N-ethylmaleimide sensitive fusion protein (NSF) and soluble NSF attachment proteins (SNAPs) are involved in many vesicular transport steps. It has been proposed that SNAPs and NSF associate with their membrane receptors only when vesicles dock on the target membrane. Analysis of NSF and ~x-SNAP distribution in fractionation of organelles from adrenal medulla indicated that a substantial amount of both proteins distributed with chromaffin granules. Further fractionation of intact granules and lysed granule membranes showed exact overlap of NSF and s-SNAP distribution with chromaffin granules. These results suggest that NSF and s-SNAP are associated with chromaffin granules and support the idea that they function prior to docking of the granules on the plasma membrane.
Doc2 is a C2-domain-containing protein that is highly expressed in the nervous system and has a constitutively expressed isoform. It has been implicated as a potential Ca2+ sensor in regulated exocytosis, and has been suggested to be associated with synaptic vesicles. To examine whether Doc2 is associated with synaptic-like microvesicles (SLMVs) or dense-core granules in neuroendocrine cells, we examined the distribution of Doc2 in subcellular fractionation of chromaffin cells of the adrenal medulla and in PC12 cells. Doc2 did not co-distribute with SLMVs from either cell type, but did appear to co-distribute with dense-core granules from PC12 cells. In contrast, it was not associated with the dense-core granules during subcellular fractionation of the adrenal medulla, and nor did it appear to be associated with endosomes, cis-Golgi or the trans-Golgi network. In contrast, Doc2 co-distributed under all conditions with a mitochondrial marker. We conclude that Doc2 is not a general component of regulated secretory vesicles, but may instead be associated with mitochondria.
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