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 protein that cycles on and off the cytoplasmic face of the endosome membrane (Traub et al., Biochem. J. 280 (1991) 171–178). Here we present evidence that the inositol polyphosphates, inositol 1,4,5-trisphosphate (IP₃) and inositol hexakisphosphate (IP₆), release p100 from light-density microsomal membranes and inhibit rebinding of p100 through receptors, 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 inositol polyphosphates as do untreated microsomes. Soluble p100 self-aggregates and this aggregation is blocked by both IP₃ and IP₆. Stimulation of permeabilized rat basophilic leukemia (RBL-2H3) cells with carbachol, via transfected muscarinic m1 receptors, results in increased levels of inositol polyphosphates and the quantitative release of p100 into the cytosol. This effect is reversible and cytosolic p100 rebinds to the membrane as the levels of inositol polyphosphates decline. These findings suggest that p100 may belong to a family of IP-binding proteins whose intracellular localization is determined by extracellular signals.