cytoskeleton Flashcards
why does a cell need cytoskeleton
- helps maintain shape
- provides mechanical strength
- transports vesicles
- movement
what are microfilaments
- twisted chain of the monomer units of the protein G-actin
- F- actin is filamentous polymer composed of G-actin monomers
- thinner - 7nm
- 3 iso-forms of G-actin with different isoelectric points
- alpha in muscle cells
- beta and gamma in non muscle cells
- actin filaments can grow by addition of actin monomers
- the length of the filament is determined by
the conc of G actin
and the presence of G actin binding proteins
what are G actin levels controlled by
2 actin binding proteins
- thymosin - prevents the additional actin monomers to F actin
- profilin - faciliates actin polymerisation
describe actin protein binding to filaments
actin bundling proteins - keep F actin in parallel bundles
cross-linking proteins - maintain F actin in a gel like meshwork
F- acting severing proteins- Break F actin into smaller filaments
motor proteins - transport of vesicles and organelle through actin filaments
what is the function of actin filaments ?
Skeletal muscle
- arranged in para-crystalline array integrated with different ABPs
- interaction with myosin motors allow muscle contraction
NON muscle cells
- actin filaments form a thin sheath beneath the plasma membrane associated with myosin for the cleavage of mitotic cells
-In cytokinesis actin filaments are involved in the actin-myosin ring
-
function of actin filaments In cell migration
In cell migration the cell pushes out protrusions at its front [actin polymerization] which adhere to the surface [integrins link actin filaments to extracellular matrix surrounding the cell]. Cell contraction and retraction of the rear part of the cell [interaction between actin filaments and myosin]
what is intermediate filaments
- provide mechanical strength -
- ropelike
- form a network through out the cytoplasm joining up cell-cell junction
- ## surrounding nucleus
what is the structure of intermediate filaments
- N terminal globular head
- C terminal globular tail
- central elongated rod like domain
- ## units form stable dimers , every 2 dimers form tetramers and the bind to each other and twist to form a rope like filament
function of intermediate filaments
- tensile strength
- structual support by forming 3D strucutac framework
-present in all nucleated
eukaryotic cells - form mesh rather than “rope-
like” structure - line in the inner face of the
nuclear envelope to:
– strengthen it
– provide attachment sites for
chromatin - disassemble and reform at each
cell division as nuclear envelope
disintegrates
– i.e. very different from the stable
cytoplasmic IFs
– process controlled by post-
translational modifications (mainly
phosphorylation and
dephosphorylation
where are intermediate filaments found
Intermediate filaments found in cytoplasm and nucleus. In the cytoplasm keratins, vimenatins, neurofilaments provide tensile strength and structural support. In the nucleus lamins Form mesh rather than ‘rope-like’ structure. Line in the inner face of the nuclear envelope to stengthen it and provide attachment sites for chromatin. Disassemble and reform at each cell division as nuclear envelope disintegrates. IFBP (intermediate filaments binding proteins) link intermediate filaments structures and reinforce them into 3D networks. Examples include fillagrin (binds keratin filaments into bundles) and plakins (keep contact between desmosomes).
what are microtubules
Hollow tubes made up from the protein tubulin. Thicker of the filaments. Each filament is polarized. Assemble and disassemble in response to cell needs.
what is the polymerisation of microtubules?
Microtubule organizing centre
(MTOC) are specialized
protein complexes from
where assembly of tubulin
units starts.
* Centrosome (in the
perinuclear region) is the
MTOC in most of the cells
– Contains g-tubulin ring that
initiates the microtubule
growth.
* Heterodimers of a and b
tubulin constitute the
microtubule.
* It is a polarized growth (i.e.
there is an end that grows
faster (+end) than the other (-
end).
what are the function of microtubules?
intracellular transport
– act like railway tracks on which molecular motors run
– different motors for different cargoes
– directionality of filaments is vital (each motor only moves in one
direction)
* organises position of organelles
– hence, provides polarisation of cells
– directionality of filaments is vital
Rhythmic beating of cilia and
flagella
– Motile processes, with highly
organized microtubule core.
– Core consist of 9 pairs of
microtubules around 2 central
microtubule (axoneme).
– Bending of cilia & flagella is
driven by the motor protein
Dynein.
– The basal body, at the base of
the tubule, controls the
assembly of the axoneme
Cilia in the
respiratory tract,
sweeping mucus
and debris from
lungs
– Flagella on
spermatozoa
