An Open Conformation of Switch I Revealed by the Crystal Structure of a Mg2+-free Form of RHOA Complexed with GDP

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-Å 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 structure 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.