The subcellular localizations of atypical synaptotagmins III and VI - Synaptotagmin III is enriched in synapses and synaptic plasma membranes but notin synaptic vesicles

Multiple synaptotagmins are expressed in brain, but only synaptotagmins I a nd II have known functions in fast, synchronous Ca2+-triggered neurotransmi tter release. Synaptotagmin III was proposed to regulate other aspects of s ynaptic vesicle exocytosis, particularly its slow component. Such a functio n predicts that synaptotagmin IU. should be an obligatory synaptic vesicle protein, as would also be anticipated from its high homology to synaptotagm ins I and II. To test this hypothesis, we studied the distribution, develop mental expression, and localization of synaptotagmin III and its closest ho molog, synaptotagmin VI. We find that synaptotagmins III and VI are present in all brain regions in heterogeneous distributions and that their levels increase during development in parallel with synaptogenesis, Furthermore, w e show by immunocytochemistry that synaptotagmin III is concentrated in syn apses, as expected, Surprisingly, however, we observed that synaptotagmin I II is highly enriched in synaptic plasma membranes but not in synaptic vesi cles. Synaptotagmin VI was also found to be relatively excluded from synapt ic vesicles. Our data suggest that synaptotagmins III and VI perform roles in neurons that are not linked to synaptic vesicle exocytosis but to other Ca2+-related nerve terminal events, indicating that the functions of synapt otagmins are more diverse than originally thought.