Slac2-c (synaptotagmin-like protein homologue lacking C2 domains-c), a novel linker protein that interacts with Rab27, myosin Va/VIIa, and actin

Slac2-a (synaptotagmin-like protein (Slp) homologue lacking C2 domains-a)/melanophilin is a melanosome-associated protein that links Rab27A on melanosomes with myosin Va, an actin-based motor protein, and formation of the tripartite protein complex (Rab27A(.)Slac2a(.)myosin Va) has been suggested to regulate melanosome transport (Fukuda, M., Kuroda, T. S., and Mikoshiba, K. (2002) J. Biol. Chem. 277, 12432-12436). Here we report the structure of a novel form of Slac2, named Slac2-c, that is homologous to Slac2-a. Slac2-a and Slac2-c exhibit the same overall structure, consisting of a highly conserved N-terminal Slp homology domain (about 50% identity) and a less conserved C-terminal myosin Va-binding domain (about 20% identity). As with other Slac2 members and the Slp family, the Slp homology domain of Slac2-c was found to interact specifically with the GTP-bound form of Rab27A/B both in vitro and in intact cells, and the C-terminal domain of Slac2-c interacted with myosin Va and myosin VIIa. In addition, we discovered that the most C-terminal conserved region of Slac2-a (amino acids 400-590) and Slac2-c (amino acids 670-856), which is not essential for myosin Va binding, directly binds actin and that expression of these regions in PC12 cells and melanoma cells colocalized with actin filaments at the cell periphery, suggesting a novel role of Slac2-a/c in capture of Rab27-containing organelles in the actin-enriched cell periphery.