Tissue-specific expression and endogenous subcellular distribution of the inositol 1,3,4,5-tetrakisphosphate-binding proteins GAP1(IP4BP) and GAP1(m)

GAP1(IP4BP) and GAP1(m) belong to the GAP1 family of Ras GTPase-activating proteins that are candidate InsP(4) receptors. Here we show they are ubiqui tously expressed in human tissues and are likely to have tissue-specific sp lice variants. Analysis by subcellular fractionation of RBL-2H3 rat basophi lic leukemia cells confirms that endogenous GAP1(IP4BP) is primarily locali sed to the plasma membrane, whereas GAP1(m) appears localised to the cytopl asm (cytosol and internal membranes) but not the plasma membrane. Subcellul ar fractionation did not indicate a specific colocalisation between membran e-bound GAP1(m) and several Ca2+ store markers, consistent with the lack of co-localisation between GAP1(m) and SERCA1 upon co-expression in COS-7 cel ls. This difference suggests that GAP1(m) does not reside at a site where i t could regulate the ability of InsP(4) to release intracellular Ca2+. As G AP1(m) is primarily localised to the cytosol of unstimulated cells it may b e spatially regulated in order to interact with Ras at the plasma membrane, (C) 1999 Academic Press.