SORTING OF SYNAPTOPHYSIN INTO SPECIAL VESICLES IN NONNEUROENDOCRINE EPITHELIAL-CELLS

Synaptophysin is a major transmembrane glycoprotein of a type of small vesicle with an electron-translucent content (SET vesicles), includin g the similar to 50-nm presynaptic vesicles in neuronal cells, and of similar, somewhat larger (less than or equal to similar to 90 nm) vesi cles (SLMV) in neuroendocrine (NE) cells. When certain epithelial non- NE cells, such as human hepatocellular carcinoma PLC cells, were cDNA transfected to synthesize synaptophysin, the new molecules appeared in specific SET vesicles. As this was in contrast to other reports that only NE cells were able to sort synaptophysin away from other plasma m embrane proteins into presynaptic- or SLMV-type vesicles, we have furt her characterized the vesicles containing synaptophysin in transfected PLC cells. Using fractionation and immunoisolation techniques, we hav e separated different kinds of vesicles, and we have identified a dist inct type of synaptophysin-rich, small (30-90-nm) vesicle that contain s little, if any, protein of the constitutive secretory pathway marker hepatitis B surface antigen, of the fluid phase endocytosis marker HR P, and of the plasma membrane recycling endosomal marker transferrin r eceptor. In addition, we have found variously sized vesicles that cont ained both synaptophysin and transferrin receptor. A corresponding res ult was also obtained by direct visualization, using double-label immu nofluorescence microscopy for the endocytotic markers and synaptophysi n in confocal laser scan microscopy and in double-immunogold label ele ctron microscopy. We conclude that diverse non-NE cells of epithelial nature are able to enrich the ''foreign'' molecule synaptophysin in a category of SET vesicles that are morphologically indistinguishable fr om SLMV of NE cells, including one type of vesicle in which synaptophy sin is sorted away from endosomal marker proteins. Possible mechanisms of this sorting are discussed.