cytoskeleton Flashcards
Which three polymers make up the cytoskeleton
microtubules, intermediate filaments, actin filaments
general functions of microtubules
organelle positioning,
intracellular transport
general functions of intermediate filaments
mechanical strength
general functions of actin filaments
cell shape
organelle shape
cell migration
What do accessory proteins regulate?
- size and rate of filament formation (nucleation)
- polymerization/depolymerization
- function
Describe the structure of microfilaments
helical polymers made of actin, flexible, organsied into 2D networks and 3D gels
describe the structure of IF’s
heterogeneous group of filamentous proteins, rope like structure
describe the structure of microtubules
hollow tubes made of tubulin
rigid, long, straight
what monomer makes up actin filaments
g actin
how does g actin polymerize
twisted chain - structural polarity
how thick are actin filaments
7nm
What proteins are actin filaments associated with
actin binding proteins
what are the three isoforms of g actin
alpha - muscle cells
beta and gamma - non muscle cells
how do actin filaments (f-actin) grow
addition of g actin at either end
what two factors determine the length of an actin filament
concentration of g actin, presence of actin binding proteins (ABPs)
Which 2 ABPs mainly control G actin levels
PROFILIN - facilitates polymerization
THYMOSIN B4 - prevents addition of G actin to F actin
What do actin bundling proteins do
keep f actin in parallel bundles eg microvilli in epithelial cells
what do cross linking proteins do
maintain f actin in a gel like mesh eg cell cortex beneath plasma membrane
what do f actin severing proteins do
break f actin into smaller filaments
what do motor proteins do (myosin)
transport of vesicles/organelles through actin filaments
function of actin filaments in skeletal muscle
arranged in para-crystalline array integrated with different ABPs.
interaction with myosin motors allow muscle contraction
function of actin filaments in non muscle cells
cell cortex - forms thin sheath around plasma membrane
associated with myosin forms a purse string ring resulting in cleavage of mitotic cells
how do actin filaments aid in cell migration
- cell pushes out lamellipodia and filopodia at its front - actin polymerization
- protrusions adhere to surface - INTEGRINS link actin filaments to ECM surrounding cell
- cell contraction and retraction at rear - interaction between myosin and actin filaments
what makes IFs tough
resistant to detergents, high salt etc.
width of IFs
8-12nm
nuclear function of IFs
strengthen nuclear envelope
how do IFs form a network
join up to desmosomes throughout cytoplasm - withstands mechanical strength when stressed
what are the three parts of an IF unit
N terminal globular head
C terminal globular tail
central elongated rod like domain
what do to IF units form
a dimer
what do every 2 IF dimers form
tetramere
how do IF tetrameres form a filament
bind to each other and twist to constitute rope like filament
what types of IF are found in the cytoplasm
KERATIN - in epithelia
VIMENTIN (related) - connective tissue, muscle cells, neuroglial cells
NEUROFILAMENTS - in nerve cells
what are the nuclear IFs
nuclear lamins - in all nucleated cells
what do Intermediate Filament Binding Proteins do
linkers of IF structures/stabilize and reinforce IF into 3D networks
what is fillagrin
an IFBP than binds keratin filaments into bundles
what are synamin and plectin
IFBPs that bind desmin and vimentin, link IF to other cytoskeleton compounds eg actin/desmosomes
what are Plakins
IFBPs that keep contact between desmosomes of epithelial cells
functions of IFs in the cytoplasm
provide tensile strength
structural support - create deformable 3D network, reinforce cell shape and fix organelle localization
functions of IFs in nucleus
form mesh, line inner face of NE to strengthen and provide attachment sites for chromatin
role of IFs in cell division
dissemble and reform at each cell division as nuclear envelope disintegrates
structure of microtubules
hollow tubes made of tublin, relatively stiff 25nm, polarized
microtubules are dynamic - what does this mean
assesmble and disassemble in response to cell needs
how is tubulin distributed in the cell
roughly 50:50 free or in filament
how do microtubules polymerize
starts at Microtubule Organizing Centre (MTOC) - αandβ heterodimers - growth is polarized; faster at +ve end
where is the MTOC in most cells
centrosome - contains gamma tubulin ring that initiates microtubule growth
function of microtubules in intracellular transport
act like railway tracks for molecular motors - different motors for different cargoes - directionality is vital
function of microtubules in organelle positioning
organizes their position - providing polarisation of cells - directionality is vital
how do microtubules contribute to rhythmic beating of cilia and flagella
motile processes w/ highly organized microtubule core
what is an axoneme
core of cilia and flagella
what constitutes an axoneme
9 pairs of microtubules around 2 central axonemes
what is bending of cilia and flagella driven by q
motor protein dynein
whats the basal body
at base of tubule, controls axoneme assembly
big microtubule examples
cilia - resp tract - sweeps mucus and debris from lungs
flagella on spermatozoa
