Target antigens of the SSA/Ro and SSB/La system

The SSA/Ro and SSB/La antigens are polypeptides which serve as autoantigens in systemic lupus erythematosus and Sjogren's syndrome. The SSA/Ro contains two major isoforms of 60 kD and 52 kD. The former is the main native antigen while the latter is a major autoantigen in its denatured form. The SSB/La is a single phosphorylated protein of 48 kD. Recently a new protein of 46 kD, termed calregulin, was suggested as an additional component of the SSA/Ro antigens. However, extensive investigations failed to confirm its relation to the SSA/Ro system. Based on molecular techniques and cDNA cloning of these antigens, it was demonstrated that the 60 kD protein is capable of binding RNA and DNA molecules, suggesting a regulatory role in transcription for this antigen. The 52 kD polypeptide contains multiple zinc finger motifs and its sequence is homologous to the mouse rptl protein, which is a T-cell regulating peptide. The SSB/La is associated with precursors of 5S RNA and tRNA, implying that it has a role in the synthesis and maturation of RNA polymerase III transcripts. The 60 kD and 52 kD SSA/Ro components may be associated within the cell. The SSA/Ro and SSB/La may also be in complex in some points of the cell cycle.