Hominini-specific regulation of cell cycle by stop codon readthrough of FEM1B

Abstract

In certain mRNAs ribosomes continue translation beyond the canonical stop codon to generate longer isoform with a C-terminal extension. This phenomenon is termed stop codon readthrough (SCR). Previously, a genome-wide bioinformatic analysis was performed to identify several readthrough candidate genes and FEM1B was one of them. FEM1B is a substrate-recognition component of the CRL2 E3 ubiquitin protein ligase. This multi-protein complex targets specific proteins for ubiquitylation, which leads to their degradation. FEM1B independently ubiquitinate the stem-loop binding protein (SLBP) during the G1 phase, thus orchestrates histone metabolism with the cell cycle. In our study, we have demonstrated the SCR of FEM1B by using fluorescence-based, luminescence-based, and western blot assays. Translating ribosomes readthrough the stop codon of FEM1B to generate an unstable isoform. 81-nucleotides in the proximal 3′UTR of FEM1B constitute the necessary and sufficient cis-signal for SCR, and also for the degradation of the SCR product. CRISPR-edited cells lacking this region, and therefore FEM1B SCR, showed increased FEM1B expression, which in turn resulted in reduced expression of SLBP (target of FEM1B for degradation), and histone (target of SLBP for stability) causing cell cycle delay. Tumor studies in athymic nude mice using these SCR-defective cells showed reduced tumor growth rate. Evolutionary analysis revealed that this phenomenon is specific to the genus Pan and Homo (hominini). Overall, our study shows a relatively recently evolved SCR process that relieves the cell cycle from the negative regulation by FEM1B. This could partly explain why humans are more susceptible to cancer among primates.