Characterization of Rab12 Interactions with its Rab-Interacting Lysosomal Protein (RILP) Family Effectors in Mast Cells

Triggered release by exocytosis of inflammatory mediators that are prestored in secretory granules (SGs) is the central mechanism by which mast cells elicit their physiological functions in innate immunity and their pathological functions in allergic disorders and chronic inflammatory conditions. Mast cell SGs move bidirectionally on microtubule tracks. However, the underlying mechanisms of mast cell SG positioning are not fully understood. Recently, we identified the small GTPase Rab12 as a regulator of the bidirectional transport of the SGs in triggered mast cells. We showed that by binding of RILP and recruiting the dynein motor protein, Rab12 controls retrograde transport of the SGs and negatively regulates their exocytosis. Here, we demonstrate that Rab12 also interacts with RILP-like-2 (RILP-L2), another member of the RILP family of proteins. We show that their interaction defines the cellular location of RILP-L2. Further, our results implicate RILP-L2 in regulating anterograde transport of the SGs towards the plasma membrane. Finally, by combining bioinformatics analyses and site directed mutagenesis, we have mapped Rab12 binding sites and demonstrate unique regions of interaction of Rab12 with its effectors. Taken together, our results identify RILP-L2 as a novel regulator of mast cell functions that acts downstream of Rab12 to control SG positioning.