Cellubrevin and synaptobrevins: similar subcellular localization and biochemical properties in PC12 cells

There is strong evidence to indicate that proteins of the synaptobrevin family play a key role in exocytosis. Synaptobrevin 1 and 2 are expressed at high concentration in brain where they are localized on synaptic vesicles. Cellubrevin, a very similar protein, has a widespread tissue distribution and in fibroblasts is localized on endosome-derived, transferin receptor-positive vesicles. Since brain cellubrevin is not 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. However, cellubrevin is coexpressed with the two synaptobrevins in PC12 cells, a neuroendocrine cell line which contains synaptic vesicle-like microvesicles. In PC12 cells, cellubrevin has a distribution very similar to that of synaptobrevin 1 and 2. The three proteins are targeted to neurites which exclude the transferrin receptor and are enriched in synaptic-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 coprecipitated by anti-synaptobrevin 2 immunobeads. Finally, cellubrevin, like the synaptobrevins, interact with the neuronal t-SNAREs syntaxin 1 and SNAP-25. These results suggest that cellubrevin and the synaptobrevins have similar function and do not play a specialized role in constitutive and regulated exocytosis, respectively.