The features and functions of influenza B virus BM2 protein in the process of virus particle formation.

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A bicistronic mRNA transcribed from the influenza B virus RNA segment 7 encodes two viral proteins, matrix protein M1 and uncharacterized small protein BM2. Our previous findings indicate that BM2 is synthesized as a phosphoprotein at the late phase of infection, transported from the perinuclear region to the plasma membrane, and finally incorporated into the virion. In the present study, we focused on the cytoplasmic transport and cellular membrane association of BM2. Immunofluorescence studies of virus-infected and BM2 expression plasmid-transfected cells indicated that BM2 accumulated at the Golgi apparatus immediately after synthesis and then was transported to the plasma membrane through the trans-Golgi network. Localization of a set of BM2 deletion mutants revealed that the N-terminal half of BM2 (residues 2-50) was crucial for its transport ; in particular, the deletion of residues 2-23, deduced to be a transmembrane domain, resulted in diffused distribution of protein throughout the entire cells. Sucrose gradient flotation of the membrane showed that BM2 alone can associate with cellular membranes. Biochemical analysis of the membranes expressing BM2 further indicated that BM2 was tightly associated with cellular membranes as an integral membrane protein. Oligomerization of BM2 was demonstrated by co-precipitation of differentially epitope-tagged BM2 proteins. Furthermore, proteolytic analysis of the purifed virion showed that most part of the BM2 molecule exists in the interior of the virion. Taken together, these results strongly suggest that BM2 is integrated into the plasma membrane at the N-terminal hydrophobic domain in similar fashion to small proteins M2 and NB of influenza A and B viruses, respectively.