unit 6 - cytosketelon - part 2 Flashcards
structure of actin filaments
- made up of linked globular actin monomers (G actin) with a pointed end and a barbed (notched) end that come together in a double helix-like structure (F actin)
- pointed end: minus end
- barbed end: plus end
what is dynamic instability?
growing and shrinking at a rapid rate. actin filaments can grow by the addition of monomers at both ends but it grows faster on the plus end (barbed) than the minus end (pointed)
treadmilling of actin
simultaneous gain of monomers at the plus end of actin filament and loss at the minus end. when rates are equal, filaments remain the same size. allows rapid switch from growth to shrinkage
proteins that stabilize actin filaments
profilins: sequestering proteins that keep actin monomers from binding until needed (also keeps it from wasting ATP)
formins: nucleating proteins - help initiate and stimulate elongation of filaments. promotes long, unbranched filaments
ARPs: nucleating proteins - help initiate and stimulate elongation of filaments. promote branching by attaching near the plus end of a filament and acting as a new nucleation site
capping proteins: bind to the ends of filaments to stabilize them, or prevent them from growing/dissociating
crosslinking proteins: connect several actin filaments together, in parallel bundles or in overlapping webs that form large sheet structures.
functions of actin filaments
- actin filaments form a branched web around the outer edge of the cell– cell cortex, which provides shape and mechanical strength to the cell
- help form structures like microbilli in the small intestine, or form the shape of red blood cells - provide the structure for cell protrusions: lamellipodia (cell crawling), filopodia (antenae), and pseudopodia (phagocytosis)
function of Rho protein family
GTP-binding formin. acts as a molecular switch and promotes rapid assembly of long, unbranched bundles of actin filaments
drugs that affect actin filaments
what causes dynamic instability in actin filaments?
- free actin monomers are tightly bound to ATP
- actin monomers bind to the plus end and hydrolyze the ATP to ADP
- monomers with ADP aren’t bound as tightly and begin to disassociate at the minus end
role of actin in cell crawling
- ARPs bind to the actin filaments at the leading edge of the cell cortex. promotes the formation of a web of branched actin filaments that protude into lamellipodia
- actin forms anchorage points with the surface the cell is crawling on
- plus ends of the actin filaments are capped. minus ends continue to disassemble. causes the rear end of the cell to contract and the cell shifts forward
- old anchorage points are severed, cell forms new lamellipodia and repeats the process
role of actin in filopodia formation
relys on formins that attach to the plus end of the actin filament and promote addition of new monomers to form straight, unbranched filaments
