Although some members of Ras family small GTPases, including M-Ras, share the primary structure of their effector regions with Ras, they exhibit vastly different binding properties to Ras effectors such as c-Raf-1. We have solved the crystal structure of M-Ras in the GDP-bound and guanosine 5'-[,-imido]triphosphate (GppNHp)-bound forms. The overall structure of M-Ras resembles to those of H-Ras and Rap2A, except that M-Ras-GppNHp exhibits a distinctive switch I conformation, which is caused by impaired intra-molecular interactions between Thr-45 (corresponding to Thr-35 of H-Ras) of the effector region and the -phosphate of GppNHp. Previous ³¹P-nuclear magnetic resonance (NMR) studies showed that H-Ras-GppNHp exists in two interconverting conformations, state 1 and 2. While state 2 is a predominant form of H-Ras and corresponds to the on conformation found in the complex with effectors, state 1 is thought to represent the off conformation, whose tertiary structure remains unknown. ³¹P-NMR analysis shows that free M-Ras-GppNHp predominantly assume the state 1 conformation, which undergoes conformational transition to state 2 upon association with c-Raf-1. These results indicate that the solved structure of M-Ras-GppNHp corresponds to the state 1 conformation. The predominance of state 1 in M-Ras is likely to account for its weak binding ability to the Ras effectors, suggesting the importance of the tertiary structure factor in small GTPase-effector interaction. Further, the first determination of the state 1 structure provides a molecular basis for developing novel anti-cancer drugs as compounds that hold Ras in the state 1 off conformation.