Selective expression of a sec1/munc18 member in sea urchin eggs and embryos.  Regulated secretion is mediated by SNAREs (soluble NSF attachment receptors) and their regulators and effectors, which include the SM (sec1/munc18) family of proteins. Homologs of the SNAREs have been identified in sea urchins, associated with cortical granule exocytosis at fertilization, with membranes of the cleavage furrow, and in secretory cells later in development. To contribute to the understanding of regulated secretion in sea urchins we have cloned the single SM protein homolog from two species of sea urchin, Lytechinus variegatus and Strongylocentrotus purpuratus. In oocytes and eggs, we find that it localizes to the plasma membrane and the cortical region of the egg, consistent with a role in one of the steps leading to cortical granule exocytosis. The protein is also expressed throughout development, enriched in membranes of the cleavage furrow in early embryos, and in cells of the gut in advanced embryos. Furthermore, we find that sec1/munc18 co-localizes with its cognate binding partner syntaxin. Finally, our biochemical analysis shows that the protein associates with rab3 in high molecular weight complexes, suggesting that the exocytotic machinery functions as a multi-protein subunit to mediate regulated secretion in sea urchins. These results will be instrumental in the future to functionally test the SNARE regulators associated with multiple membrane fusion events.