New Catalytic\nResidues and Catalytic Mechanism of\nthe RNase T1 Family

The ribonuclease T1 family, including RNase Po1 secreted\nby <i>Pleurotus ostreatus</i>, exhibits antitumor activity.\nHere,\nwe resolved the Po1/guanosine-3′-monophosphate complex (3′GMP)\nstructure at 1.75 Å. Structure comparison and fragment molecular\norbital (FMO) calculation between the apo form and the Po1/3′GMP\ncomplex identified Phe38, Phe40, and Glu42 as the key binding residues.\nTwo types of the RNase/3′GMP complex in RNasePo1 and RNase\nT1 were homologous to Po1, and FMO calculations elucidated that the\nbiprotonated histidine on the β3 sheet (His36) on the β3\nsheet and deprotonated Glu54 on the β4 sheet were advantageous\nto RNase activity. Moreover, tyrosine (Tyr34) on the β3 sheet\nwas elucidated as a crucial catalytic residues. Mutation of Tyr34\nwith phenylalanine decreased RNase activity and diminished antitumor\nefficacy compared to that in the wild type. This suggests the importance\nof RNase activity in antitumor mechanisms.