Monoclonal antibody specific to a subclass of polyproline-Arg motif provides evidence for the presence of an snRNA-free spliceosomal Sm protein complex in vivo: Implications for molecular interactions involving proline-rich sequences of SmB/B ' proteins

The human spliceosomal Sm B/B' proteins are essential for the biogenesis of the snRNP particles. B/B' proteins contain several clusters of the PPPPGM/ IR sequence, which occurs within the C-terminus of Sm B/B'. This sequence i s very similar to the PPPPPGHR sequence of the cytoplasmic rail of the CD2 receptor and closely resembles the class II of SH3 ligands, suggesting a si milarly important role. We report that a monoclonal antibody (3E10) against the PPPPPGHR sequence recognizes spliceosomal Sm B/B' proteins. Proteins t hat are specifically immunoprecipitated by 3E10 include Sm B, B', D1, D2, D 3, E, F, and G. However, unlike Y12 and other anti-Sm immunoprecipitates, 3 E10 immunoprecipitates appear to lack the U1 snRNP-specific proteins A and C and U snRNAs. These findings indicate that 3E10 recognizes a subset of Sm protein core and suggest the presence of snRNA-free Sm protein complex(es) in vivo. We propose that the epitope binding for 3E10 may become unaccessib le upon interactions of Sm proteins and their subsequent incorporation into the care particles. The Sm proline rich sequences may have an important ro le in mediating protein-protein interactions necessary for the proper snRNP core assembly or function, or both. To our knowledge, 3E10 is the first we ll characterized mAb specific for a subclass of polyproline-arg motif recog nizing Sm B/B' and CD2 proteins. 3E10 antibody can be used to further chara cterize the nature of protein components in the snRNA-free Sm subcore prote in complex(es) that are formed during the snRNP core assembly steps. J. Cel l. Biochem. 74:168-180, 1999. (C) 1999 Wiley-Liss, Inc.