Rab27 and its effectors in secretory granule exocytosis: a novel docking machinery composed of a Rab27.effector complex.

A small GTPase Rab27 is present on secretory granules in a wide variety of secretory cells and on melanosomes in melanocytes, and it is involved in controlling the trafficking of these organelles through interaction with a cell-type- or tissue-specific Rab27 effector(s). Slps (synaptotagmin-like proteins) and rabphilin contain an N-terminal Rab27-binding domain and C-terminal tandem C2 domains, and some of the Rab27-binding proteins have recently been shown to promote docking of Rab27-bound organelles to the plasma membrane. This mini-review presents a model for how the Rab27.effector complex controls the docking step in the trafficking of Rab27-bound organelles. Our results indicate that Slp2-a, Slp4-a/granuphilin-a and rabphilin are capable of interacting with the plasma membrane directly or indirectly, and thus that these Rab27 effectors form a bridge between Rab27-bound organelles and the plasma membrane.