Species distribution of a phosphoprotein (parafusin) involved in exocytosis.

A cytosolic phosphoprotein that appears to function in membrane fusion during exocytosis of secretory products has previously been isolated from Paramecium tetraurelia. This phosphoprotein, parafusin, with Mr 63,000, is rapidly dephosphorylated via a Ca2+-dependent process when secretagogues induce exocytosis in competent cells. Dephosphorylation does not occur in exocytosis-incompetent cells. Polyclonal antibodies against purified parafusin have now been used to show that this protein is present in unicellular organisms and cells of metazoan groups of wide evolutionary divergence, such as yeast, insects, and mammals, including humans. These results suggest that parafusin was present early in the history of eukaryotes and that it is of functional importance in the general mechanism of exocytosis and membrane fusion.