrane-dependent processes such as
Mb1 subfamilies, were shown to be true
Reviews | Molecular
Cell Biology
exocytosis, endocytosis, phagocytosis, cell motility (for example, Nature
using lamellipodia),
cytokinesis
and
actin homologues that can form actin-like
cell polarity. Actin (red) and its prokaryotic homologues have many intimate connections with
membranes in eukaryotes (see figure; top panel) and prokaryotes (bottom panel). Membranes can
filaments in vitro and in bacteria42,43. As in
nucleate the assembly of actin and, in all known cases, the topology is such that the fast-growing end
eukaryotic cells, these filaments polymerize
(barbed end or plus end (+)) is localized adjacent to the membrane — this means that the insertion of
while being attached to the membrane
new monomers occurs at this site. Of the different mechanisms and processes shown, only two can be
surface, providing a force that contributes to
considered to be well understood. First, myosin motors attached to membrane organelles can walk
prokaryote shape44,45. When these proteins
along actin; most myosins move towards the plus ends. Second, actin comets, which were first
are knocked out in rod-shaped bacteria,
described in the intracellular transport of Listeria monocytogenes and other cytoplasmic pathogens,
for example, the cells are converted into
transport vesicles such as endocytic vesicles and phagosomes. For the most part, however, the
spheres44. One actin homologue, FtsA, is
mechanistic details of how actin or its homologues in prokaryotes interact with membranes is poorly
also involved in bacterial cell division and
understood49,52. Even less is known about the interactions of microtubules with membranes, with the
has recently been shown to interact directly
exception of motor proteins that can be bound to membrane organelles; kinesin transports cargo
towards the plus end of the microtubule, whereas dynein transports cargo in the opposite direction.
with FtsZ; thus, the actin- and tubulin-based
In prokaryotes, actin homologues interact with the cell membrane, whereas the tubulin homologue
systems work together to drive bacterial
FtsZ, which also assembles into filaments (green), interacts with the membrane at the site of cell
cytokinesis46 (BOX 2).
division (it forms a ring structure at the septum known as the Z ring) and is essential to carry out
In eukaryotes, the actin family is involved
cytokinesis (bottom panel). There are no comprehensive models to predict how FtsZ–membrane
in many membrane-dependent processes
interactions operate mechanistically in this process but, recently, the actin homologue FtsA has
such as exocytosis, endocytosis, phagobeen shown to interact directly with FtsZ46. A similar lack of understanding pertains to the role of
cytosis, cell motility, cytokinesis and cell
actin in the much more complex process of cytokinesis in eukaryotic cells; microtubules and
polarity47,48. In many fusion processes, actin
microtubule-associated proteins are also implicated in this process.
polymerizes on membranes and somehow
provides a force to pull membranes together
might have emerged that interacted first
with their extracellular domains facing the
and/or push them or keep them apart49–52
38
with the outer surface of the vesicles. Small
inside of the vesicle . To develop normal
(BOX 2). Recent data have shown direct
molecules may indeed have crossed sponcell-membrane topology, the vesicle would
interactions between membrane nucleation
taneously into the vesicle lumen and I will
have to invert upon itself and fuse to form a
mechanisms for actin assembly (N-WASP)
revisit this issue below.
double-layered vesicle (FIG. 2d). The outer of
and machinery for non-clathrin-mediated
If ribosomes could make hydrophobic
the two vesicles would then need to lyse to
endocytic vesicle formation and for
polypeptides, these would interact with the
dorsal cell ruffling. In these processes,
release the cell. This clever idea by Blobel38
vesicle outer surface and, in time, the system is rarely cited by origin-of-life specialists
sorting nexin-9 (SNX9) was identified as a
would evolve the capacity to insert membut has been extensively championed by
membrane scaffold protein that stimulates
brane proteins that spanned the membrane,
Cavalier-Smith39, who conceived the idea
N‑WASP and ARP2/3-dependent actin
Cytokinesis
Membrane
nucleation –
of actin
nature reviews | molecular cell biology
volume 8 | December 2007 | 1021
© 2007 Nature Publishing Group
Perspectives
a
b
c
Pre-cytoplasmic
environment
External milieu
Extracellular space
Ribosome
Lumen
Protocell
Actin
Channel
Microtubule/FtsZ
+
+
+
+
+
+
e
+
+
+
Cytokinesis
d
Extracellular space
Luminal
fusion
Cytoplasmic
fusion
Autonomous cells
Inverted vesicle
Figure 3 | The outside-in model of cellularization. Shown is a possible evolutionary mechanism by which the outside-in model (FIG. 2d) may have
occurred. a | An evolutionary stage exists in which a complex protocytoplasm
has a genetic code that is RNA- or even DNA-based, ribosomes for synthe­
sizing proteins and a relatively advanced biochemistry. The system has evolved
the capacity to insert membrane proteins such that the future extracellular or
luminal domains are inside the liposome. In parallel, an actin- and tubulinbased cytoskeleton evolved the capacity to interact with the liposome surface.
