The BM2 protein of influenza B virus is synthesized in the late phase of infection and incorporated into virions as a subviral component

The influenza B virus genome RNA segment 7 encodes the M1 and BM2 proteins. The BM2 protein is synthesized by a coupled translational termination-rein itiation mechanism at the overlapping stop-start pentanucleotide in a bicis tronic mRNA transcribed from RNA segment 7, However, features and functions of this protein remain unclear. In this study the BM2 protein was characte rized by using an antiserum raised to the BM2 protein of influenza virus st rain B/Yamagata/1/73, In cells infected with B/Yamagata virus the alpha BM2 antibody specifically detected the BM2 protein with a molecular mass of 12 kDa and also a polypeptide with a molecular mass of 17 kDa, When infected cells were labelled with P-32(i) and immunoprecipitated with the alpha BM2 antibody, the P-32-labelled 17 kDa polypeptide was specifically precipitate d. In the presence of casein kinase inhibitor CKI-7 the synthesis of the 17 kDa and BM2 proteins was completely suppressed, although other viral prote ins, except for the polymerase protein, were synthesized normally. These re sults suggest that the 17 kDa species is a phosphorylated form of the BM2 p rotein. These species were substantially synthesized in the late phase of i nfection and localized in the cytoplasm throughout infection, Moreover, the y were transported to the plasma membrane and thereafter were incorporated into virions. These results therefore suggest that the BM2 and the 17 kDa p roteins are necessary for the life-cycle of influenza B virus.