We have previously shown that addition of Ins(1,3,4,5)P4 to permeabilized L1210 cells increases the amount of Ca2+ mobilized by a submaximal concentration of Ins(2,4,5)P3, and we suggested that, in doing this, Ins(1,3,4,5)P4 is not working via an InsP3 receptor but indirectly via an InsP4 receptor [Loomis-Husselbee, Cullen, Dreikhausen, Irvine and Dawson (1996) Biochem. J. 314, 811–816]. Here we have investigated whether this effect might be mediated by GAP1IP4BP, recently identified as a putative receptor for Ins(1,3,4,5)P4. GAP1IP4BP is a protein that interacts with one or more monomeric G-proteins, so we sought evidence for involvement of monomeric G-proteins in the effects of Ins(1,3,4,5)P4 in permeabilized L1210 cells. Guanosine 5´-[γ-thio]triphosphate (GTP[S]) enhanced the effect of Ins(1,3,4,5)P4 on Ins(2,4,5)P3-stimulated Ca2+ mobilization, but had no effect on the action of Ins(2,4,5)P3 alone. A specific enhancement of only the action of Ins(1,3,4,5)P4 was also seen with GTP[S]-loaded R-Ras or Rap1a (two G-proteins known to interact with GAP1IP4BP), whereas H-Ras was inactive at similar concentrations. Guanosine 5´-[β-thio]diphosphate (GDP[S]) did not alter the action of either Ins(2,4,5)P3 or Ins(1,3,4,5)P4. Finally, the addition of exogenous GAP1IP4BP, purified from platelets, markedly enhanced the effect of Ins(1,3,4,5)P4, and again, the amount of Ca2+ mobilized by Ins(2,4,5)P3 alone was unaltered. We conclude that the increase in Ins(2,4,5)P3-stimulated Ca2+ mobilization by Ins(1,3,4,5)P4 may be mediated by GAP1IP4BP or a closely related protein (such as GAP1m), and if so, the action of the GAP1 is not solely to regulate GTP loading of a G-protein, but rather it acts with a G-protein to cause its effect.

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