THE SMALL GTP-BINDING PROTEIN, RAB6P, IS ASSOCIATED WITH BOTH GOLGI AND POST-GOLGI SYNAPTOPHYSIN-CONTAINING MEMBRANES DURING SYNAPTOGENESISOF HYPOTHALAMIC NEURONS IN CULTURE

We have recently localized a small GTP-binding protein (Rab6p) thought to be involved in vesicular membrane transport, to the medial and tra ns-cisternae of the Golgi apparatus in NRK (normal rat kidney) cells. Here, we have localized and quantified Rab6p during the development in culture of embryonic neurons, up to synapse formation, and compared i ts subcellular distribution and level of expression to that of synapto physin, a major integral membrane protein of small synaptic vesicles. Using immunocytochemistry (laser scanning confocal microscopy, immunoe lectron microscopy), fractionation and immunoisolation methods, we sho w that during the early phase of synaptogenesis, Rab6p is associated w ith synaptophysin-containing membranes of a trans-Golgi subcompartment , post-Golgi vesicles and small synaptic vesicles or their precursors. Concomitantly, Rab6p undergoes translocation from cytosol to membrane s and its level of expression increases. However, at late stages, the association of Rab6p to small synaptic vesicles sharply decreases and its level of expression plateaus. These findings suggest a role for Ra b6p in the post-Golgi transport of synaptophysin, at an early step of the bio-genesis of small synaptic vesicles.