C-reactive protein (CRP) binding to the Sm-D protein of snRNPS. Identification of a short polypeptide binding region

C-reactive protein (CRP) binds to chromatin, histones, and small nuclear ribonucleoproteins (snRNPs) through a phosphocholine (PC)-inhibitable, calcium-dependent binding site. snRNPs process pre-mRNA to mature mRNA and are composed of small uridine-rich RNAs (designated U1, U2, U5 and U4/U6) and associated proteins. We have shown that CRP binds to snRNPs in intact cells and to the U1 snRNP-specific 70 K protein in cell extracts. To determine whether CRP bound to other snRNP proteins, snRNPs were purified from rabbit thymus extract and CRP binding was assessed by blotting. CRP bound to a protein with the same mobility as Sm-D as well as to the 70 K protein. CRP specifically bound to and precipitated a fusion protein containing full-length Sm-D, confirming the binding of CRP to Sm-D. Binding was inhibited by PC and by EDTA. Binding studies using deletion mutants of the Sm-D fusion protein revealed that CRP binding was mediated by the C-terminal region of Sm-D, a region which binds autoantibodies and is proposed to bind to RNA. A comparison of the peptide regions on different autoantigens suggests that there is a shared motif to which CRP binds.