Using high-resolution Mono-Q anion-exchange chromatography, we purified four distinct GTP-binding proteins from bovine brain. Each consists of a and associated β/γ subunits, and each is a substrate for pertussis toxin catalyzed ADP-ribosylation. We defined the relationship between the a subunits of the purified proteins and cloned cDNAs encoding putative a subunits (1) by performing immunoblots with peptide antisera with defined specificity and (2) by comparing the migration on two-dimensional gel electrophoresis of the purified proteins, and of the in vitro translated products of cDNAs encoding a subunits. Purified G proteins with a subunits of 39, 41, and 40 kDa (G39, G41, and G40in order of abundance) correspond to the products of Go, Gi1, and Gi2cDNAs. We purified a novel G protein with an α subunit slightly above 39 kDa (G39*). G39* is less abundant than G39, elutes earlier than G39on Mono-Q chromatography, and has a more basic pI (6.0 vs 5.6) than G39. G39and G39*, however, are indistinguishable on immunoblots with a large number of specific antisera. The data suggest that G39* may represent a novel form of Go, differing in posttranslational modification rather than primary sequence.
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