Compare and contrast actin filaments and microtubules.

Many reviews have been written on the biochemistry and cell biology of actin filaments and microtubules individually (see Korn, 1982; Kirschner and Mitchison, 1986; Pollard, 1986a; Pollard and Cooper, 1986; Gelfand and Bershadsky, 1991; Caplow, 1992). Most labs work on one polymer or the other, which has led to something of a gulf between the actin and microtubule research fields. As a broad (and therefore dangerous) generalization, actin research over the last 10 y has focused on detailed biochemistry, in particular of actin binding proteins, whereas microtubule research has focused on trying to understand behavior and function in vivo. These studies have produced intellectual insights and technical tools that can be usefully transferred between the two polymers. The title of this essay "compare and contrast. . . ," is intended to recall the typical finals exercise, where the task is as much to define the question as to answer it. The discussion will focus on polymerization dynamics of the two polymers, both in vitro and in vivo, and what we can learn from comparing them. My goal is to abstract common principles and to focus attention on areas where recent insights into one polymer should lead us to question assumptions about the other. For another comparison of dynamics, see Carlier, 1989.