The 72/74-kDa polypeptides of the 70-110 S large heterogeneous nuclear ribonucleoprotein complex (LH-nRNP) represent a discrete subset of the hnRNP Mprotein family

Pre-mRNA processing in eukaryotes is thought to take place on a multitude o f nuclear ribonucleoprotein (RNP) complexes, the most abundant of them bein g the heterogeneous nuclear (hn) RNP complexes. The identification in mamma lian nuclear extracts of a novel, less-abundant 70-110 S heterogeneous RNP, named large heterogeneous nuclear RNP (LH-nRNP), has previously been repor ted by Aidinis, Sekeris and Guialis (1995) Nucleic Acids Res. 23, 2742-2753 . The structural composition of the LH-nRNP complex has been determined fol lowing the production of polyclonal antibodies against the major protein co nstituents of the complex, the pair of the 72/74-kDa polypeptides. In the p resent study evidence is shown to prove that the 72/74-kDa proteins are mem bers of the hnRNP M protein family, hereafter referred to as 72/74(M) polyp eptides. The extensive application of two-dimensional gel electrophoresis, combined with specific immunoprecipitation and immunoblotting assays, has a llowed the assignment of the 72/74(M) proteins to a subset of the hnRNP M f amily, characteristic of the presence of the LH-nRNP complex and distinct f rom the hnRNP-associated M1-M4 components. Moreover, the immunoselection of the LH-nRNP complex from [P-32]orthophosphate-labelled HeLa cells, with th e parallel application of UV irradiation, has permitted the identification of the 72/74(M) polypeptides as the sole protein constituents of the comple x in direct contact with the RNA. It is proposed that LH-nRNP constitutes a discrete subset of hnRNP complexes, having a possible role in establishing specific interactions between hnRNP and nuclear-matrix protein components.