Studies on the interaction of α subunits of GTP‐binding proteins with βγ dimers

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Abstract
The interaction of several preparations of purified beta gamma dimers with two types of guanosine-nucleotide-binding-regulatory-(G)-protein alpha subunits, a recombinant bv alpha i3, made in Sf9 Spodoptera frugiperda cells by the baculovirus (bv) expression system, and alpha s, either purified from human erythrocyte Gs-type GTP-binding protein, and activated by NaF/AlCl3, or unpurified as found in a natural membrane, were studied. The beta gamma dimers used were from bovine rod outer segments (ROS), bovine brain, human erythrocytes (hRBC) and human placenta and contained distinct ratios of beta subunits that, upon electrophoresis, migrated as two bands with approximate M(r) of 35,000 and 36,000, as well as distinct complements of at least two gamma subunits each. When tested for their ability to recombine at submaximal concentrations with bv alpha i3, ROS, brain, hRBC and placental beta gamma dimers exhibited apparent affinities that were the same within a factor of two. When bovine brain, placental and ROS beta gamma dimers were tested for their ability to promote deactivation of Gs, brain and placental beta gamma dimers were equipotent and at least 10-fold more potent than that of ROS beta gamma dimers; likewise, brain beta gamma and placental dimers were equipotent in inhibiting GTP-activated and GTP-plus-isoproterenol-activated adenylyl cyclase, while ROS beta gamma dimers were less potent when assayed at the same concentration. The possibility that different alpha subunits may distinguish subsets of beta gamma dimers from a single cell was investigated by analyzing the beta gamma composition of three G proteins, Gs, Gi2 and Gi3, purified to near homogeneity from a single cell type, the human erythrocyte. No evidence for an alpha-subunit-specific difference in beta gamma composition was found. These findings suggests that, in most cells, alpha subunits interact indistinctly with a common pool of beta gamma dimers. However, since at least one beta gamma preparation (ROS) showed unique behavior, it is clear that there may be mechanisms by which some combinations of beta gamma dimers may exhibit selectivity for the alpha subunits they interact with.