TETANUS TOXIN-MEDIATED CLEAVAGE OF CELLUBREVIN IMPAIRS EXOCYTOSIS OF TRANSFERRIN RECEPTOR-CONTAINING VESICLES IN CHO CELLS

Cellubrevin is a member of the synaptobrevin/VAMP family of SNAREs, wh ich has a broad tissue distribution. In fibroblastic cells it is conce ntrated in the vesicles which recycle transferrin receptors but its ro le in membrane trafficking and fusion remains to be demonstrated. Cell ubrevin, like the synaptic vesicle proteins synaptobrevins I and II, c an be cleaved by tetanus toxin, a metallo-endoprotease which blocks ne urotransmitter release. However, nonneuronal cells are unaffected by t he toxin due to lack of cell surface receptors for its heavy, chain. T o determine whether cellubrevin cleavage impairs exocytosis of recycli ng vesicles, we tested the effect of tetanus toxin light chain on the release of preinternalized transferrin from streptolysin-O-perforated CHO cells. The release was found lo be temperature and ATP dependent a s well as NEM sensitive. Addition of tetanus toxin light chain, but no t of a proteolytically inactive form of the toxin, resulted in a parti al inhibition of transferrin release which correlated with the toxin-m ediated cleavage of cellubrevin. The residual release of transferrin o ccurring after complete cellubrevin degradation was still ATP dependen t. Our results indicate that cellubrevin plays an important role in th e constitutive exocytosis of vesicles which recycle plasmalemma recept ors. The incomplete inhibition of transferrin release produced by the toxin suggests the existence of a cellubrevin-independent exocytotic m echanism, which may involve tetanus toxin-insensitive proteins of the synaptobrevin/VAMP family.