A cotranslational role for selenocysteine in productive protein folding

Selenocysteine (Sec) is generally regarded as the catalytic determinant that distinguishes selenoproteins from their cysteine (Cys)-containing counterparts. Consequently, the biological significance of Sec has largely been attributed to its unique chemical properties within mature proteins. Here, we uncover an unexpected role for Sec in protein biogenesis. Using the endoplasmic reticulum selenoprotein SELENOM as a model, we show that replacement of Sec with Cys redirects the protein into an alternative assembly pathway, resulting in formation of a stable domain-swapped homodimer. Biochemical and structural analyses demonstrate that this phenotype cannot be explained solely by altered active-site chemistry, as dimerisation persists following extensive active-site mutagenesis. Remarkably, reintroduction of Sec at alternative positions suppresses dimer formation, including when Sec is placed within a cleaved signal peptide and is absent from the mature protein. Thus, identical protein sequences can adopt distinct folding outcomes depending on whether Sec incorporation occurred during biosynthesis. These findings reveal a previously unrecognised role for Sec in productive protein folding and raise the possibility that evolutionary retention of the Sec incorporation pathway reflects advantages not only in catalysis but also in protein folding.