TRANSPORT ROUTE FOR SYNAPTOBREVIN VIA A NOVEL PATHWAY OF INSERTION INTO THE ENDOPLASMIC-RETICULUM MEMBRANE

Synaptobrevin/vesicle-associated membrane protein is one of the solubl e N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE ) proteins. It is proposed to provide specificity for the targeting an d fusion of vesicles with the plasma membrane. It belongs to a class o f membrane proteins which lack a signal sequence and contain a single hydrophobic segment close to their C-terminus, leaving most of the pol ypeptide chain in the cytoplasm (tail-anchored). We show that in neuro endocrine PC12 cells, synaptobrevin is not directly incorporated into the target organelle, synaptic-like vesicles. Rather, it is first inse rted into the endoplasmic reticulum (ER) membrane and is then transpor ted via the Golgi apparatus. Its insertion into the ER membrane in vit ro occurs post-translationally, is dependent on ATP and results in a t rans-membrane orientation of the hydrophobic tail. Membrane integratio n requires ER protein(s) different from the translocation components n eeded for proteins with signal sequences, thus suggesting a novel mech anism of insertion.