Download What determines the size and shape of a cell?

Cell Physiology in Health and Disease
Learning objectives
BIOL2174
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Cell structure and function
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[email protected]
Cells are complex!
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‘to build the most basic yeast cell .. you
would have to miniaturize the same number of
components as are found in a Boeing 777 and
fit them in a sphere just 5 Pm across; then
somehow you would have to persuade that
sphere to reproduce.’
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To understand the limitations on cell size
To understand why eukaryotic cells are
compartmentalized
To understand the role of the cytoskeleton
To understand different aspects of molecular
movement within cells
What determines the size and
shape of a cell?
Bryson B (2003). A short history of nearly everything.
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Cells have very different shapes
but are usually about the same size
Structure of a motor neuron
Motor neurons can be 1 metre long
Neurons communicate with other
cells through synapses
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An axon terminal releases chemical signals which
result in electrical changes in the post synaptic cell
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The axon is surrounded by a myelin
sheath which provides insulation
Neurons
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Neurons have very different plasma membrane
protein composition at different sites
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Charcot-Marie-Tooth disease
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Most common inherited disease of the peripheral
nervous system
Affects 1 in 2500 people
Loss of motor and sensory function: muscle weakness
in extremities, loss of senses, loss of reflexes, wasting
Due to reduced function of neurons in the peripheral
nervous system. Two types:
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cell body, axon, synapse
Molecules and organelles travel very long
distances within a single cell
What limits the size of a cell?
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Surface to volume ratio
Rate of diffusion
Need to maintain adequate concentrations of
metabolites
Demyelinating neuropathies ( mutations affecting myelin
production), 50% cases
Axonal neuropathies (mutations affecting structure or function of
the axon), 20-40% cases
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Some cells have increased surface
area
Surface to volume ratio
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As size increases,
surface to volume ratio
decreases
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Surface area increases as
the square of length but
volume increases as the
cube
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Cells that are specialized for nutrient uptake eg
epithelial cells use membrane folds to increase
surface area
S/V = 6
Cells need large surface
areas to allow import of
nutrients and export of
waste
S/V = 2
Source: Alberts et al., Molecular Biology of the Cell
Membranes have other functions
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The plasma membrane is specialised to
interact with the environment
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Transport and signalling
Eukaryotic cells use internal membranes for
some functions
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The plasma membrane
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Interface between the cell and its environment
Nutrient uptake and waste export
Cell-cell signalling and contact
Excitability
Secretion
Respiratory chain
Synthesis of lipids and proteins
Campbell et al, Biology
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Mammalian cells are compartmentalised
Eukaryotic cells require large
amounts of internal membrane
Golgi stacks
Source: Alberts et al., Molecular Biology of the Cell
Eukaryotic cells are
compartmentalized
Alberts
Eukaryotic cells are
compartmentalized
Plus all the other
stuff
Golgi plus ER
(yellow)
Marsh et al., 2001
Vesicles
(purple and white)
Mitochondria
(green)
Ribosomes
(orange)
Marsh et al., 2001
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Internal membranes are also
limiting
Eukaryotic cells are crowded
Diffusion of molecules inside cells
Rate of diffusion: how long does a
molecule take to cross a cell?
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Molecules are in constant motion
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Molecules can only interact when they bump into each
other, eg
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Enzyme and substrate
Receptor and ligand
Protein/protein interaction (formation of complexes)
Diffusion is important within a compartment; other
mechanisms are needed to move molecules between
compartments
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Diffusion is random movement
Rate of diffusion depends on
the size of the molecule
Time taken increases as the
square of the distance travelled
A small organic molecule takes
about 0.02 sec to travel across
a small eukaryotic cell (10 Pm)
BUT, 9.3 hours to move 1 cm!
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Mitochondrial distribution in cells
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Mitochondrial distribution represents a
compromise between distance for diffusion of
O2 from blood and distance for diffusion of ATP
to where it is needed
Need to maintain adequate
concentrations of metabolites
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The larger the cell, the more metabolite
molecules there need to be to maintain the
same concentration
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For a 10 nM concentration:
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10 molecule/cell in E. coli
10,000 molecules/cell in HeLa (human) cell
Kinsey, S. T. et al.(2007)
J Exp Biol ;210:3505-3512
Cells are compartmentalized
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Can increase concentrations of metabolites in an
organelle (smaller volume)
Can separate potentially harmful or interfering
reactions
Increased specialization
Generate more membrane
BUT, there must be coordination between the
functions of different organelles
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Transport, communication, regulation
Functions of organelles
¾Mitochondria
¾Peroxisome
¾Cytosol
Energy metabolism
Oxidative metabolism
Metabolism, anabolism, protein
synthesis
¾Nucleus
DNA replication, transcription
¾Endoplasmic reticulum Protein synthesis, quality control
¾Golgi
Protein delivery, glycosylation
¾Endosome
Protein degradation, recycling
¾Lysosome
Degradation
¾Plasma membrane
Signal transduction, excitability
cell-cell contact, secretion
¾Cytoskeleton
Cell shape, compartmentation
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How is the compartmentalization
maintained?
