The C-terminal RG dipeptide repeats of the spliceosomal Sm proteins D1 andD3 contain symmetrical dimethylarginines, which form a major B-cell epitope for anti-Sm autoantibodies

The Sm proteins B/B', D1, D2, D3, E, F, and G are components of the small n uclear ribonucleoproteins U1, U2, U4/U6, and U5 that are essential for the splicing of pre-mRNAs in eukaryotes. D1 and D3 are among the most common an tigens recognized by anti-Sm autoantibodies, an autoantibody population fou nd exclusively in patients afflicted with systemic lupus erythematosus. Her e we demonstrate by protein sequencing and mass spectrometry that all argin ines in the C-terminal arginine-glycine (RG) dipeptide repeats of the human Sm proteins D1 and D3, isolated from HeLa small nuclear ribonucleoproteins , contain symmetrical dimethylarginines (sDMAs), a posttranslational modifi cation thus far only identified in the myelin basic protein. The further fi nding that human D1 individually overexpressed in baculovirus-infected inse ct cells contains asymmetrical dimethyl-arginines suggests that the symmetr ical, dimethylation of the RG repeats in D1 and D3 is dependent on the asse mbly status of D1 and D3. In antibody binding studies, 10 of 11 anti-Sm pat ient sera tested, as web as the monoclonal antibody Y12, reacted with a che mically synthesized C-terminal. peptide of DI containing sDMA, but not with peptides containing asymmetrically modified or nonmodified arginines. Thes e results thus demonstrate that the sDMA-modified C terminus of D1 forms a major linear epitope for anti-Sm autoantibodies and Y12 and further suggest that posttranslational modifications of Sm proteins play a role in the eti ology of systemic lupus erythematosus.