Identification of autoantibodies to a macromolecular structure containing u small nuclear ribonucleoproteins

Mouse monoclonal antibody (mAb) F78 has been reported to immunoprecipitate U1, U2, U4, U5, and U6 small nuclear RNAs (snRNAs). In immunoblotting (IB), F78 recognizes a large (coincides with 170 kDa protein), heat-labile particle (F78P) which contains Sm core proteins, but does not recognize individual Sm core proteins. To identify human autoantibodies similar to F78, IB with unheated HeLa cell extracts was performed on 50 sera which immunoprecipitated U1, U2, and U4-6 snRNAs. Two sera identified the band with Mr=170 kDa without recognition of other polypeptides. These anti-170kDa(+) sera reacted with immunoprecipitates of F78 in IB, and F78 also identified immunoprecipitates of these sera. The reactivities of F78 and these two sera were further compared by IB after native polyacrylamide gel electrophpresis of HeLa cell extracts incubated with -globin pre-mRNA under splicing condition. F78 and anti-170kDa(+) sera reacted with slower migrating structures corresponding to the splicing complex and also recognized faster migrating particles. To confirm that F78 and anti-170kDa(+) sera recognized spliceosomal structure, immunoprecipitation was performed with cell extracts after splicing reaction with 32P-labeled pre-mRNA. Both F78 and anti-17pkDa(+) sera significantly precipitated the pre-mRNA. These results indicate that these two sera have autoantibodies with similar reactivities to F78, and they recognize the splicing complex. This study has revealed the presence of new human autoantibodies against the U snRNP complex, F78P. Furthermore, utilizing the autoantibodies and mAb F78, the association of F78P with spliceosome assembly has been confirmed.