An open conformation of switch I revealed by the crystal structure of a Mg2+-free form of RHOA complexed with GDP - Implications for the GDP/GTP exchange mechanism

Mg2+ ions are essential for guanosine triphosphatase (GTPase) activity and play key roles in guanine nucleotide binding and preserving the structural integrity of GTP-binding proteins. We determined the crystal structure of a small GTPase RHOA complexed with GDP in the absence of Mg2+ at 2.0-Angstro m resolution Elimination of a Mg2+ ion induces significant conformational c hanges in the switch I region that opens up the nucleotide-binding site. Si milar structural changes have been observed in the switch regions of Ha Ras bound to its guanine nucleotide exchange factor, Sos. This RHOA-GDP struct ure reveals an important regulatory role for Mg2+ and suggests that guanine nucleotide exchange factor may utilize this feature of snitch I to produce an open conformation in GDP/GTP exchange.