Role of Nuclear Matrix Proteins in the Regulation of Pre-mRNA Splicing

Abstract : Deregulation of alternative splicing has been linked to malignant transformation in breast cancers. Therefore, to fully understand breast cancer, it will be important to identify and characterize factors that regulate the splicing process. Nuclear matrix proteins (NMp's) containing serine/arginine (SR) rich domains that associate with splicing complexes were isolated and characterized. The NMp's B1C8 (160kDa) and B4Al1 (300kDa) are novel SR phosphoproteins of 820 and 2245 amino acids, respectively. Unlike previously defined SR family proteins (SRp's), which function as constitutive and regulatory splicing factors, B1C8 and B4A11 lack RNA Recognition Motifs (RRMs). B1C8 and B4A11 form a complex (B1C8/B4A11) that associates with SR family proteins SRp40 and SRp75. B1C8/B4A11 is required for the splicing of a subset of pre-mRNAs in vitro. B1C8/B4A11 binds to pre-mRNA and promotes splicing in combination with SR family proteins. The association of B1C8/B4A11 with pre-mRNA also requires binding of U1 snRNP to the 5' splice site and is stabilized by binding of U2 snRNP to the branch site. Thus, B1C8/B4A11 appears to form critical interactions that span intron sequences. These results suggest that B1C8/B4A11 functions as a splicing 'coactivator', by augmenting the activity of factors bound to pre-mRNA. These results are providing new insights into mechanisms by which pre-mRNA splicing may be regulated.