NOVEL NUCLEAR AUTOANTIGEN WITH SPLICING FACTOR MOTIFS IDENTIFIED WITHANTIBODY FROM HEPATOCELLULAR-CARCINOMA

A patient with liver cirrhosis who progressed to hepatocellular carcin oma was found to develop novel antinuclear antibodies. The serum was u sed to isolate full-length cDNA clones encoding related proteins of 53 0 amino acids (representative clone HCC1.4) and 524 amino acids (repre sentative clone HCC1.3). Affinity-purified antibodies eluted from reco mbinant proteins recognized a 64-kD nuclear protein in Western blottin g and decorated the nucleoplasm in a speckled-network fashion in immun ofluorescence, colocalizing with antibodies to pre-mRNA splicing facto r SC35 and uridine-rich small nuclear RNAs. The deduced amino acid seq uence contained an arginine/serine-rich (RS) domain and three-ribonucl eoprotein consensus sequence domains, two classes of motifs present in several splicing factors. A repeating octapeptide of Arg-Ser-Arg-Ser- Arg(Lys)-Glu(Asp)-Arg-Lys(Arg) was present in RS region of HCC1. This octapeptide sequence called RS-ERK motif was also found in splicing fa ctors U2AF 35- and 65-kD proteins and 70-kD U1 small nuclear ribonucle oprotein. The molecular features and immunolocalization data suggest t hat the HCC1 autoantigen may be associated with splicing activities an d are consistent with observations that autoantibody responses frequen tly target molecules involved in important cellular biosynthetic funct ions.