Crystal structures of the G protein Gi alpha 1 complexed with GDP and Mg2+: a crystallographic titration experiment.
The effect of Mg2+ binding on the conformation of the inactive GDP-bound complex of the heterotrimeric G protein alpha subunit Gi alpha 1 has been investigated by X-ray crystallography. Crystal structures of the Gi alpha 1.GDP complex were determined after titration with 5, 10, 100, and 200 mM Mg2+. Comparison of these structures with that of the Mg2+-free complex revealed Mg2+ bound at the same site as observed in the structure of the active, Gi alpha 1. GTP gamma S.Mg2+-bound complex of Gi alpha 1, with a similar coordination scheme except for the substitution of a water molecule for an oxygen ligand of the gamma-phosphate of Gi alpha 1.GTP gamma S. Mg2+. In contrast to the GDP.Mg2+ complex of Gt alpha and of other G proteins, switch I residues of Gi alpha 1 participate in Mg2+ binding and undergo conformational changes as a consequence of Mg2+ binding. Partial order is induced in switch II, which is disordered in the Mg2+-free complex, but no order is observed in the switch III region. This contrasts with the GDP.Mg2+ complex of Gt alpha in which both switch II and III switch are ordered. Mg2+ binding also induces binding of an SO42- molecule to the active site in a manner which may mimic a Gi alpha 1.GDP.PO42-.Mg2+ product complex. Implications of these findings are discussed.
Howard Hughes Medical Institute, Department of Biochemistry, The University of Texas Southwestern Medical Center, Dallas 75235-9050, USA.
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D000818:Animals; D001665:Binding Sites; D002384:Catalysis; D003198:Computer Simulation; D003460:Crystallization; D018360:Crystallography, X-Ray; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000737:chemistry* / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000737:chemistry* / Q000378:metabolism; D046911:Macromolecular Substances; D008274:Magnesium / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D011487:Protein Conformation; D051381:Rats; D013431:Sulfates / Q000378:metabolism; D017186:Titrimetry
D046911:Macromolecular Substances; D010446:Peptide Fragments; D013431:Sulfates; D006153:Guanosine Diphosphate; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D008274:Magnesium