[ODP4C] c. Protein Structure and Function

[ODP-067] Structural basis of bacterial actin MreB involved in Spiroplasma swimming

○Daichi Takahashi¹, Ikuko Fujiwara^1,2, Katsumi Imada³, Makoto Miyata^1,2 (¹Grad. Sch. Sci., Osaka City Univ., ²OCARINA, Osaka City Univ., ³Grad. Sch. Sci., Osaka Univ.)

Bacterial actin homologs are involved in various important cellular processes, such as cell shape determination and cell division. MreB, an actin-like ATPase, is widely found in walled bacteria as one copy on the genome and functions as a scaffold of a peptidoglycan synthesis complex. However, Spiroplasma eriocheiris, a helical wall-less bacterium, has five types of MreB homologs (MreB1-5) that assemble into an internal ribbon-like structure for swimming motility. MreB3 binds to the cell membrane and forms a double-stranded filament with a wide gap between protofilaments. The P_i release activity of MreB3 is slower than that of other MreBs. To elucidate the structural basis of these unique properties, we determined the crystal structure of MreB3 at AMPPNP binding state. MreB3 is composed of four sub-domains like other actin homologs and forms a protofilament in the crystal. A helix at the protofilament interface is tilted toward the other protofilament, thus the distance between the protofilaments of MreB3 is wider than that of other MreBs. The conserved threonine residue that interacts with the catalytic water molecule in the active site of other MreBs is replaced by lysine (K174) in MreB3. However, K174 does not binds to the water, and no hydrophilic residue that can be a substitute for the threonine is present near the water molecule, probably leading to the slow P_i release.