Purification of SR Protein Splicing Factors

In recent years, the SR protein family of precursor messenger RNA splicing factors has emerged as a key player in the assembly of the spliceosomal machinery onto pre-mRNA. The SR proteins are essential splicing factors and different family members can direct usage of alternative splice sites in vitro and in vivo (reviewed in refs. 1 and 2 ). SR proteins are required for the earliest spliceosomal interactions in recruitment of U1snRNP (3 ,4 ), they can interact with the splicing factors that recruit U2 snRNP (5 ), and they appear to be essential for recruitment of the tri-snRNP to the pre-mRNA (6 ). When present in excess in a U1snRNP-depleted extract, they can function to bypass a U1 snRNP requirement for splicing (7 ,8 ). They have been identified as trans -acting factors that interact with cis regulatory sequences in alternatively spliced exons (3 ,9 –13 ). Because they are involved in activating, enhancing, and repressing splicing for constitutive and alternatively spliced messages, they have become the focus of a great number of recent studies.