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How is the organelle generated or
maintained?
How do organelles receive a specific set of
proteins and lipids?
How do metabolites (substrates) enter and
leave organelles?
Delivering proteins to organelles
Organelles divide and are inherited
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‘the spatial organization in a cell is not
(entirely) written in the genetic blueprint; it
emerges
from the interplay of genetically
specified molecules
. constrained by
heritable structures’
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Harold, FM (2005) Microbiol Mol Biol Rev: 69: 544
Some organelles have their own
genome
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Mitochondria and chloroplasts
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Needs communication between the nuclear
and other genome
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Replication of the organelle involves DNA
replication as well as division of the organelle
Gated transport
Transmembrane transport
Vesicular transport
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Mitochondria: powerhouse of the
cell
Mitochondria generate ATP by
oxidative phosphorylation
Source: Lodish et al., Molecular Cell Biology
Evolution of mitochondria: the
endosymbiont hypothesis
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Mitochondria evolved
when an ancestral
eukaryotic cell engulfed
a bacterium
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Mitochondria surrounded
by a double membrane
Mitochondria have their
own genome (circular)
Protein synthesis in
mitochondria is bacterialike (sensitive to the same
antibiotics)
The mitochondrial genome: coordination of
expression between mitochondria and
nucleus
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Small – just 13 proteins encoded by mammalian
mitochondrial DNA
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Mitochondria require complete transcription and
translation machinery to produce these 13 proteins
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All 13 are components of the respiratory chain
Estimated that about 25% of the proteins found in
mitochondria are there to produce these 13 proteins
RNA polymerase, ribosomal proteins etc are encoded
by nuclear genes and the proteins imported into
mitochondria
Mitochondrial DNA encodes 2 rRNAs and 22 tRNAs
Source: Alberts et al., Molecular Biology of the Cell
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Nuclear-encoded mitochondrial proteins are
synthesised in the cytosol and then delivered
to mitochondria
Coding of mitochodrial complex
subnunits
Blue: nuclear
Orange: mito
Pink: either,
in different
species
Gated transport
Transmembrane transport
Vesicular transport
Cytoplasmic inheritance of
mitochondria
Genetics of
mitochondrial diseases
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Mitochondrial diseases may show:
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Mendelian inheritance if the gene affected is in the nuclear
genome
Maternal inheritance if the gene affected is in the
mitochondrial genome
Mitochondrial dysfunction is important in aging
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Mitochondrial genomes accumulate mutations in somatic
tissue
DNA in mitochondria has a higher error rate than nuclear
DNA (fewer repair mechanisms and increased oxidative
stress)
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Mitochondrial diseases
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Symptoms of mitochondrial
diseases are usually seen in
organs with high energy
requirements
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Muscle
Nervous system
eyes
Leber’s hereditary optic
neuropathy
Electron transport chain
component
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MERRF (myoclonic epilepsy
with ragged red fibres)
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Mitochondrial mutations are a
factor in aging
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Mitochondrial tRNA
Mitochondria fuse and divide
Source: Alberts et al., Molecular Biology of the Cell
Mitochondrial DNA polymerase was replaced with a
mutant form that lacks proofreading
Higher mutation rate of mitochondrial DNA leads to
premature aging and death.
