ASSOCIATION OF NUCLEAR MATRIX ANTIGENS WITH EXON-CONTAINING SPLICING COMPLEXES

mAbs raised against the human nuclear matrix (anti-NM)(1) mAbs have be en used to investigate the role of nuclear matrix antigens in pre-mRNA processing. The three anti-NM mAbs used in this study recognize antig ens that are highly localized to nuclear matrix speckles. Surprisingly , all three of these mAbs preferentially immunoprecipitate splicing co mplexes containing exon sequences. The anti-NM mAbs efficiently immuno precipitate the exon product complex but not complexes containing the lariat product after the second step of splicing. Two of the anti-NM m Abs completely inhibit pre-mRNA splicing in vitro. However, none of th e anti-NM mAbs appear to recognize factors stably associated with spli cing snRNPs. The three anti-NM mAbs predominantly react with distinct high molecular weight antigens, which belong to a class of nuclear pro teins that selectively precipitate with Ser-Arg protein-splicing facto rs in the presence of high Mg2+ concentrations. Immunological, biochem ical, and cell biological data indicate that two of the NM antigens ar e related to the defined set of Ser-Arg proteins. The results suggest the existence of an extended Ser-Arg family as a component of the nucl ear matrix.