INOSITOL POLYPHOSPHATES REGULATE THE MEMBRANE INTERACTIONS OF THE ENDOSOMAL P100, G-PROTEIN-RELATED PROTEIN

The protein, p100, was previously identified as a G-protein related pr otein that cycles on and off the cytoplasmic face of the endosome memb rane (Traub et al., Biochem. J. 280 (1991) 171-178). Here we present e vidence that the inositol polyphosphates, inositol 1,4,5-trisphosphate (IP3) and inositol hexakisphosphate (IP6), release p100 from light-de nsity microsomal membranes and inhibit rebinding of p100 through recep tors, which are specific for IP3 or for IP6. These receptors can be co -extracted with p100 from the microsomes by 0.5 M Tris-HCl and, in the soluble state, they exhibit similar binding activity towards the inos itol polyphosphates as do untreated microsomes. Soluble p100 self-aggr egates and this aggregation is blocked by both IP3 and IP6. Stimulatio n of permeabilized rat basophilic leukemia (RBL-2H3) cells with carbac hol, via transfected muscarinic mi receptors, results in increased lev els of inositol polyphosphates and the quantitative release of p100 in to the cytosol. This effect is reversible and cytosolic p100 rebinds t o the membrane as the levels of inositol polyphosphates decline. These findings suggest that p100 may belong to a family of IF-binding prote ins whose intracellular localization is determined by extracellular si gnals. (C) 1998 Published by Elsevier Science B.V. All rights reserved .