Topic 29: Actin Binding Proteins & Intermediate Filaments

what are the 5 mechanisms by which cells regulate the actin cytoskeleton?

• controlling monomer availability

• controlling timing and localization of nucleation

• capping

• serving

• stabilizing and crosslinking

what do profilin and tymosin bind to? What do they do?

• bind to actin monomers and regulate their availavility

cells maintain a large excess of what?

• soluble actin subunits (well above critical concentration)

how is uncontrolled polymerization prevented?

• prevented by regulating monomer availability

describe what way profilin bind monomers

• profilin binds monomers in a way that inhibits spontaneous polymerization but can promote rapid addition to the plus ends of pre-existing filaments

describe what way tymosin binds monomers

• tymosin binds monomers in a way that inhibits addition at both ends of actin filaments

how can cells spatiotemporally control actin polymeration?

• by regulating the activity of filament nucleating complexes (and by regulating the competition of profilin and thymosin for actin monomers)

what are major actin filament nucleators we talked about in class?

• Actin realted proteins 2 & 3 (Arp2/3)

structure of Arp2 and Arp3

• very similar to actin

• they don’t form filaments, but they provide a complex that functions like a pre-formed seed to promote polymerization

what stimulates the Arp2/3 complex?

• NPFs (nucleation promoting factros)

what does Arp2/3 do?

• nucleates new actin filaments

  • they adopt a shape that creates an ideal platform for new filament growth

While Arp2/3 complex is bound, what does it also prevent?

• prevents depolymerization at the minus end

What does Arp2/3 promote?

• promotes formation of branched actin networks

How does ARp2/3 promote the formation of branched actin networks?

• by binding the sides of existing filaments

What do formins do?

• nucleate linear arrays of polymerized actin

What end of actin filaments do activated formins interact with? What does this promote?

• interact with plus ends of actin filaments

• promote the growth of linear arrays of actin filaments

what does profilin stimulate? How?

• actin filament elongation

• profilin binding sites on NPFs and formins position actin monomers near the plus end of actively elongating actin filaments

What do capping proteins do if added to (+) ends?

• If plus ends are all capped, growth rate, shrinkage rate, and critical concentration will be controlled by the minus end

what do actin serving proteins promote?

• promote more rapid actin disassembly OR assembly

what happens when cofilin binds actin?

• indices conformational strain, leading to severing

what supports the organization of actin in different regions of migrating cells?

• different actin binding proteins

what are 4 different actin configurations?

1. anti-parallel linear array, looser

2. mixed (branched and unbranched)

3. branched array

4. parallel linear array, tighter

What can establish different geometries of actin filament networks?

• activation of different actin binding proteins in different regions of the cell

actin-parallel filaments can be pulled in both directions by what?

• myosin motor proteins

what can highjack the actin control machinery?

• pathogens (ex: listeria)

  • In listeria, ActA proteins recruits and activates the Arp2/3 complex

  • this promotes cell-to-cell spreading

    • uptake of bacterium from one cell by a neighboring cell via phagocytosis

Describe structure of Intermediate filaments. Key feature?

• form rope-like fibers

• lack polarity

• Soluble: domers and tetramers are the soluble subunits

Key feature: very high tensile strength and no polarity (no plus or minus end)

what does keratin provide for epithelial cell layes?

• mechanical support

  • keratin filaments form an interconnected network that mechanically couples epithelial cells within epithelial cell layers

What does a defective keratin network result in?

• very fragile epidermal cell layers

What is the result of a keratin mutant that can incorporated into keratin filaments but fails to make all of the proper contacts?

• results in a blistering phenotype when co-expressed in cells expressing wild type keratin

the cytoskeleon is physically coupled to the “nuceloskeleton” by what?

• SUN-KASH complexes

what are Nuclear lamins?

• ancient ancestros of cytoplasmic intermediate filaments

what links nuclear lamins to all 3 cytosolic filaments systems?

• SUN-KASH complexes

What does Plectin provide?

• crosslinks to diverse cytoskeletal elements, including intermediate filaments