Most loss of function mutations of GNAS identified in different forms of pseudohypoparathyroidism disrupt not only the stimulatory G protein alpha-subunit (Gsalpha), but also its paternally expressed variant, XLalphas. However, the possibility that XLalphas deficiency contributes to disease pathogenesis has remained unexplored. We therefore examined the signaling property of human XLalphas and the effects of one novel (XLalphas(H704P) or Gsalpha(H362P)) and two previously described (XLalphas(DelI724) and XLalphas(Y733X) or Gsalpha(DelI382) and Gsalpha(Y391X), respectively) GNAS mutations on either XLalphas or Gsalpha activity. Confocal immunofluorescence microscopy detected human XLalphas immunoreactivity at the plasma membrane of transduced mouse embryonic fibroblasts null for endogenous Gsalpha and XLalphas (Gnas(E2-/E2-) cells). Cholera toxin- and isoproterenol-induced cAMP accumulation in Gnas(E2-/E2-) cells transiently expressing wild-type human XLalphas was similar to that in cells transiently expressing wild-type Gsalpha. Human XLalphas, like Gsalpha, mediated PTH-induced cAMP accumulation in Gnas(E2-/E2-) cells coexpressing PTH receptor type 1 and either of these proteins. Moreover, overexpression of human XLalphas or Gsalpha markedly enhanced the PTH-induced cAMP accumulation in opossum kidney cells that endogenously express PTH receptor type 1. In contrast, each XLalphas mutant failed to mediate isoproterenol- and PTH-induced cAMP accumulation in transduced Gnas(E2-/E2-) cells. XLalphas(DelI724) showed a reduced cholera toxin response over the basal level compared with wild-type XLalphas, and XLalphas(H704P) completely failed to respond to cholera toxin. These findings were comparable to those observed with each corresponding Gsalpha mutant transiently expressed in Gnas(E2-/E2-) cells. Thus, mutations that typically inactivate Gsalpha also impair XLalphas activity, consistent with a possible role for XLalphas deficiency in diseases caused by paternal GNAS mutations.