RGSZ1, A G(Z)-SELECTIVE RGS PROTEIN IN BRAIN - STRUCTURE, MEMBRANE ASSOCIATION, REGULATION BY G-ALPHA(Z) PHOSPHORYLATION, AND RELATIONSHIP TO A G(Z) GTPASE-ACTIVATING PROTEIN SUBFAMILY

We cloned the cDNA for human RGSZ1, the major G(z)-selective GTPase-ac tivating protein (GAP) in brain (Wang, J., Tu, Y., Woodson, J., Song, IL, and Ross, E. M. (1997) J. Biol, Chem. 272, 5732-5740) and a member of the RGS family of G protein GAPs. Its sequence is 83% identical to RET-RGS1 (except its N-terminal extension) and 56% identical to GAIP. Purified, recombinant RGSZ1, RET-RGS1, and GAIP each accelerated the hydrolysis of G alpha(z)-GTP over 400-fold with K-m values of similar to 2 nM. RGSZ1 was 100-fold selective for G alpha(z) over G alpha(i), unusually specific among RGS proteins. Other enzymological properties of RGSZ1, brain G(z) GAP, and RET-RGS1 were identical; GAIP differed o nly in Mg2+ dependence and in its slightly lower selectivity for G alp ha(z). RGSZ1, RET-RGS1, and GAIP thus define a subfamily of G(z) GAPs within the RGS proteins. RGSZ1 has no obvious membrane-spanning region but is tightly membrane-bound in brain. Its regulatory activity in me mbranes depends on stable bilayer association. When co-reconstituted i nto phospholipid vesicles with G(z) and m2 muscarinic receptors, RGSZ1 increased agonist-stimulated GTPase >15-fold with EC50 <12 nM, but RG SZ1 added to the vesicle suspension was <0.1% as active. RGSZ1, RET-RG S1, and GAIP share a cysteine string sequence, perhaps targeting them to secretory vesicles and allowing them to participate in the proposed control of secretion by G(z). Phosphorylation of G alpha(z) by protei n kinase C inhibited the GAP activity of RGSZ1 and other RGS proteins, providing a mechanism for potentiation of G(z) signaling by protein k inase C.