REGULATION OF ADENOVIRUS ALTERNATIVE RNA SPLICING BY DEPHOSPHORYLATION OF SR PROTEINS

SR proteins are a family of essential splicing factors required for ea rly recognition of splice sites during spliceosome assembly(1,2). They also function as alternative RNA splicing factors when overexpressed in vivo or added in excess to extracts in vitro(1,2). SR proteins are highly phosphorylated in vivo, a modification that is required for the ir function in spliceosome assembly(3,4) and splicing catalysis(5,6). Here we show that SR proteins purified from late adenovirus-infected c ells are inactivated as splicing enhancer or splicing repressor protei ns by virus-induced dephosphorylation. We further show that the virus- encoded protein E4-ORF4 activates dephosphorylation by protein phospha tase 2A of HeLa SR proteins and converts their splicing properties int o that of SR proteins purified from late adenovirus-infected cells. Ta ken together, our results suggest that E4-ORF4 is an important factor controlling the temporal shift in adenovirus alternative RNA splicing, We conclude that alternative pre-mRNA splicing, like many other biolog ical processes, is regulated by reversible protein phosphorylation.