The human hnRNP M proteins: identification of a methionine/arginine-rich repeat motif in ribonucleoproteins

Recent reports indicate that proteins which directly bind to nascent RNA polymerase II transcripts, the heterogeneous nuclear ribonucleoproteins (hnRNPs), play an Important role In both transcript-specific packaging and alternative splicing of pre-mRNAs. Here we describe the isolation and characterization of a group of abundant hnRNPs, the M1- M4 proteins, which appear as a cluster of four proteins of 64,000-68,000 daltons by two-dimensional electrophoresis. The M proteins are pre-mRNA binding proteins In vivo, and they bind avidly to poly(G) and poly(U) RNA homopolymers In vitro. Covalently associated polyadenylated RNA-protein complexes, generated by irradiating living HeLa cells with UV light, were purified and used to elicit antibodies in mice. The resulting antlsera were then employed to Isolate cDNA clones for the largest M protein, M4, by Immunologlcal screening. The deduced amlno acid sequence of M4 indicates that the M proteins are members of the ribonucleoprotein consensus sequence family of RNA-blnding proteins with greatest similarity to a hypothetical RNA-binding protein from Saccharomyces cerevlslae. The M proteins also possess an unusual hexapeptlde-repeat region rich In methionlne and arglnine residues (MR repeat motif) that resembles a repeat in the 64,000 darton subunit of cleavage stimulation factor, which is Involved In 3'-end maturation of pre-mRNAs. Proteins Immunologically related to M exist in divergent eukaryotes ranging from human to yeast