Epstein-Barr virus SM protein functions as an alternative splicing factor.

Alternative splicing of RNA increases the coding potential of the genome and allows for additional regulatory control over gene expression. The full extent of alternative splicing remains to be defined but is likely to significantly expand the size of the human transcriptome. There are several examples of mammalian viruses regulating viral splicing or inhibiting cellular splicing in order to facilitate viral replication. Here, we describe a viral protein that induces alternative splicing of a cellular RNA transcript. Epstein-Barr virus (EBV) SM protein is a viral protein essential for replication that enhances EBV gene expression by enhancing RNA stability and export. SM also increases cellular STAT1 expression, a central mediator of interferon signal transduction, but disproportionately increases the abundance of the STAT1beta splicing isoform, which can act as a dominant-negative suppressor of STAT1alpha. SM induces splicing of STAT1 at a novel 5' splice site, resulting in a STAT1 mRNA incapable of producing STAT1alpha. SM-induced alternative splicing is dependent on the presence of an RNA sequence to which SM binds directly and which can confer SM-dependent splicing on heterologous RNA. The cellular splicing factor ASF/SF2 also binds to this region and inhibits SM-RNA binding and SM-induced alternative splicing. These results suggest that viruses may regulate cellular gene expression at the level of alternative mRNA splicing in order to facilitate virus replication or persistence in vivo.