actin and actin binding proteins (microfilaments)

• actin is a globular protein monomer (G actin) (42kD) that polymerizes to form long filamentous polymers (F actin)
• actin microfilaments provide structural support, generates force and restricts diffusion of organelles
• two major types of actin filaments: cortical actin and stress fibers
• cortical actin: at cell periphery (also called lamellipodia)

  • can form long projections (filopodia)
• stress fibers: structural support consisting of actin, myosin and accessory proteins throughout the cell
• cell motility and movement is driven by actin
• actin creates a scaffolding within the cell which provides places for the vescicle and other organelles to attach onto
• actin: several isoforms exist which are > 90% identical
• humans have 6 actin genes: beta and gamma isoform in non muscle cells and 4 alpha isoforms in muscle cells
• in muscle, actin and myosin >60% of total protein
• in non muscle, actin + actin associated proteins >25% of total
• >60 families of actin associated proteins

  • this is what regulates the proteins
• if you inhibit actin you're dead
• actin associated proteins: interact with actin in some way
• F actin is polarized: has a barbed (+) end (at membrane) and a pointed (-) end (interior of cell)
• plus end is being added to
• polymerizes from plus end
• minus end is not polymerizing
• there is a single ATP per actin molecule
• actin is very prevalent at cell cortex
• actin is added to plus end as cell is moving, actin polymerization is what is driving the movement of the cell
• have filopodia on either side of the lamellipodia
• actin binds ATP, ADP and Mg++ (very weak ATPase activity)
• actin binds ATP with higher affinity than ADP
• unpolymerized actin is in ATP bound form
• nucleotide exchange on actin is regulated by actin binding proteins
• in low salt conditions, actin is a monomer
• actin polymerizes quickly in presence of salt
• F actin shrinks and grows at both ends (but is faster growth at plus end)
• ADP filament is very unstable

  ATP filament is very stable
• majority of G actin is bound to ATP

  • called G actin pools
  • huge excess compared to what the cell needs
  • cell needs can change so it needs a lot of actin in case it needs it
• F actin polymerization occurs at plus end and depolymerization is predominantly from the minus end
• 3 monomers of actin together at the beginning = trimer (generates F actin)
• actin trimers act as nucleus for further polymerization
• ATP hydrolysis occurs randomly throughout the filament, leaving filament with ADP bound actin
• hydrolysis modifies behavior of filament

  • allows other actin binding proteins to bind
• actin treadmilling: loss of monomers from the pointed end are used for growing the barbed end (after recharging with ATP)

  • if loss at the minus end is faster than the gain at th eplus end, you get filaments
• actin starts as a trimer and every actin molecule added after is a monomer.

  as polymer grows into F actin, the ATP hydrolysis rate will start to pick up

  growing at plus end while shrinking at minus end
• actin polymerization is controlled by actin associated proteins: monomer (G actin) binding, severing (breaks F actin), capping (caps filament ends), cross linking (holds filaments together)
• once an F actin filament has grown to the right length it can be capped, which can stop polymerization from growing from the plus end