UCSF

Tropomyosin, which binds along the length of actin filaments, has long been considered the master regulator over the binding of other proteins to actin. In particular, non-muscle tropomyosin is considered the key to the transition between the lamellipod and the lamellum in motile cells by inhibiting Arp2/3 complex nucleation and cofilin disassembly. This in vitro study of a D. melangaster tropomyosin isoform, TM1A, which localizes to the lamellum of S2 cells, shows that Arp2/3 and cofilin both affect the binding of TM1A. Labeled TM1A binds preferentially near the pointed end of actin filaments, and Arp2/3 blocks TM1A from binding to branched networks. Surprisingly, cofilin promotes the binding of TM1A to a branched actin network. Our data provides an exciting look at how actin, tropomyosin, cofilin and Arp2/3 complex together can self-organize to create two structurally and dynamically different actin networks at the leading edge. We also explore the actin-binding characteristics of another non-muscle tropomyosin isoform, TM1J, and show that its binding is dependent on TM1A. Capping protein also contributes to the exclusion of tropomyosin from a branched network. Attempts at measuring the mechanical properties of a branched network ultimately failed, but are presented for potential future inspiration. Finally, the results of two collaborations are presented.