Both the sec6/8 complex and septin filaments have been implicated in directing vesicles and proteins to sites of active membrane addition in yeast. The rat brain sec6/8 complex coimmunoprecipitates with a filament composed of four mammalian septins, suggesting an interaction between these complexes. One of the septins, CDC10, displays broad subcellular and tissue distributions and is found in postmitotic neurons as well as dividing cells. Electron microscopic studies showed that the purified rat brain septins form filaments of 8.25 nm in diameter; the lengths of the filaments are multiples of 25 nm. Glutaraldehyde-fixed rat brain sec6/8 complex adopts a conformation resembling the letter "T" or "Y". The sec6/8 and septin complexes likely play an important role in trafficking vesicles and organizing proteins at the plasma membrane of neurons.
rsec6 and rsec8 are two components of a 17S complex in mammalian brain that is homologous to the yeast 834 kDa Sec6/8/15 complex which is essential for exocytosis. Purification and partial amino acid sequencing of the mammalian rsec6/8 complex reveals that it is composed of eight novel proteins with a combined molecular weight of 743 kDa. The complex is broadly expressed in brain and displays a plasma membrane localization in nerve terminals. Membrane associated rsec6/8 complex coimmunoprecipitates with syntaxin, a plasma membrane protein critical for neurotransmission. These data suggest a role for the mammalian rsec6/8 complex in neurotransmitter release via interactions with the core vesicle docking and fusion apparatus.
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