PSEUDOHYPOPARATHYROIDISM, A NOVEL MUTATION IN THE BETA-GAMMA-CONTACT REGION OF G(S)ALPHA IMPAIRS RECEPTOR STIMULATION

Pseudohypoparathyroidism, type Ia (PHP-Ia), is a dominantly inherited endocrine disorder characterized by resistance to hormones that act by stimulating adenylyl cyclase. It is caused by inheritance of an autos omal mutation that inactivates the alpha subunit (alpha(s)) of G(s), t he stimulatory regulator of adenylyl cyclase. In three members of a fa mily, the PHP-Ia phenotype is associated with a mutation (R231H) that substitutes histidine for an arginine at position 231 in alpha(s). We assessed signaling function of alpha(s)-WT versus alpha(s)-R231H trans iently transfected in HEK293 cells. Hormone receptor-dependent stimula tion of cAMP accumulation in cells expressing alpha(s)-R231H is reduce d by similar to 75% in comparison to cAMP accumulation in cells expres sing alpha(s)-WT. A second mutation, alpha(s)-R201C, inhibits the GTPa se turnoff reaction of alpha(s), thus producing receptor-independent s timulation of cAMP accumulation. The double mutant, alpha(s)-R231H/R20 1C, stimulates cAMP accumulation almost as well (similar to 80%) as do es alpha(s)-R201C itself, indicating that the R231H mutation selective ly impairs receptor-dependent signaling. In three-dimensional structur es of G protein heterotrimers, Arg-231 is located in a region, switch 2, that is thought to interact with the beta gamma subunit rather than with the hormone receptor. Thus, the R231H phenotype suggests that sw itch 2 (perhaps in concert with beta gamma) mediates G protein activat ion by receptors at a site distant from the receptor-G protein contact surface.