Cytochalasin B and the structure of actin gels

We analyzed the structure of gels formed when macrophage actin-binding protein crosslinks skeletal muscle actin polymers and the effect of the fungal metabolite cytochalasin B on this structure. Measurement of the actin filament length distribution permitted calculation of the critical concentration of crosslinker theoretically required for gelation of actin polymer networks. The experimentally determined critical concentration of actin-binding protein agreed sufficiently with the theoretical to conclude that F-actin-actin-binding protein gels are networks composed of isotropically oriented filaments crosslinked at intervals. The effects of cytochalasin B on these actin networks fits this model. Cytochalasin B (1) bound to F-actin (but not to actin-binding protein), (2) decreased the length of actin filaments without increasing the quantity of monomeric actin, (3) decreased the rigidity of actin networks both in the presence and absence of crosslinking proteins and (4) increased the critical concentration of actin-binding protein required for incipient gelation by a magnitude predicted from network theory if filaments were divided and shortened by the extents observed. The effects of cytochalasin B on gelation were highly dependent on actin concentration and were inhibited by the actin-stabilizing agent phalloidin. Therefore, cytochalasin B diminishes actin gel structure by severing actin filaments at limited sites. The demonstration of gel-sol transformations in actin networks caused by limited actin filament cleavage suggests a new mechanism for the control of cytoplasmic structure.