Molecular basis of mRNA cap recognition by influenza B polymerase PB2 subunit

Abstract

Influenza virus polymerase catalyzes the transcription of viral mRNAs by a process known as "cap-snatching," where the 5'-cap of cellular pre-mRNA is recognized by the PB2 subunit and cleaved 10-13 nucleotides downstream of the cap by the endonucleasePAsubunit. Although this mechanism is common to both influenza A (FluA) and influenza B (FluB) viruses, FluB PB2 recognizes a wider range of cap structures including m7GpppGm-,m7GpppG-, and GpppG-RNA, whereas FluA PB2 utilizes methylated G-capped RNA specifically. Biophysical studies with isolated PB2 cap-binding domain (PB2cap) confirm that FluB PB2 has expanded mRNA cap recognition capability, although the affinities toward m7GTP are significantly reduced when compared with FluA PB2. The x-ray co-structures of the FluB PB2cap with bound cap analogsm7GTP and GTP reveal an inverted GTP binding mode that is distinct from the cognate m7GTP binding mode shared between FluA and FluB PB2. These results delineate the commonalities and differences in the cap-binding site between FluA and FluB PB2 and will aid structure- guided drug design efforts to identify dual inhibitors of both FluA and FluB PB2.