PANTOPHYSIN IS A UBIQUITOUSLY EXPRESSED SYNAPTOPHYSIN HOMOLOG AND DEFINES CONSTITUTIVE TRANSPORT VESICLES

Certain properties of the highly specialized synaptic transmitter vesi cles are shared by constitutively occurring vesicles. We and others ha ve thus identified a cDNA in various nonneuroendocrine cell types of r at and human that is related to synaptophysin, one of the major synapt ic vesicle membrane proteins, which we termed pantophysin. Here we cha racterize the gene structure, mRNA and protein expression, and intrace llular distribution of pantophysin. Its mRNA is detected in murine cel l types of nonneuroendocrine as well as of neuroendocrine origin. The intron/exon structure of the murine pantophysin gene is identical to t hat of synaptophysin except for the last intron that is absent in pant ophysin. The encoded polypeptide of calculated mol wt 28,926 shares ma ny sequence features with synaptophysin, most notably the four hydroph obic putative transmembrane domains, although the cytoplasmic end doma ins are completely different. Using antibodies against the unique carb oxy terminus pantophysin can be detected by immunofluorescence microsc opy in both exocrine and endocrine cells of human pancreas, and in cul tured cells, colocalizing with constitutive secretory and endocytotic vesicle markers in nonneuroendocrine cells and with synaptophysin in c DNA-transfected epithelial cells, By immunoelectron microscopy, the ma jority of pantophysin reactivity is detected at vesicles with a diamet er of <100 nm that have a smooth surface and an electron-translucent i nterior. Using cell fractionation in combination with immunoisolation, these vesicles are enriched in a light fraction and shown to contain the cellular vSNARE cellubrevin and the ubiquitous SCAMPs in epithelia l cells and synaptophysin in neuroendocrine or cDNA-transfected nonneu roendocrine cells and neuroendocrine tissues. Pantophysin is therefore a broadly distributed marker of small cytoplasmic transport vesicles independent of their content.