DISTINCT FUNCTIONS OF SR PROTEINS IN RECRUITMENT OF U1 SMALL NUCLEAR RIBONUCLEOPROTEIN TO ALTERNATIVE 5' SPLICE SITES

Alternative splicing of precursor messenger RNAs (pre-mRNAs) is an imp ortant mechanism for the regulation of gene expression. The members of the SR protein family of pre-mRNA splicing factors have distinct func tions in promoting alternative splice site usage, Here we show that SR proteins are required for the first step of spliceosome assembly, int eraction of the U1 small nuclear ribonucleoprotein complex (U1 snRNP) with the 5' splice site of the pre-mRNA. Further, we find that individ ual SR proteins have distinct abilities to promote interaction of U1 s nRNP with alternative 5' splice junctions. These results suggest that SR proteins direct 5' splice site selection by regulation of U1 snRNP assembly onto the pre-mRNA.