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-A resolution. Elimination of a Mg²⁺ ion induces significant conformational changes in the switch I region that opens up the nucleotide- binding site. Similar structural changes have been observed in the switch regions of Ha-Ras bound to its guanine nucleotide exchange factor, Sos. This RHOA-GDP structruce reveals an important regulatory role for Mg²⁺ and suggests that guanine nucleotide exchange factor may utilize this feature of switch I to produce an open conformation in GDP/GTP exchange.