hnRNP C1/C2 is required for localized translation at the cell cortex in mitotic cells

Regulation of mRNA translation is critical for cell growth and proliferation. RNA-binding proteins mediate selective up- or down-regulation of active translation of specific transcripts in response to signals. We previously demonstrated that 5’TOP mRNA transcripts are translated during mitosis by polyribosomes associated with hnRNP C, a splicing inhibitor nuclear protein. Moreover, Ribo-Seq experiments showed that hnRNP C is required for 5`TOP mRNA translation during mitosis (Aviner et al., NAR 2017). Further in-vivo puromycilation combined with immunofluorescence experiments showed that during metaphase, hnRNP C is strictly localized to the cell periphery and co-localizes with actively translating polyribosomes. Upon the formation of nascent nuclei in telophase, hnRNP C returns back to its nuclear location. We used the CRISPR-Cas9 system to mutate the hnRNP C gene by eliminating the SUMOylation site at residue K237. Strikingly, not only that the mutated K237R hnRNP C protein does not co-localize with ribosomes during mitosis, it also does not localize to the cell cortex. Importantly, active translation at the cell cortex is absent in Hela cells stably expressing hnRNP C (K237R), and they undergo cell death following several passages. Moreover, the diffusive pattern of the K237R hnRNP C in mitotic cells correlates with granular appearance of ribosomes and increased binding of RPS6 to the mitotic spindle. Our data suggests that hnRNP C, following mitosis-mediated specific SUMOylation, is responsible for active translation of 5`TOP mRNA at the cell periphery prior to formation of daughter cells nuclei at telophase.