b | The emergence of the cytoplasmic fusion machinery (orange) allows liposomes to fuse together and bend membranes. c | The fusion machinery could
collaborate with the cytoskeleton to form an inwards-budding vesicle. This
budding process, which is similar to modern endocytosis but has the opposite
assembly, especially when it is allowed to
oligomerize on phosphatidylinositol-4,5bisphosphate (PtdIns(4,5)P2)-enriched
membrane domains in vitro. SNX9 contains
a BAR domain that is known to facilitate
membrane bending53. Such studies start
to resolve the long-elusive molecular links
between actin, its assembly and membrane
functions. Besides its role in cytoplasmic
fusion processes (where the cytoplasmic
leaflets of membranes first interact), a
recent study shows that N‑WASP-based
actin polymerization is also essential for
two apposing plasma membrane luminal
domains to fuse completely during
Drosophila melanogaster myoblast fusion54.
Thus, actin facilitates both cytoplasmic and
luminal fusion events (FIG. 3).
Homologues of actin and tubulin have
therefore been identified in all kingdoms
of life. The ATPase actin and the GTPase
tubulin–FtsZ protein family may have
appeared before the cell became surrounded
topology, allows the genetic material (not shown) and the protocytoplasm to
Nature—
Reviews
| Molecular
Cell Biology
enter into a vesicle within a larger vesicle
the protocell.
Channels
and
transporters that allow ions to cross the membrane would be important for
maintaining the ionic homeostasis of the protocytoplasm inside the protocells
and in the extracellular space. d | The emergence of luminal fusion mechanisms allows the process of fission out of the parental vesicle to occur.
This machinery also allows the protocells to fuse together within the extra­
cellular space. e | The cytoplasmic fusion machinery evolves into the process
of cytokinesis, which allows protocells to divide in a regulated fashion such
that each daughter cell contains everything it needs to metabolize and replicate. The protocells are eventually released when the outer membrane lyses.
These are now independent living forms that are capable of self-replication.
by a membrane and, if so, could have
functioned in cellularization. These suggestions are supported by an analysis by Davis,
who argued that the order in which coded
synthesis of the different amino acids and
lipids emerged during evolution correlates
with the number of reactions needed for their
synthesis from an already evolved biochemical system, which included the tricarboxylic
acid (TCA) cycle and the pentose pathway22,26 (FIG. 1). These reaction systems are
universally involved in the synthesis of all 20
common amino acids used by modern cells.
Davis26 identified a conserved 11-residue
sequence in the FtsZ–tubulin family that he
mapped to his evolutionary stage 7.5, a stage
he classified as occurring before the system
could self-synthesize membrane phospho­
lipids (stage 10; FIG. 1). Because microtubules
and FtsZ, and especially actin, have intricate
interactions with modern membranes,
I propose a speculative model for the role of
their precursors in cellularization.
1022 | December 2007 | volume 8
The cytoskeleton in cellularization?
My model of cellularization starts with interactions of the protocytoplasm with the outer
surface of the initially pure lipid liposomes
(FIG. 3a). In the model, I propose that proteins
(made by ribosomes present in the proto­
cytoplasm) evolved hydrophobic domains
that allowed them to interact with liposomes.
Later, membrane-spanning proteins, including channels and pumps, were inserted. The
proton and other pumps formed chemical
and electrical gradients and synthesized ATP
on the cytoplasmic side of the membrane.
The inside of the vesicle could then develop
a different composition to the outside and
might later have become the extracellular
space. Thus, in this model, even sophisticated
membrane functions such as proton and
electrochemical gradients emerged before
cellularization.
Actin and/or tubulin ancestors then
interacted with the outer surface of the membrane and facilitated membrane bending.
