Lecture Set 18

Question 1

Describe the actin filament including polarity and the different types

Answer 1

Made of 2 f-actin strands. Each f-actin strand is made of polymerized G-actin proteins
6 types of actin, 4 α in muscle, β and γ in non-muscle (structure/movement)
The filament has a - end where there is an exposed binding cleft, and a + end which is the barbed end

Question 2

How do microfilaments change morphology?

Answer 2

Very easy to assemble/disassemble so can change shape through destruction/construction
most abundant intracellular protein, concentrated beneath plasma membrane

Question 3

Describe addition and dissociation of actin (ATP/ADP, phases of addition, rates)

Answer 3

Can occur at either end, addition occurs faster at the + end, dissociation rates are equal at either end
ATP bound to free G-actin, when it polymerizes, hydrolyzes to ADP. This is less stable and promotes dissociation since it reduces affinity of the subunit for neighbouring subunits. If addition rates are faster than hydrolysis, then you form a cap
3 phases to addition: nucleation, elongation, and steady state
nucleation requires 3 actins bound together, and is a slow step

Question 4

Describe some actin-binding proteins

Answer 4

thymosin: most abundant, binds ATP-G-actin and doesn’t allow hydrolysis/exchange, prevents association at either end
profilin: binds to opposite side from binding site, forces release of ADP and binding of ATP. Only allows association at + end
cofilin: forms bridge between two actin monomers, causes twist that promotes dissociation of segments of ADP-actin

Question 5

Describe some actin-capping proteins

Answer 5

Cap-Z (caps + end) and tropomodulin (caps - end)

Question 6

What other ways can proteins bind to actin filaments?

Answer 6

cross-linking for stabilization, bundling
branching/growth –> Arp 2/3 complex activated by WASP that nucleate new branches off existing filaments –> can cause movement of the cell membrane

Question 7

Describe the structure of intermediate filaments

Answer 7

Have a section where there is a coiled-coil motif of α helices, two of these form a dimer in opposite orientation of dimers (tetramer) which overlapping to form a protofilament. 4 protofilaments form protofibril, 4 protofibrils form a filament.
Very rigid, strong
Don’t require ATP/GTP, formed through association of subunits

Question 8

What is the purpose of Rho GTPases?

Answer 8

affect dynamics, cdc42/Rac/Rho active in cell migration, phagocytosis, actin remodeling