The native pertussis toxin sensitive GTP-binding proteins (G(i) proteins) were individually resolved, and their guanine nucleotide binding and release properties were studied. G(i2) and G(i3), the two major GTP-binding proteins of human erythrocytes, were purified to apparent homogeneity by fast protein liquid chromatography. G(i1) was purified from bovine brain. The three proteins bound 0.6-0.85 mol of guanosine 5'-O-(thiotriphosphate) (GTPγS)/mol of protein with similar affinities (K(D(app)) = 50-100 nM). The rate of [35S]GTPγS binding to G(i2) were 5-8-fold faster than to G(i1) or G(i3) at 2 mM Mg2+. There were no observable differences in the binding characteristics between bovine brain G(i1) and human erythrocyte G(i3). At 50 mM Mg2+, all three G(i) proteins exhibited fast binding, although G(i1) and G(i3) were marginally slower than G(i2). All three G(i) proteins exhibited different rates of [32P]GDP release at 2 mM Mg2+. GDP release from G(i2) was severalfold faster than that from G(i1) or G(i3). GDP release rates from G(i1) and G(i3) were similar, although G(i3) was somewhat (60-80%) faster than G(i1). These data indicate that rates of GDP release and GTP binding may be independently regulated for these three proteins and that the relative proportions of G(i2)/G(i1) or G(i2)/G(i3) will be a crucial factor in determining the kinetics of signal transduction through G(i)-coupled effectors.
|Number of pages||6|
|Journal||Journal of Biological Chemistry|
|State||Published - 1990|