Mapping of the human 60,000 M(r) Ro/SSA locus: the genes for three Ro/SSA autoantigens are located on separate chromosomes

Characterization of DNA binding properties and sequence specificity of the human 52 kDa Ro/SS-A (Ro52) zinc finger protein

The Ro52 protein is an autoantigen in Sjögren's Syndrome and systemic lupus erythematosus. Although its function is not yet known, sequence similarities to other proteins suggest that it binds to DNA. In this study, the hypothesis that Ro52 recognizes particular nucleic acid sequences was tested. Ro52 bound to double stranded but not single stranded DNA. 1,10-Phenanthroline, a chelater of zinc, was found to inhibit this interaction, suggesting that the zinc fingers of Ro52 are functional. DNA sequences were selected from an oligonucleotide library by binding to Ro52 followed by amplification by Taq DNA polymerase in order to characterize the DNA sequence-binding motif for this protein. These studies support the hypothesis that Ro52 is functionally a member of a family of zinc finger proteins, many of which are known to bind to DNA or regulate gene expression. We speculate that Ro52 functions as a transcription factor, and that its disregulation may have important consequences in the expression or susceptibility of certain autoimmune diseases.

A 1.1-Mb transcript map of the hereditary hemochromatosis locus

In the process of positionally cloning a candidate gene responsible for hereditary hemochromatosis (HH), we constructed a 1.1-Mb transcript map of the region of human chromosome 6p that lies 4.5 Mb telomeric to HLA-A. A combination of three gene-finding techniques, direct cDNA selection, exon trapping, and sample sequencing, were used initially for a saturation screening of the 1.1-Mb region for expressed sequence fragments. As genetic analysis further narrowed the HH candidate locus, we sequenced completely 0.25 Mb of genomic DNA as a final measure to identify all genes. Besides the novel MHC class 1-like HH candidate gene HLA-H, we identified a family of five butyrophilin-related sequences, two genes with structural similarity to a type 1 sodium phosphate transporter, 12 novel histone genes, and a gene we named RoRet based on its strong similarity to the 52-kD Ro/SSA lupus and Sjogren's syndrome auto-antigen and the RET finger protein. Several members of the butyrophilin family and the RoRet gene share an exon of common evolutionary origin called B30-2. The B30-2 exon was originally isolated from the HLA class 1 region, yet has apparently "shuffled" into several genes along the chromosome telomeric to the MHC. The conservation of the B30-2 exon in several novel genes and the previously described amino acid homology of HLA-H to MHC class 1 molecules provide further support that this gene-rich region of 6p21.3 is related to the MHC. Finally, we performed an analysis of the four approaches for gene finding and conclude that direct selection provides the most effective probes for cDNA screening, and that as much as 30% of ESTs in this 1.1-Mb region may be derived from noncoding genomic DNA.

Expression and DNA binding of the human 52 kDa Ro/SSA autoantigen

The 52 kDa Ro/SSA protein is an intracellular autoantigen that is frequently recognized by antibodies in sera of patients with systemic lupus erythematosus or Sjogren's syndrome. While the function of this molecule is not known, zinc finger and leucine zipper motifs have been identified in its predicted amino acid sequence which suggest that it may interact with nucleic acids. To test this hypothesis, the human gene which encodes this protein was cloned in a baculovirus and expressed in Spodoptera frugipoda cells. Extracts from these infected insect cells were used as a source of protein for this study. The protein is similar in size and antigenicity to that expressed in human cells. This protein binds to DNA at physiological temperature and is eluted with high concentrations of sodium chloride. Striking similarities were found between the sequence in, and adjacent to, the nucleic acid-binding motifs of 52 kDa Ro/SSA and a growing family of zinc finger proteins which have been shown to bind to DNA or regulate gene expression. The findings presented here place this protein structurally and functionally in this family and demonstrate a biochemical assay which can be used to study its function.