CELLUBREVIN AND SYNAPTOBREVINS - SIMILAR SUBCELLULAR-LOCALIZATION ANDBIOCHEMICAL-PROPERTIES IN PC12 CELLS

There is strong evidence to indicate that proteins of the synaptobrevi n family play a key role in exocytosis. Synaptobrevin 1 and 2 are expr essed at high concentration in brain where they are localized on synap tic vesicles. Cellubrevin, a very similar protein, has a widespread ti ssue distribution and in fibroblasts is localized on endosome-derived, transferrin receptor-positive vesicles. Since brain cellubrevin is no t detectable in synaptic vesicles, we investigated whether cellubrevin and the synaptobrevins are differentially targeted when co-expressed in the same cell. We report that in the nervous system cellubrevin is expressed at significant levels only by glia and vascular cells. Howev er, cellubrevin is coexpressed with the two synaptobrevins in PC12 cel ls, a neuroendocrine cell line which contains synaptic vesicle-like mi crovesicles. In PC12 cells, cellubrevin has a distribution very simila r to that of synaptobrevin 1 and 2. The three proteins are targeted to neurites which exclude the transferrin receptor and are enriched in s ynaptic-like microvesicles and dense-core granules. They are recovered in the synaptic-like microvesicle peak of glycerol velocity gradients , have a similar distribution in isopycnic fractionation and are copre cipitated by anti-synaptobrevin 2 immunobeads. Finally, cellubrevin, l ike the synaptobrevins, interact with the neuronal t-SNAREs syntaxin 1 and SNAP-25. These results suggest that cellubrevin and the synaptobr evins have similar function and do not play a specialized role in cons titutive and regulated exocytosis, respectively.