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Molecular mechanism of the cSHMT IRES

The 5′ untranslated region (UTR) of the cytoplasmic serine hydroxymethyltransferase (cSHMT) transcript contains an internal ribosome entry site (IRES) that regulates folate‐mediated one‐carbon metabolism. In this study, we investigate the molecular mechanism of cSHMT IRES activation. We identify heterogeneous nuclear ribonucleoprotein H2 (hnRNP H2) as a novel cSHMT IRES trans‐acting factor (ITAF), and show that it interacts with the two previously identified cSHMT ITAFs, BrunoL2 and heavy chain ferritin (HCF). We investigate the role of HCF post‐translational processing and the ligand state of HCF in the activation of the cSHMT IRES, testing the hypothesis that a 14 kDa HCF fragment and a HCF ferroxidase mutant have a greater effect on IRES activity than HCF. Finally, we elucidate the interactions among all of these proteins and the 5′ and 3′ UTRs of the cSHMT transcript, and use this information to construct a model detailing the ITAF‐mRNA interactions that allow for the cap‐independent translation of cSHMT.