Cholesterol-dependent interaction of syncollin with the membrane of the pancreatic zymogen granule

Syncollin is a protein of the pancreatic zymogen granule that was isolated through its ability to bind to syntaxin. Despite this in vitro interaction, it is now clear that syncollin is present on the luminal side of the zymog en granule membrane. Here we show that there are two pools of syncollin wit hin the zymogen granule: one free in the lumen and the other tightly associ ated with the granule membrane. When unheated or cross-linked samples of me mbrane-derived syncollin are analysed by SDS/PAGE, higher-order forms are s een in addition to the monomer, which has an apparent molecular mass of 16k Da. Extraction of cholesterol from the granule membrane by treatment with m ethyl-beta -cyclodextrin causes the detachment of syncollin, and this effec t is enhanced at a high salt concentration. Purified syncollin is able to b ind to brain liposomes at pH 5.0, but not at pH 11.0, a condition that also causes its extraction from granule membranes. Syncollin binds only poorly to dioleoyl phosphatidylcholine Liposomes, but binding is dramatically enha nced by the inclusion of cholesterol. Finally, cholesterol can be co-immuno precipitated with syncollin. We conclude that syncollin is able to interact directly with membrane lipids, and to insert into the granule membrane in a cholesterol-dependent manner. Membrane-associated syncollin apparently ex ists as a homo-oligomer, possibly consisting of six subunits, and its assoc iation with the membrane may be stabilized by electrostatic interactions wi th either other proteins or phospholipids.