Mitochondria fuse and divide
Source: Alberts et al., Molecular Biology of the Cell
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Fusion of mitochondria requires
mitofusin 2
Mitochondrial fission and disease:
Charcot-Marie-Tooth Disease
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Summary
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Cell size is limited
Cells can increase in complexity by using
organelles for specialised functions
Organelles:
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May have their own genomes
Divide and are inherited by daughter cells
Receive proteins and metabolites from the
cytoplasm
Loss of motor and sensory function: muscle weakness
in extremities, loss of senses, loss of reflexes, wasting
Some patients have an axonal neuropathy due to a
mutation in the gene encoding mitofusin 2
Large clumps of mitochondria form in nerve cells,
mitochondria cannot travel down axons and this might
lead to loss of function of nerve cells
Delivering proteins, membranes and
organelles to other parts of the cell
The secretory
pathway and
cytoskeleton
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The secretory pathway: delivering
proteins and lipids to the PM
Physiological functions of the
endoplasmic reticulum (ER)
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Synthesis of plasma membrane
proteins
Synthesis of lysosomal proteins
Synthesis of secreted proteins
Quality control and protein
degradation
Synthesis of membrane lipids
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Calcium homeostasis
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Source: Alberts et al., Molecular Biology of the Cell
Moving proteins and membranes in
the cell
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Vesicles transport proteins,
signalling molecules and lipids
The ER and Golgi (secretory pathway) play a
central role in the synthesis and trafficking of
lipids as well as proteins
Proteins and lipids are moved from the ER and
Golgi to other destinations by vesicle trafficking
Vesicles are also used to internalise plasma
membrane proteins and extracellular
compounds
Alberts Fig 13.2
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Exocytosis and endocytosis
Alberts Fig 13.1
Directing lipid composition of a
membrane
The ER synthesizes phospholipids
Source: Lodish et al., Molecular Cell Biology
Mitochondria may receive their
lipids from the ER
ER
(yellow)
Mitochondria
(green)
Alberts Fig 12.58
Marsh et al., 2001
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Three exit pathways start from the
Golgi
Vesicle trafficking in neurons
Source: Lodish et al., Molecular Cell Biology
Synaptic vesicles
Alberts Fig 13.73
Problems!
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How can the vesicles move
such a long distance without
getting lost?
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How do the vesicles ‘know’
which part of the membrane
to fuse with?
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How are vesicle contents
made specific for each
destination?
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Reference for vesicle trafficking in
neurons
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http://icarus.med.utoronto.ca/neurons/index.swf
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The cytoskeleton
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Gives cells:
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Chapters 2 and 6
Chapter 1 for review of neuron structure and function
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Structure
Strength
Ability to move
Ability to move and rearrange
organelles
Dynamic: the cytoskeleton
allows cells to respond to
environmental signals by
moving, dividing, changing
shape
Source: Alberts et al., Molecular Biology of the Cell
Three types of filaments constitute
the cytoskeleton
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Actin filaments
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Cell shape and structure
Cell movement (muscles,
whole cell locomotion)
Microtubules
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Composition of the cytoskeleton
Intracellular movement
(vesicles and organelles)
Cell division
Intermediate filaments
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Mechanical strength
Source: Alberts et al., Molecular Biology of the Cell
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Cytoskeleton filaments are made from proteins
(different for each filament type)
Monomer subunits make multiple non-covalent
interactions to form a strong and stable protein filament
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The cytoskeleton determines cell
shape
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The cytoskeleton is dynamic
Muscle contraction
Cell motility
Cell shape (e.g.
microvilli)
Cell polarity
Fixation of membrane
proteins
Source: Lodish et al., Molecular Cell Biology
Membrane proteins are attached to
the cytoskeleton
Source: Alberts et al.,
Molecular Biology of the
Cell
Duchenne muscular dystrophy
¾Muscle degeneration
and fibrosis
¾Creatine kinase
levels elevated
¾Totally disabled
at the age of 12
¾Death at the age of 20
Source: Lodish et al., Molecular Cell Biology
Lodish Figure 19.35
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Membrane proteins are localised by
the cytoskeleton
Membrane proteins are localised by
the cytoskeleton
Newpher et al 2008
Luscher and Keller (2004) Pharm and Ther: 195-221, 102
The cytoskeleton and membrane
proteins
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The cytoskeleton is important in maintaining
heterogeneity in plasma membrane protein
content:
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Biology of the Cell
www.biolcell.org
Clustering proteins with the same or related
functions in the membrane (actin)
Delivering proteins to particular locations in the
plasma membrane (microtubules)
Biol. Cell (2007) 99, 297297-309
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Myosin: the actin motor protein
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The microtubules
Myosin uses ATP hydrolysis to do mechanical
work (movement)
Source: Lodish et al., Molecular Cell Biology
Microtubules contribute to different
cell shapes and to cell division
Source: Lodish et al., Molecular Cell Biology
Cytoskeleton in neurons
Source: Lodish et al., Molecular Cell Biology
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Functions of microtubules
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Kinesins: the molecular motors that
drive vesicle transport
Microtubules form
‘tracks’ and use motor
proteins to move
organelles and vesicles
within the cell
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Source: Lodish et al., Molecular Cell Biology
Kinesins use the energy from ATP hydrolysis to ‘walk’
along microtubules
Source: Lodish et al., Molecular Cell Biology Figure 18.20
Vesicle trafficking in neurons
Biology of the Cell
www.biolcell.org
Biol. Cell (2007) 99, 297297-309
Alberts Fig 13.73
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How are vesicles specifically
targetted to particular locations?
