Membrane-integrated proteins at preformed exocytosis sites.

Parameters tetraurelia cells were used for analyzing the crucial but also controversial question regarding the organization of those sites of the cell membrane that undergo fusion during exocytosis. Paramecia are unique in that they display a "rosette" of from eight to ten MIPs (Membrane-intercalated particles, as seen on freeze-fracture replicas) at each of their numerous preformed exocytosis sites. We analyzed these structures by exposing slightly fixed cells to various enzymes and by subsequent freeze fracturing. We show that "rosette" MIPs are selectively sensitive to proteolytic enzymes. Since "rosettes" are known to be necessary for membrane fusion, this cytochemical result would be in line with the assumption that membrane-integrated proteins may play an active role in regulating exocytotic membrane fusion.