Fun at the leading edge: Biochemical and biomechanical studies of the actin networks that drive cell motility
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.