Protein Kinase C (PKC) regulates the release of pro-inflammatory compounds from IgE/antigen-activated mast cells by unknown mechanisms. In this study, we show for the first time that PKC inhibitor Ro-03-0432, which inhibits RBL-2H3 exocytosis/degranulation in a concentration-dependent fashion, prevents the phosphorylation of membrane fusion factor Munc18a at Ser 313. Our study provides fresh evidence that PKC-dependent protein phosphorylation may contribute to the intricate regulation of mast cell degranulation by directly targeting the fusion factors.

Sec1-Munc18 (SM) proteins cooperate with SNAREs {SNAP [soluble NSF (N-ethylmaleimide-sensitive factor) attachment protein] receptors} to mediate membrane fusion in eukaryotic cells. Studies of Munc18a/Munc18-1/Stxbp1 in neurotransmission suggest that SM proteins accelerate fusion kinetics primarily by activating the partially zippered trans-SNARE complex. However, accumulating evidence has argued for additional roles for SM proteins in earlier steps in the fusion cascade. Here we investigate the function of Munc18a in reconstituted exocytic reactions mediated by neuronal and non-neuronal SNAREs. We show that Munc18a plays a direct role in promoting proteoliposome clustering, underlying vesicle docking during exocytosis. In the three different fusion reactions examined, Munc18a-dependent clustering requires an intact N-terminal peptide (N-peptide) motif in syntaxin that mediates the binary interaction between syntaxin and Munc18a. Importantly, clustering is preserved under inhibitory conditions that abolish both trans-SNARE complex formation and lipid mixing, indicating that Munc18a promotes membrane clustering in a step that is independent of trans-SNARE zippering and activation.

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