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, which has a broad tissue distribution. In fibroblastic cells it is concentrated in the vesicles which recycle transferrin receptors but its role in membrane trafficking and fusion remains to be demonstrated. Cellubrevin, like the synaptic vesicle proteins synaptobrevins I and II, can be cleaved by tetanus toxin, a metallo-endoprotease which blocks neurotransmitter release. However, nonneuronal cells are unaffected by the toxin due to lack of cell surface receptors for its heavy chain. To determine whether cellubrevin cleavage impairs exocytosis of recycling 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 to be temperature and ATP dependent as well as NEM sensitive. Addition of tetanus toxin light chain, but not of a proteolytically inactive form of the toxin, resulted in a partial inhibition of transferrin release which correlated with the toxin-mediated cleavage of cellubrevin. The residual release of transferrin occurring after complete cellubrevin degradation was still ATP dependent. Our results indicate that cellubrevin plays an important role in the constitutive exocytosis of vesicles which recycle plasmalemma receptors. The incomplete inhibition of transferrin release produced by the toxin suggests the existence of a cellubrevin-independent exocytotic mechanism, which may involve tetanus toxin-insensitive proteins of the synaptobrevin/VAMP family.