CYTOSKELETON: ACTIN Actin is a structural protein required for processes including cell shape, movement, polarity, and division. Actin forms polarized filaments that are more likely to add actin monomers to one end (the plus end ) than the other (the minus end ). Actin is also an ATPase ; the energy for filament assembly and disassembly is supplied by ATP. Stages of actin filament growth in vitro : • Lag phase: Without nucleating proteins, it takes a long time for the initial actin polymer to form. This is the rate-limiting step of actin filament formation . How could you test if nucleation is the rate limiting step? • Growth phase : Filaments get longer as monomers are added to the + and/or – ends. • Equilibrium : Rate of monomer addition at the + end equals the rate of loss at the – end. Actin filaments undergo treadmilling , continually adding and losing monomers, but the filaments do not get longer. Proteins that regulate actin filaments:
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