Mitotic checkpoint gene expression is tuned by codon usage bias

The mitotic checkpoint (also called spindle assembly checkpoint, SAC) is a signaling pathway that safeguards proper chromosome segregation. Correct functioning of the SAC depends on adequate protein concentrations and appropriate stoichiometries between SAC proteins. Yet very little is known about the regulation of SAC gene expression. Here, we show in the fission yeast Schizosaccharomyces pombe that a combination of short mRNA half‐lives and long protein half‐lives supports stable SAC protein levels. For the SAC genes mad2 + and mad3 + , their short mRNA half‐lives are caused, in part, by a high frequency of nonoptimal codons. In contrast, mad1 + mRNA has a short half‐life despite a higher frequency of optimal codons, and despite the lack of known RNA‐destabilizing motifs. Hence, different SAC genes employ different strategies of expression. We further show that Mad1 homodimers form co‐translationally, which may necessitate a certain codon usage pattern. Taken together, we propose that the codon usage of SAC genes is fine‐tuned to ensure proper SAC function. Our work shines light on gene expression features that promote spindle assembly checkpoint function and suggests that synonymous mutations may weaken the checkpoint. Synopsis The proper function of the mitotic spindle assembly checkpoint requires the concentrations of checkpoint proteins to be within narrow ranges. Here, the gene expression characteristics and the contribution of codon usage to the expression of key checkpoint proteins are examined Short mRNA half‐lives combined with long protein half‐lives ensure stable protein levels over time and between cells. Nonoptimal codon usage in mad2 and mad3 contributes to their short mRNA half‐lives. Mad1's codon usage bias is important for proper expression despite not greatly influencing mRNA half‐life. Mad1‐Mad1 homodimers assemble co‐translationally from a single mRNA, which may be promoted by Mad1's codon usage pattern. Graphical Abstract Optimal protein levels of the mitotic checkpoint proteins Mad1, Mad2, and Mad3 in fission yeast are the result of a combination of differential regulation of mRNA and protein stability.