Download Chapter 10 Intracellular Compartments and Transport

Chapter 10
Intracellular Compartments
and Transport
In eucaryotic cells, internal membrane create enclosed
compartments and organelles in which different
metabolic processes are segregated
Membrane-enclosed organelles
Aggregates of glycogen in liver cell
A cell from the lining of the intestine contains the
basic set of organelles found in most animal cells
(nuclear envelope)
glycosylation
steroid smooth ER
rough ER
; Ca2+ store
Nuclear membranes and the ER may have evolved
through investigation of the plasma membrane
Endomembrane system
Nuclear membrane
ER
Golgi apparatus
Endosomes
Lysosomes
Mitochondria are though to have originated when a
procaryote was engulfed by a large eucaryotic cell
symbiosis
a.
b.
c.
d.
Two membranes
Do not participate in the vesicle traffic
Have their own small genomes
Can make some of their own proteins similar to bacteria
Chloroplasts almost certainly evolved
from engulfed bacteria
Membrane-enclosed organelles import proteins
1. Require energy
by one of three mechanisms
2. Sorting signal:
amino acid sequence
nuclear pores
(folded proteins)
protein translocators
few proteins are synthesized on ribosomes
inside these organelles
(unfolded proteins)
Proteins sorting
Cytosol
Protein
ER
Mitochondria
Chloroplasts
Interior of the nucleus
Golgi apparatus
Lysosomes
Endosomes
Nuclear membranes
Plasma membrane
(folded proteins)
15-60 amino acids
KDEL
1. Physical properties (hydrophobicity)
2. Charged amino acids
Signal sequences direct proteins to the correct organelle
The outer nuclear membrane is continuous with the ER
A finely woven meshwork
of protein filaments (lamin)
Molecules in the cell membrane
About 30 different proteins
(1) Small water soluble molecules
→ pass freely and non-selectively
(2) Large molecules (RNAs, proteins) &
macromolecular complexes (ribosomal subunits)
→ sorting signal
Proteins bound for the nucleus are actively transported
through nuclear pores
Several positively charged
lysines or arginines
In their fully folded conformation
The energy supplied by GTP hydrolysis
drives nuclear transport
Proteins are imported into mitochondria
in an unfolded form
Unfolded form
Chaperone
proteins
Other
sorting
signals
The endoplasmic reticulum is the most extensive
membrane network in eucaryotic cells
ER
Nucleus
Ribosome
A common pool of ribosomes is used to synthesize both
the proteins that stay in the cytosol and those that are
transported into membrane-enclosed organelles,
including the ER
An ER signal sequence and an SRP direct a ribosome
to the ER membrane
A soluble protein crosses the ER membrane
and enters the lumen
A single-pass transmembrane protein is integrated
into the ER membrane
A double-pass transmembrane protein uses an internal
star-transfer sequence to integrate into the ER membrane
Membrane-based vesicle transport
Vesicles bud from one membrane and fuse with another,
carrying membrane components and soluble proteins
between cell compartments
(digestion)
(1) endosome: the main sorting station in the inward endocytotic pathway
(2) trans Golgi: the main sorting station in the outward secretory pathway
Clathrin molecules form basketlike cages that
help shape membrane into vesicles
Clathrin-coated pit
Clathrin-coated vesicle
Golgi  Plasma membrane
Clathrin-coated pits and vesicles
Clathrin-coated vesicles transport
selected cargo molecules
COP (coat protein)-coated vesicles
Golgi  ER
Rab proteins and SNAREs help direct transport vesicles
to their target membranes
Vesicle-specific SNAP receptor
or
Vesicle-soluble NSF attachment protein receptor
Target membrane-specific SNAP receptor
SNARE proteins play a central role in membrane fusion
H2O out
1.5 nm
Many proteins are glycosylated in the ER
Asparagine-X-Serine
Asparagine-X-Threonine
Oligosaccharide
protein transferase
(oligosaccharyl transferase)
Asparagine
14 sugars
N (NH2)-linked
glycosylation
Chaperones prevent misfolded or partially assembled
proteins from leaving the ER
ER retention signal
Misfolded proteins in the ER lumen trigger the production
of chaperones and the expansion of the ER
Unfolded protein response (UPR)
ER stress and UPR
The Golgi apparatus is made of a stack of flatted,
membrane-enclosed sacs
ER
entry
3-20
cisterna
exit
Plasma membrane
Several methods can be used to determine whether
a protein bearing a particle signal sequence
is transported into a preparation of isolated organelles
Temperature-sensitive mutants have been used to dissect
the protein secretory pathway in yeast
GFP fusion allows proteins to be tracked
GFP-viral coat protein
throughout the cell
ER
ER
Golgi
Golgi → Plasma membrane
Exocytosis
In secretory cells, the regulated and constitutive pathways
of exocytosis diverge in the trans Golgi network
Exocytosis
Default pathway
Regulated exocytosis pathway
Hormones
Mucus
Digestive enzymes
Secretory vesicles package and discharge concentrated
aggregates of protein
A pancreatic  cell
Endocytosis
Phagocytic cells
(cellular eating)
All eucaryocytic cells
(cellular drinking)
< 150 nm
> 250 nm
Molecules in the cell membrane
Phagocytosis
pseudopod
Red blood cell
macrophage
neutrophil
Pinocytosis by clathrin-coated pits and vesicles
LDL enters cells via receptor-mediated endocytosis
e.g. B12, iron, virus
Cholesterol + proteins
Acidic environment
PH = 5-6
The fate of the receptor proteins involved in endocytosis
depends on the type of receptor
Materials destined for degradation follow
different pathways to the lysosome
amino acids
sugars
nucleotides
H+ ATPase
Materials destined for degradation follow
different pathways to the lysosome