Treadmilling, diffusional exchange and cytoplasmic structures.

Microfilaments and microtubules exchange monomers from solution by at least two mechanisms; treadmilling and diffusional exchange. Refined kinetic analysis of both mechanisms shows that this exchange may be nonlinear under certain conditions. The two mechanisms of exchange differ in some of their predictions for the behaviour of cytoplasmic structures. Studies of assembly of cytoplasmic structures in vivo suggest that diffusional exchange is probably predominant for steady-state structures and further suggest that additional mechanisms may be operating in the cell.