CRYSTAL-STRUCTURES OF THE G-PROTEIN G(I-ALPHA-1) COMPLEXED WITH GDP AND MG2- A CRYSTALLOGRAPHIC TITRATION EXPERIMENT( )

The effect of Mg2+ binding on the conformation of the inactive GDP-bou nd complex of the heterotrimeric G protein alpha subunit G(i alpha 1) has been investigated by X-ray crystallography. Crystal structures of the G(i 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, G(i alpha 1). GTP gamma S . Mg2+-boun d complex of G(i alpha 1), with a similar coordination scheme except f or the substitution of a water molecule for an oxygen ligand of the ga mma-phosphate of G(i alpha 1). GTP gamma S . Mg2+. In contrast to the GDP . Mg2+ complex of G(t alpha) and of other G proteins, switch I res idues of G(i alpha 1) participate in Mg2+ binding and undergo conforma tional changes as a consequence of Mg2+ binding. Partial order is indu ced 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 G DP Mg2+ complex of G(t alpha) in which both switch II and III switch a re ordered. Mg2+ binding also induces binding of an SO42- molecule to the active site in a which may mimic a G(i alpha 1). GDP . PO42-. Mg2 product complex. Implications of these findings are discussed.