Interactions between the three major cytoskeletal filament systems are critical for normal cell function. Interactions between actin and microtubules, for example, underpin cell migration; similarly, association with microtubules appears to be important for translocation of intermediate filaments (IFs). But to what extent do IFs and actin interact? Kari Weber and William Bement have addressed this question, using Xenopus egg extracts to avoid problems presented by multipolymer analysis in intact cells(see ). They show that cytokeratin assembles on and moves in concert with forming actin cables. If actin assembly is blocked, the IF forms disorganized aggregates rather than cables. Such actin-directed IF assembly is probably relevant in vivo, since disruption of the actin cytoskeleton induces formation of cytokeratin aggregates in intact eggs. Interestingly, the authors show that during later stages of assembly cytokeratin often dissociates from F-actin to form independent networks. They therefore conclude that F-actin acts as a template for assembly of a cytokeratin network in these cells but is not necessarily important for its maintenance.
