2 IMMUNOLOGICALLY RELATED POLYPEPTIDES OF 72 74 KDA SPECIFY A NOVEL 70-110S HETEROGENEOUS NUCLEAR RNP/

Evidence suggesting the presence in rat liver nuclear extracts of a ne w RNP complex of 70-110S has been provided [Hatzoglou, M., Adamtziki, E., Margaritis, L. and Sekeris, C.E (1985) Exp. Cell. Res. 157, 227-24 1]. Biochemical features unique to this RNP were its stability to salt and mild RNase digestion and the presence of a pair of polypeptides o f 72/74 kDa. By producing antibodies against the 72/74 kDa polypeptide s these proteins have been defined as integral components of the 70-11 0S RNP complex. They comprise two immunologically related polypeptides with an exclusively nucleoplasmic localization, giving a speckled pat tern in a diffuse background, similar, but not identical, to the Sm an tigen. The 70-110S RNP complex, referred to as large heterogeneous nuc lear RNP (LH-nRNP), has a simple protein pattern that includes, in add ition to the 72/74 kDa proteins, three stably associated polypeptides of apparent molecular size 110, 61 and 59 kDa. The bulk of its RNA com ponent represents a discrete RNA population of 10-20S, belonging to a subset of the RNA detected within immunopurified HeLa hnRNP complexes. These RNA species are RNA polymerase II transcripts of greater stabil ity relative to the bulk of hnRNA, containing oligo(A) or poly(A) sequ ences. Immunodepletion and/or antibody addition studies in HeLa splici ng extracts using antibodies with specificity for the 72/74 kDa protei ns revealed a rather strong inhibition of splicing activity, suggestin g participation of the LH-nRNP complex in in vitro splicing.