lec 4- cytoskeleton: actin 1 Flashcards
(38 cards)
what is actin?
-one of the 4 components of the cytoskeleton
-one of the most abundant proteins in eukaryotic cells
-highly conserved sequence among all species
what are the 4 cytoskeleton components?
-actin
-microtubules
-intermediate filaments
-spectrin
of all the proteins in a normal eukaryotic cell, what percentage of it is actin?
15%
what are actin filaments involved in?
-generating/maintaining the cell structure
-adhesion to substrates and other cells
-cell division
-intracellular motility
-whole cell motility
actin filaments get crosslinked, resulting in a structure that:
-resists deformation
-transmits forces
-restricts organellar movement
how strong are actin filaments?
-very strong
-if expanded 1 million times they could be as strong as steel, but they are thin, so they can break if too long
what is the function of cortical actin (actin at the cell border, the cortex)?
-restricts the movement of some membrane proteins
-reinforces the plasma membrane
-excludes organelles from the cell periphery
-ranges in thickness from 1 layer thick (RBC) to 1 micron thick (amoeba)
what are the two forms of actin and explain them?
- globular actin (G-actin): monomeric form of actin, monomers have 2 lobes that house Mg-ATP in the cleft at the center of the monomers
- Filamentous actin (F-actin): when filaments form, the filament grows faster from 1 end (the + end, AKA the barbed end), the slower growing end is the - end (AKA the pointed end)
why is the plus end and minus end of actin called barbed and pointed?
-when decorated with myosin heads (from skeletal muscle) the twist in the actin filaments shows arrowheads by electron microscopy
why is the barbed and pointed end of the actin filaments important?
because it makes the filament polar which is crucial for molecular motors
how are filaments formed?
-filaments grow from the incorporation of G-actin subunits onto the ends of the filament through the rapid hydrolysis of ATP to ADP and Pi
-actin filaments need a site of nucleation to start polymerization of filaments
-most often this is in the form of a trimer of actin
-this is an unfavourable conformation for actin and is dealt with by using other actin-associated proteins
what are the 3 general functions of profilin?
- binds to actin monomers
- catalyzes ADP/ATP exchange in actin monomers
- binds to polyproline residues in other actin-associated proteins
what is the main goal of prolin?
-it adds monomers to the barbed ends of actin filaments
what happens to profilin once the actin/profilin complex completes its binding to the barbed end of the actin filament?
profilin dissociates, allowing the inhibition of spontaneous nucleation of the actin filaent from the cytoplasmic pool of monomeric actin
what is Actin Depolymerizing Factor (ADF/cofilin) and what does it do?
-small proteins
-the opposite of profilin because they bind , destabilize and sever ADP-actin filaments, leads to actin filament disassembly
-can also bind to monomeric G-actin and nucleate actin filaments
what are the 3 main members in mammals of Actin Depolymerizing Factor?
cofilin 1, cofilin 2, and ADF (AKA Destrin)
how are ADF/cofilin regulated? what does it cause?
-through phosphorylation on serine 3 by kinases including Limkinases (LIMK) 1 and 2
-phosphorylation blocks their interaction with actin
what activates cofilin?
dephosphorylation of Ser3
what activates LIMK?
-LIMK is activated by many of the Rho-GTPasees (Rac, Rho, cdc42) through an assortment of branched pathways
what causes the dephosphorylation of Ser3, which activates cofilin?
-a phosphotase called slingshot, it can also dephosphorylate LIMK
what is gelsonin and what does it do? what activates it?
-a protein with 6 domains
-binds to the sides of actin filaments and wraps around the filaments
-functions as an actin severing and capping protein
-activated by micromolar concentrations of calcium
what is CapZ and what does it do? what does it promote?
-a heterodimer of a and B subunits making a proteins 64kDa
-the heterodimers are intertwined and behave as a single protein
-binds to the free barbed ends of actin filament to act as a capping protein
-one molecule is enough to cap
-promotes the nucleation of new actin filaments by stabilizing small actin oligomers at the pointed ends
is actin in the nucleus?
yes, just not visible most of the time in images
what are the two types of actin in the nucleus?
monomeric and polymeric filamentous actin
