14.1 Structure and Organization of Actin Filaments Flashcards
() in the most abundant cytoskeletal protein of most cells (5-10% of total proteins in eukaryotic cells)
actin
actin polymerizes to form () → thin, flexible fibers
actin filaments (microfilaments)
each actin monomer () has tight binding sites that mediate head-to-tail interactions with 2 other monomers → forms a trimer (nucleation)
globular [G] actin
polymerization of G actin leads to formation of ()
filamentous (F) actin
why are actin filaments polar (i.e. have plus and minus ends)?
all actin monomers are oriented in the same direction
describe “treadmilling” of actin filaments
ATP-actin is added to plus end while ADP-actin is dissociated from the minius end of a microfilament
describe the general dynamics of actin filaments
- the plus end of actin filaments elongates by the addition of ATP-actin monomers
- actin bound to ATP associates with plus ends, ATP is then hydrolyzed to ADP
- actin-ATP binds more readily to actin filaments compared to actin-ADP
- ADP-actin found at the - end is less tightly bound, and thus actin monomers are more readily disassociated from this end
actin-binding proteins regulate the ff:
- assembly and disassembly of actin filaments
- cross-linking of actin filaments into bundles and networks
- association of actin filaments to cell structures (e.g. plasma membrane)
what are the principal proteins that stimulate the initiation and elongation of actin filaments?
- formin
- Arp2/3 complex (actin-related protein)
proteins involved in the nucleation, initiation, and growth of actin filaments → they move along growing filament and add new ATP-acting monomers at plus end
formins
binds actin monomers and stimulates exhange of bound ADP for ATP, increasing local concentration of ATP-actin → promotes actin polymerization
profilin
initiates growth of branched actin filaments → important in driving cell movement at the plasma membrane
Arp2/3 complex
actin-binding proteins that stabilize actin filaments by binding lengthwise along filament groove
tropomyosins
actin-binding proteins that stabilize actin by binding to the plus or minus ends
capping proteins
actin-binding proteins that associate microfilaments into bundles or networks
cross-linking proteins
() are filaments cross-linked into parallel arrays
actin bundles
() are filaments cross-linked in arrays that form 3D meshworks (have properties of semisolid gels)
actin networks
actin-binding protein that severs filaments → generates new ends which are then available for polymerization or depolymerization
cofilin
a drug that affects actin polymerization by binding to plus ends and block elongation → inhibits cell movement and even cell division
cytochalasins
a drug that affects actin polymerization by binding to actin filaments and prevents dissociation
phalloidin
network of microfilaments and associated proteins under the plasma membrane
cell cortex
member of calponin family and is the major protein that provides the structural basis for the cortical cytoskeleton; a tetramer of two polypeptides, ⍺ and β
spectrin
spectrin ends associate with short actin filaments → forms the ()
spectrin-actin network
links the spectrin-actin network and the plasma membrane by binding to spectrin and band 3 (transmembrane protein)
ankyrin
ankyrin binds to spectrin and ()
band 3 transmembrane protein
() is another link that binds spectrin-actin junctions and glycophorin (transmembrane protein)
protein 4.1
protein 4.1 binds spectrin-actin junctions and ()
glycophorin
links actin filaments to transmembrane proteins that link to the extracellular matrix → maintains cell stability during muscle contraction
dystrophin
what is the cause of Duchenne’s muscular dystrophy?
absence (complete KO) of dystrophin
what is the cause of Becker’s muscular dystrophy?
abnormal expression of dystrophin
specialized plasma membrane regions of fibroblasts that form contacts with the adjacent cells’ extracellular matrix
focal adhesions
in focal adhesions, fibroblast cells attach to extracellular matrix by binding to transmembrane proteins called ()
integrins
for fibroblasts, focal adhesions are also the attachment sites for large actin bundles called ()
stress fibers
stress fibers are contractile fibers, cross-linked by (1) and stabilized by (2)
- alpha-actinin
- tropomyosin
what proteins are involved in stress fiber binding in focal adhesions
- talin - interacts with vinculin and integrin
- vinculin - interacts with talin and actin filament
cell-cell contacts in epithelial cell sheets
aderens junctions
the continuous belt of cell-cell contacts around each epithelial cell in epithelial cell sheets
adhesion belt
what are the proteins that are involved cell-cell contact in adherens junctions
- cadherins - transmembrane proteins that mediate cell-cell contact by binding to cytoplasmic catenins
- catenins - interact with actin filament and cadherins, anchoring the actin filaments to the plasma membrane
extensions of the cell surface are usually based on (), either in permanent or transient conformations; the latter usually form in response to environmental signals
actin filaments
cell surface extensions of moderate width; responsible for phagocytosis and movement of amoebas
pseudopodia
broad, sheet-like extensions at the leading edge of fibroblasts
lamellipodia
projections of the plasma membrane supported by actin bundles
filopodia
fingerlike extensions on plasma membranes that are abundant on cell surfaces involved in absorption (e.g. epithelial cells); serve to increase surface area of the cell
microvilli
in the epithelial cells lining the intestine, microvilli form a () (about 1000 microvilli per cell) on apical surface of cell
brush border
intestinal microvilli contain parallel bundles of 20-30 microfilaments cross-linked by (1) and (2)
- fimbrin
- villin
actin bundles of intestinal microvilli are attached to the plasma membrane by the calcium-binding protein (1), in association with (2)
- calmodulin
- myosin I
how are extensions of the plasma membrane driven by branching and polymerization of underlying actin filaments
- growing actin filaments push against plasma membranes and drive formation of protrusions
- inhibition of actin polymerization blocks the formation of cell surface protrusions
cell movement across a surface proceeds in 3 stages
- extension of leading edge
- attachment of leading edge to substratum
- retraction of trailing edge
like other cell surface protrusions, extension of the leading edge during cell motility involves
branching and polymerization of actin filaments
formation of cell surface protrusions in response to external stimuli is mediated by small (), which are activated by signals stimulating cell movement
GTP-binding Rho proteins
Rho proteins promote actin polymerization by:
- stimulating Arp2/3 complex (initiates growth of branched actin filaments)
- activating formins (initiates growth of linear filaments)
- activating WASP
what is the role of cofilin in cell motility
- by cleaving existing filaments, it creates new plus ends to support filament branching and growth
as new actin filaments extend into the growing cell, they also provide pathways for vesicles containing ()
lipids and proteins needed for continued extensions
for slow-moving cells, cell attachment to surface requires ()
rebuilding cell-substratum adhesions (e.g. focal adhesions)
retraction of the trailing edge into the cell body involves the action of (1) and (2)
- small GTP-binding Arf proteins
- Rho proteins
what is the role of Arf GTPases in the retraction of the trailing edge
control membrane endocytosis and regulate Rho proteins to modulate actin polymerization and dynamic reorganization
what is the role of Rho GTPases in the retraction of the trailing edge
causing the formation of stress fibers and actomyosin contractility providing tension for the cell to retract its tail and move forward
