Abstract
Syncollin, a novel pancreatic zymogen granule protein, is present on the luminal side of the granule membrane. To address the function of syncollin, we searched for putative binding partners. Cross-linking experiments with purified syncollin, and granule content and membrane proteins revealed a direct interaction between syncollin and GP-2, a major glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein. An interaction was also observed when cross-linking was performed with recombinant GP-2. In addition, syncollin could be cross-linked to itself, supporting the suggestion that it exists as a homo-oligomer. Cleavage of the GPI anchor of GP-2 by treatment of granule membranes with phosphatidylinositol-specific phospholipase C had no effect on the membrane attachment of syncollin, indicating that it is not mediated exclusively via an interaction with GP-2. Syncollin was found to be associated with detergent-insoluble cholesterol/glycolipid-enriched complexes. These complexes floated to the lighter fractions of sucrose-density gradients and also contained GP-2, the lectin ZG16p, sulphated matrix proteoglycans and the soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs) syntaxin 3 and synaptobrevin 2. Our results indicate that membrane-associated syncollin is a component of lipid rafts, where it interacts both with GP-2 and membrane lipids. We suggest that the syncollin-GP-2 complex might play a role in signal transduction across the granule membrane.