NOVEL EXPLOITATION OF A NUCLEAR FUNCTION BY INFLUENZA-VIRUS - THE CELLULAR SF2 ASF SPLICING FACTOR CONTROLS THE AMOUNT OF THE ESSENTIAL VIRAL M2 ION-CHANNEL PROTEIN IN INFECTED-CELLS/

We show that a cellular nuclear protein, the SR splicing factor SF2/AS F, controls the level of production of an essential influenza virus pr otein, the M2 ion channel protein, The M2 mRNA that encodes the ion ch annel protein is produced by alternative splicing of another viral mRN A, M1 mRNA, The production of M2 mRNA is controlled in two ways, First , a distal (stronger) 5' splice site in M1 mRNA is blocked by the comp lex of viral polymerase proteins synthesized during infection, allowin g the cellular splicing machinery to switch to the proximal (weaker) M 2 5' splice site, Second, utilization of the weak M2 5' splice site re quires its activation by the cellular SF2/ASF protein, This activation is mediated by the binding of the SF2/ASF protein to a purine-rich sp licing enhancer sequence that is located in the 3' exon of hll mRNA, W e demonstrate that activation of the M2 5' splice site is controlled b y the SF2/ASF protein in vivo during influenza virus infection, Utiliz ing four cell lines that differ in their levels of production of the S F2/ASF protein, me show that during virus infection of these cell line s both M2 mRNA and the M2 ion channel protein are produced in amounts that are proportional to the different expression levels of the SF2/AS F protein.