Distinct guanine nucleotide binding and release properties of the three Gi proteins.
The native pertussis toxin sensitive GTP-binding proteins (Gi proteins) were individually resolved, and their guanine nucleotide binding and release properties were studied. Gi2 and Gi3, the two major GTP-binding proteins of human erythrocytes, were purified to apparent homogeneity by fast protein liquid chromatography. Gi1 was purified from bovine brain. The three proteins bound 0.6-0.85 mol of guanosine 5'-O-(thio-triphosphate (GTP gamma S)/mol of protein with similar affinities (KD(app) = 50-100 nM). The rate of [35S]GTP gamma S binding to Gi2 was 5-8-fold faster than to Gi1 or Gi3 at 2 mm Mg2+. There were no observable differences in the binding characteristics between bovine brain Gi1 and human erythrocyte Gi3. At 50 mM Mg2+, all three Gi proteins exhibited fast binding, although Gi1 and Gi3 were marginally slower than Gi2. All three Gi proteins exhibited different rates of [32P]GDP release at 2 mM Mg2+. GDP release from Gi2 was severalfold faster than that from Gi1 or Gi3. GDP release rates from Gi1 and Gi3 were similar, although Gi3 was somewhat (60-80%) faster than Gi1. 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 Gi2/Gi1 or Gi2/Gi3 will be a crucial factor in determining the kinetics of signal transduction through Gi-coupled effectors.
Carty
D J
DJ
Department of Pharmacology, Mount Sinai School of Medicine, City University, New York, New York 10029.
Padrell
E
E
Codina
J
J
Birnbaumer
L
L
Hildebrandt
J D
JD
Iyengar
R
R
D000818:Animals; D001921:Brain / Q000378:metabolism*; D002417:Cattle; D004912:Erythrocytes / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000097:blood / Q000302:isolation & purification / Q000378:metabolism*; D006150:Guanine Nucleotides / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006153:Guanosine Diphosphate / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000031:analogs & derivatives* / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D012984:Software; D013873:Thionucleotides / Q000378:metabolism*
D006150:Guanine Nucleotides; D013873:Thionucleotides; D006153:Guanosine Diphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D019204:GTP-Binding Proteins