HnRNPA1 and HuR, but not PTB, modulate cap-independent translation initiation from sense and antisense human T-cell lymphotropic virus type 1 mRNAs.

The human T-cell lymphotropic virus type 1 (HTLV-1) causes adult T-cell leukemia and HTLV-associated myelopathy/tropical spastic paraparesis. HTLV-1 transcribes sense and two antisense messenger RNAs (mRNAs). The spliced and unspliced antisense mRNA encode the HTLV-1 basic zipper factor (HBZ). The sense mRNA can initiate translation using a cap- or internal ribosomal entry site (IRES)-mediated mechanism (HTLV-1 IRES). The spliced antisense mRNA initiates translation via the sHBZ IRES. Using bicistronic vectors harboring the 5'-untranslated region of the HTLV-1 sense or spliced antisense mRNAs in their intercistronic space, we evaluated the role of heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1), ELAV-like protein 1 (ELAVL1; also known as HuR), and polypyrimidine tract-binding protein 1 (PTB) as regulators of the HTLV-1 IRES and sHBZ IRES function. Results show that PTB is not an IRES-transacting factor (ITAF) for either IRES, whereas hnRNPA1 promotes both. However, protein arginine N-methyltransferase 5 (PRMT5)-induced symmetrical dimethylation of arginine residues in hnRNPA1, required to stimulate the HTLV-1 IRES, plays no role in sHBZ IRES activity. HuR overexpression reduces both HTLV-1 IRES and sHBZ IRES activity. The effect of hnRNPA1 and HuR over the IRESs is lost when both proteins are overexpressed together. When heterogeneous nuclear ribonucleoprotein K (hnRNPK), an ITAF for the HTLV-1 IRES but not for the sHBZ IRES, is overexpressed with HuR, HTLV-1 IRES is stimulated, whereas sHBZ IRES activity is reduced. Consequently, ITAFs or combinations of ITAFs differentially modulated the HTLV-1 IRES and sHBZ IRES activity, indicating that these IRESs are under translational control mediated by different subsets of ITAFs.