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A cytoplasmic machinery evolved that fused
the vesicles together (FIG. 3b). A coordinated
effort between the cytoskeleton and the
cytoplasmic fusion machinery then allowed
apposing membrane lipid coalescence and
the inwards pinching of vesicles to form
protocells (FIG. 3c). These ‘inverted’ vesicles
within the larger vesicle enclose the protocytoplasm and the genetic material (DNA or
RNA); it is implicitly assumed that this evolving genetic material is intimately associated
with the cytoplasmic surface of the vesicle
(not shown in FIG. 3).
The formation of these vesicles is
topologically equivalent to the budding of
vesicles into the lumen of specialized endocytic organelles — multivesicular bodies
— in modern eukaryotic cells. Additional
machinery must have evolved that allowed
the luminal domains of the protocell membranes to fuse together. The same machinery
could allow cells to bleb off a membrane
vesicle by fission, a process equivalent to
blebbing in modern cells. The protocells
could undergo selection by content mixing driven by fusion with themselves and
with newly made protocells bringing in
new comp­onents from the external milieu
(FIG. 3d).
At a later stage, the cytoplasmic fusion
machinery facilitates the separation of
daughter protocells in a regulated fashion
(cytokinesis) (FIG. 3e). The final stage is lysis
of the first outer membrane to release independent cells. These are expected to have
probably several hundred DNA-encoded
genes in order for the minimal cell functions
to be permitted.
The extracellular compartment (FIG. 3c)
would offer several advantages to the
emerging cells bathing within this space.
The evolution of mechanisms to transfer
glycoconjugates bound to lipids or proteins
onto and beyond the luminal surface of the
boundary membrane of protocells could
provide a reserve energy source for cells
if sugar hydrolases were also secreted into
this space. A high-viscosity environment
here could also protect the enclosed cells
from extreme environmental changes
in the external environment. Ion channels in
the boundary membrane and the protocell
membrane could concentrate some ions,
such as Ca2+ and protons, which would be
stored in the extracellular buffering compartment and injected into the protocells as
required. It is conceivable that many of the
mechanisms proposed for permeation of the
membrane in the ‘inside the vesicle’ scenario
(FIG. 2c) could be operational in the ‘outside
the vesicle’ model (FIG. 3). However, instead
of molecules entering the future cytoplasm,
in the latter model, the transport of mole­
cules into the lumen of the vesicles would
allow the future extracellular space to evolve.
The stage of protocells within a vesicle
might, perhaps, have existed for a relatively
long time. The evolutionary split between
eukaryotes and prokaryotes55 could also have
developed in this system, protected by two
membranes.
Conclusions and perspectives
I have outlined a plausible scenario for
the co-evolution of the cytoplasm and the
membrane based on existing cell biological
principles. I have discussed the two principal
theories with respect to the evolution of the
membrane and suggested that the ‘cytoplasm
outside’ model (FIG. 3) is more plausible than
the more favoured ‘cytoplasm inside the
vesicle’ model. If the inside-out (or obcell)
model is correct, it seems reasonable to suggest that it needed the active participation of
a cytoskeleton to ‘invert’ the topology of the
membrane system and initiate the cellular­
ization process. The model predicts that the
actin and tubulin family preceded the LCA
and that they are probably as universal as the
vacuolar ATPase family.
Experimentally, one could ask how a
liposome system with a subset of reconstituted membrane proteins would behave
with cytosolic extracts and factors such as
glucose, GTP and ATP. If the cytoplasmic
domain of these membrane proteins all faced
outwards, some aspects of the ‘life outside
the vesicle’ model presented here could be
tested. Alternatively, if they had the opposite
orientation, they could perhaps be used
to test some aspects of the ‘life within the
vesicle’ scenario.
Gareth Griffiths is at the
European Molecular Biology Laboratory,
Heidelberg, Germany.
e-mail:
[email protected]
doi:10.1038/nrm2287 Published online 31 October 2007
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Acknowledgements
I thank T. Gibson, M. Knop, S. Trachtenberg, D. Lancet,
G. van Meer, L. Mayorga and J. Reich for their comments
and discussion, and C. Bleck for preparing the figures.
DATABASES
Entrez Genome Project: http://www.ncbi.nlm.nih.gov/sites/
entrez?db=genomeprj
Listeria monocytogenes | Mycoplasma genitalium
UniProtKB: http://beta.uniprot.org/
FtsA | FtsZ | N-WASP | SNX9
FURTHER INFORMATION
Gareth Griffiths’s homepage:
http://www-db.embl.de/jss/EmblGroupsOrg/g_79.html
All links are active in the online pdf
1024 | December 2007 | volume 8
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