Isolation and characterization of cDNA encoding the antigenic protein of the human tRNP (Ser)Sec complex recognized by autoantibodies from patients withtype‐1 autoimmune hepatitis

We previously described autoantibodies against a UGA serine tRNA–protein complex (tRNP (Ser)Sec ) in patients with type‐1 autoimmune hepatitis [1] and now define the specificity and frequency of this autoantibody and the DNA sequence encoding the tRNA (Ser)Sec ‐associated antigenic protein. The presence of anti‐tRNP (Ser)Sec antibodies was highly specific for type‐1 autoimmune hepatitis, as 47·5% of patients were positive compared with none of the control subjects. To characterize the antigenic protein(s), we immunoscreened a human cDNA library with anti‐tRNP (Ser)Sec ‐positive sera. Two clones (19 and 13) were isolated. Clone 19 encodes a protein with a predicted molecular mass of 48·8 kD. Clone 13 is a shorter cDNA, almost identical to clone 19, which encodes a 35·9‐kD protein. Expression of both cDNAs was accomplished in Escherichia coli as His‐tagged recombinant proteins. Antibodies eluted from both purified recombinant proteins were able to immunoprecipitate the tRNA (Ser)Sec from a HeLa S 3 cell extract, demonstrating their cross‐reactivity with the mammalian antigenic complex. Recent cloning data relating to the target antigen(s) of autoantibodies in autoimmune hepatitis patients that react with a soluble liver antigen (SLA) and a liver‐pancreas antigen (LP) have revealed that these two autoantibodies are identical and that the cloned antigen shows 99% amino acid sequence homology with tRNP (Ser)Sec .