actin

Question 1

what are the main types of actin accessory proteins?

Answer 1

polymerizing
nucleating
bundling
capping
severing
gelating
motor proteins

Question 2

what are polymerizing proteins? example

Answer 2

they add actin monomers to the filament. Profilin adds to the plus end

Question 3

what are nucleating protein? example

Answer 3

they allow the cell to control the time and place of de novo actin filament assemble. Arp2/3 and Formin nucleate actin filaments in vivo

Question 4

What do Profilin and Formin do and how?

Answer 4

together they add actin monomers to the plus end of the filament. Formin proteins have long whiskers that bind profilin-actin complexes which adds actin to filament end. Formin only nucleates unbranched filaments

Question 5

what is the difference between Formin and Arp2/3?

Answer 5

Formin nucleates unbranched filaments

Arp2/3 nucleates branched filament networks. always creates a 70 degree angle in branches

Question 6

what did Arps arise from?

Answer 6

gene duplication and divergence of actin. Arps are 45% identical to actin in amino acid structure

Question 7

how do Arps work? describe steps in process

Answer 7

Arp is activated by a signaling protein and will come together with other proteins to form a complex. The complex attaches to the side of a filament and adds actin monomers in branches at 70 degree angle to the original filament

Question 8

How can bacteria exploit actin polymerization?

Answer 8

the assembly of actin on a bacterias back surface propels the bacterial forward, helping them invade neighboring cells and maintain infection. Listeria monocytogenes is an example of bacteria doing this

Question 9

what are capping proteins?

Answer 9

they stabilize actin filament length and inhibit further elongation. They can bind to either the plus or minus end

Question 10

what are severing and depolymerizing proteins? example

Answer 10

they accelerate the disassembly of actin filaments by severing and depolymerizing. Cofilin

Question 11

how does Cofilin work?

Answer 11

Cofilin binds to actin filaments and increases the apparent twist, weakening non covalent bonds and destabilizing filament.

Question 12

what are bundling proteins?

Answer 12

they organize actin filaments into bundles. some pack them tightly and some loose.

Question 13

what are cross linking proteins? example

Answer 13

they connect actin filaments into lattice like structures important for cell crawling (filopodia and pseudopodia). Filamin

Question 14

what diseases can be caused from defective filamin?

Answer 14

Periventricular Heteropia is caused by a mutation in filamin A gene. This causes failure of neurons to migrate during development and patients often have epilepsy.

Question 15

what causes Duchenne Muscular Dystrophy?

Answer 15

mutation of the protein Dystrophin, a cross linking protein

Question 16

what are motor proteins? example

Answer 16

they can form contractile structures of the actin cytoskeleton. Myosin

Question 17

what is myosin structure?

Answer 17

a motor protein with two globular heads and a tail, result of dimerization of two heavy chains.

Question 18

how does myosin work?

Answer 18

myosin heads bind and hydrolyze ATP. This causes conformational changes that lead to walking of the heads along actin filaments. Then ADP is released and the original conformation is restored, moving back to this conformation involves a “power stroke” which moves the filament

Question 19

what do the myosin tails do?

Answer 19

myosin tails can come together and aggregate. they bundle into thick bipolar filaments. the bipolar arrangement allows for sliding of actin filaments together, contributing to muscle contraction.