Testis-specific RNA methyltransferase NSUN7 contains a re-arranged catalytic site

The RNA methyltransferase NSUN7 has been reported to be involved in the regulation of longitudinal columns positioning in sperm flagella, but its catalytic mechanism remains unclear. In this study, we investigated the functional role of NSUN7's methylation in the longitudinal column positioning by generating a mouse strain with a substitution of the putative catalytic cysteine in motif IV to alanine (Nsun7^C382A). Contrary to predictions based on the typical reaction mechanism of NOP2/Sun family methyltransferases, Nsun7^C382A mice did not exhibit any phenotypical characteristics of knockouts and had normal fertility, sperm motility, and longitudinal column positioning, similar to wild-type mice. Structural modelling suggests that NSUN7 possesses an unusual catalytic site composition, including three highly conserved cysteines from motifs IV, VI and VIII. The distance between the canonical catalytic cysteines in motifs IV and VI is significantly greater than in related methyltransferases, while a cysteine from motif VIII is positioned closer to motif VI. These findings allow us to assume that NSUN7 may have an essential function in the spermatogenesis of mice independent on its methyltransferase activity.