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Coated vesicle budding
Protein/protein interactions
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Vesicles have protein coats (important for the
budding process and for targetting)
Vesicles contain destination-specific binding
proteins that bind ‘cargo’ proteins and deliver them
to the appropriate destination
Receptors in destination membranes bind vesicles
Budding and fusion of vesicles is regulated by
GTPases
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Coat proteins bind to
membranes in such a
way as to bend them
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Coat proteins also bind
transmembrane
receptors for cargo
proteins
Lodish Figure 14.6a
Different coat proteins have
different functions
Budding and uncoating of a vesicle
Alberts Fig 13.4
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GTPases are
involved
Vesicle budding
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Fusion of a vesicle to a target
membrane
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Caused by distortion of membrane by binding
of coat proteins (clathrin, COPI, COPII)
Coat proteins interact with transmembrane
receptors that bind cargo molecules
GTP hydrolysis drives conformational changes
Once a vesicle has budded, it is uncoated and
interacts with kinesins for transport
Docking and fusion of transport
vesicles
Vesicles dock through
the interaction of SNARE
proteins
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v-SNAREs on the vesicle
t-SNAREs on the target
membrane
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Vesicle fusion
Vesicle fusion
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Different coat proteins mediate
trafficking to different compartments
Specific membrane proteins on the vesicle
exterior interact with docking proteins on the
target membranes (SNAREs)
This leads to fusion with the target membrane
(may be regulated eg Ca2+ regulates fusion of
synaptic vesicles)
Soluble contents are released and membrane
proteins can diffuse laterally into the target
membrane
Specificity
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All of the proteins involved in vesicle budding
and fusion are members of gene families
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All perform the same function but different
members target vesicles to different locations
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Charcot-Marie-Tooth disease
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Caused by mutations in genes that affect
trafficking of organelles and vesicles
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Kinesin-like protein, probably involved in
mitochondrial transport
GTPase from vesicle fusion (Rab) family
Keeping cells the same size
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Vesicle fusion leads to an increase in cell
surface area which must be balanced by loss
of membrane (if the cell is not growing)
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Endocytosis and exocytosis must be balanced
Neurons cannot function without axonal
transport
Intermediate filaments
Diseases due to mutations in
intermediate filament genes: keratins
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Source: Lodish et al., Molecular Cell Biology
Mutations in keratins
cause epidermal
blistering due to
rupturing of cells
Consistent with a role for
keratins in the response
to mechanical stress
Source: Lodish et al., Molecular Cell Biology
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Diseases due to mutations in
intermediate filament genes: lamins
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More than 230 mutations in the lamin A gene
cause at least 13 different diseases
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Muscular dystrophy
Cardiomyopathies
Premature aging
Charcot-Marie-Tooth
Cytoskeleton summary
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The cytoskeleton filaments contribute to cell shape,
structure and strength
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Actin and microtubules have associated motor proteins
that drive movement
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Links between function and disease not well
understood
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Actin, whole cell locomotion, muscle contraction
Microtubules, cell division, intracellular trafficking
Intermediate filaments provide structure and strength to
cells and do not have associated motor proteins
Charcot-Marie-Tooth subtypes
Cell size and shape
Disease
subtype
CMT2A1
Gene
mutated
KIF1B
Protein encoded Likely function
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Kinesin-like
protein
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CMT2A2
MFN2
Mitofusin 2
CMT2B
RAB7
CMT2B1
LMNA
Ras-related
protein Rab-7
Lamin A/C
Mitochondrial
transport
Mitochondrial
fusion
GTPase –
vesicles
Cell structure
Cells vary in shape more
than in size
Size is constrained by:
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Surface area: volume ratio
Diffusion
Concentration of molecules
Shape is determined by
the cytoskeleton
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Subcellular specialisation
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Cells are compartmentalised: different parts of cells
perform different functions
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Organelles
Different parts of the same structure, eg plasma membrane
Internal and external membranes are specialised for different
functions
Energy is required to maintain subcellular organisation
and communication (diffusion is not sufficient for the
movement of vesicles and organelles)
The cytoskeleton plays an important role in maintaining
cell size, shape and function
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