Sorting out the complexity of SR protein functions.

Members of the serine/arginine-rich (SR) protein family have multiple functions in the pre-mRNA splicing reaction+ In addition to being required for the removal of constitutively spliced introns, SR proteins can function to regulate alternative splicing both in vitro and in vivo (Ge & Manley, 1990; Krainer et al+, 1990a; Fu et al+, 1992; Zahler et al+, 1993a; Caceres et al+, 1994; Wang & Manley, 1995)+ In the cell, SR proteins migrate from speckles—subnuclear domains that may function as storage sites for certain splicing factors—to sites of active transcription (Misteli et al+, 1997; Misteli & Spector, 1999) and some SR proteins have been found to shuttle in and out of the nucleus (Caceres et al+, 1998)+ The subcellular localization of SR proteins can be modulated by phosphorylation (Misteli & Spector, 1998;Misteli et al+, 1998) and this undoubtedly underlies some regulated splicing events+ However, once in the nucleus and localized to the nascent pre-mRNA, exactly how SR proteins engage the general splicing machinery to recognize specific splice sites is unclear and is an area of intense investigation+ All SR proteins have a modular organization and contain an N-terminal RNA-binding domain that interacts with the pre-mRNA and a C-terminal RS domain that functions as a protein interaction domain+ SR proteins have been proposed to function by binding to the pre-mRNA and recruiting a number of different general splicing factors to the pre-mRNA during spliceosome assembly (Wu & Maniatis, 1993; Kohtz et al+, 1994; Roscigno & Garcia-Blanco, 1995; Zuo & Maniatis, 1996)+ In addition, SR proteins are thought to mediate interactions between splicing factors bound to the 59 and 39 splice sites (Wu & Maniatis, 1993)+ However, it has not been clear whether all of these activities occur during the removal of each intron+ Recent studies now suggest that all of the proposed SR protein functions are carried out during each round of splicing, and at least some of these functions are performed by independent SR protein molecules+ This review discusses recent advances in understanding the diverse functions of SR proteins in metazoan pre-mRNA splicing and presents a model that takes these new findings into account+ Although the reader should keep in mind that the activity of SR proteins in vivo can be influenced by modulating their subcellular localization, this review focuses on biochemical experiments that specifically address the mechanisms by which SR proteins directly function in the splicing reaction+ (The reader may also refer to related reviews recently published elsewhere (Tacke & Manley, 1999; Blencowe, 2000)+)