LOCALIZATION OF RAB5 TO SYNAPTIC VESICLES IDENTIFIES ENDOSOMAL INTERMEDIATE IN SYNAPTIC VESICLE RECYCLING PATHWAY

After exocytosis, synaptic vesicles rapidly endocytose and recycle but little is known about the molecular mechanisms involved. Rab5 is a ub iquitous low molecular weight GTP-binding protein required for endosom al fusion in fibroblasts. We have now raised polyclonal and monoclonal antibodies to rat Rab5 and show that in rat brain. Rab5 is a major sy naptic vesicle protein. Immunoisola tion of vesicular organelles from brain with antibodies to either Rab3A and Rab5 as small GTP-binding pr oteins or with synaptophysin as general synaptic vesicle marker demons trates that there are overlapping populations of synaptic vesicles con taining either Rab5 or Rab3A or both, suggesting a stage-specific asso ciation of these low-molecular weight GTP-binding proteins with synapt ic vesicles. Our data provide the first biochemical evidence that syna ptic vesicle recycling involves an endosomal intermediate similar to t hat of the receptor mediated endocytosis pathway.