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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 GAP1(IP4BP), recently identified as a putative receptor for Ins(1,3, 4,5)P4. GAP1(IP4BP) 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'-[gamma-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 GAP1(IP4BP)), whereas H-Ras was inactive at similar concentrations. Guanosine 5'-[beta-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 GAP1(IP4BP), 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 GAP1(IP4BP) or a closely related protein (such as GAP1(m)), 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.

We have previously found that for permeabilized L1210 cells, low micromolar concentrations of Ins(1,3,4,5)P4 added prior to Ins(2,4,5)P3 enhance the effects of suboptimal concentrations of Ins(2,4,5)P3 in causing Ca2+ release from InsP3-sensitive Ca2+ stores [Cullen, Irvine and Dawson (1990) Biochem J. 271, 549-553]. If this was due either to some conversion of added Ins(1,3,4,5)P4 into Ins(1,4,5)P3 by the 3-phosphatase, or to Ins(1,3,4,5)P4 acting as a weak (or partial) agonist on the InsP3 receptor it would be expected that,in the presence of thimerosal to sensitize the InsP3 receptor, the dose-response curve to Ins(1,3,4,5)P4 would be left-shifted by the same extent as that of Ins(1,4,5)P3. This was found not to be the case; the dose-response curve to Ins(1,3,4,5)P4 was not shifted at all by thimerosal. Furthermore, L-Ins(1,3,4,5)P4, which can displace radiolabelled D-Ins(1,3,4,5)P4 but not D-Ins(1,4,5)P3 from their respective high-affinity binding sites, mimicked the effects of D-Ins(1,3,4,5)P4 in enhancing the slow phase of Ins(2,4,5)P3-stimulated Ca2+ release. Ins(1,3,4,5)P4 caused an increase in magnitude of the slow phase of InsP3-stimulated Ca2+ release leaving the magnitude of the fast phase unaltered, in contrast to increasing Ins(2,4,5)P3 concentrations which increased the size of both phases. In addition, Ins(1,3,4,5)P4 decreased the rate constant for the slow phase of Ca2+ release. These findings point strongly to the conclusion that InsP4 is not working directly via the InsP3 receptor but indirectly via an InsP4 receptor.