A function in splicing versus a function in translational control – why not both?
U2AF proteins are best known for their functions in spliceosomal processing of pre-mRNAs where a homodimer of U2AF65 and U2AF35 functions in recognition of the 3′ splice site. The smaller subunit, U2AF35, contains two zinc fingers (ZnFs). Mutations therein have recently been associated with malignant transformation. The molecular function(s) of the two domains have, however, not been studied in great detail.
A collaborative effort, spearheaded by the Heyd lab at the Free University of Berlin, now revealed that the two ZnFs have remarkably different activity. Both are required for splicing regulation, whereas only ZnF2 controls protein stability and contributes to the interaction with U2AF65.
Intriguingly, a naturally occuring splice variant of U2AF26, a paralog of U2AF35, lacks the second ZnF. It is upregulated upon activation of primary mouse T cells and localizes to the cytoplasm, suggesting a splicing-independent function. Employing ribosome profiling in a model T cell line, we provide evidence for a role of U2AF26 in activating cytoplasmic steps in gene expression, notably translation. Consistently, an MS2 tethering assay shows that cytoplasmic U2AF26/35 increases translation when localized to the 5ʹUTR of a model mRNA. This regulation is partially dependent on ZnF1 thus providing a connection between a core splicing factor, the ZnF domains and the regulation of translation. Altogether, our work reveals unexpected functions of U2AF26/35 in mammalian cells beyond the regulation of splicing.
Check out the exciting paper in RNA Biology: https://doi.org/10.1080/15476286.2020.1732701