Albumin: a Gαs-specific guanine nucleotide dissociation inhibitor and GTPase activating protein

Heterotrimeric GTP binding protein (G protein)-mediated signal transduction events are regulated by their effectors and regulators of G protein signaling (RGS) protein family. The latter proteins function as GTPase activating proteins (GAPs) for G protein α subunits and terminate signaling events. In a search for proteins that modulate the activity of the stimulatory G protein of adenylyl cyclase (Gαs), we found that bovine serum albumin (BSA) inhibits the steady-state GTPase activity of Gαs, but not the inhibitory G protein (Gαi1). This effect of BSA is mediated by decreasing the rate of GDP dissociation from Gαs and decreasing the rate of GTP binding. Thus, BSA functions as a guanine nucleotide dissociation inhibitor for Gαs. Moreover, BSA also increased the intrinsic GTPase activity of Gαs, but not Gαi1, demonstrating that BSA functions as a Gαs-specific GAP. Using mutants of Gαs (Q227L, Q227N, R201C, and R201K), we demonstrate that BSA mediates its GAP function by modulating the ability of R201 to increase GTPase activity. Moreover, using wild-type and Q227N forms of Gαs, our studies demonstrate that the GDI function of BSA decreases the ability of Gαs to stimulate adenylyl cyclase. These findings assign a novel function to BSA as a regulator of G protein signaling.