The discoidin domain receptor (DDR} is a new class of receptor tyrosine kinase that is distinguished by a unique extracellular domain homologous to the lectin Discoidin I found Dictyostelium discoideum. A cosmid was isolated from a human chromosome 6 cosmid library containing the DDR gene. A complete genomic contig of the DDR gene was constructed from seven subclones of the cosmid. The cosmid fragments were analyzed by PCR, sequencing, and comparison of genomic/cDNA sequence. The DDR gene is composed of 17 exons, ranging in size from 96 to 1014 bp, distributed along -12 kb of genomic DNA. The extracellular domain is encoded by 8 exons of which three code for the discoidin domain. The transmembrane domain is encoded by I exon, the juxtamembrane domain by 3 exons, and the catalytic domain by 5 exons. The generation of the two splice variants of DDR, EDDRI and EDDR2 are explained by the genomic structure. Exon 11 [111 bp in the juxtamembrane domain} is present in DDR and absent in the splice variant EDDRI. An inverted repeat of 20 bp was identified at the 3' exon-intron junction of exon 11, which results in a lariat loop-like secondary structure. EDDR2 is generated because of a cryptic splice acceptor site that results in an extra 18 bp [6 amino acids) inserted 5' of exon 14 in the catalytic domain. A polymorphic [GT)t7 repeat was identified in intron 5 with a hererozygosity of 0.71. The exon-intron structure of the DDR gene will be helpful in further understanding of its function and explains the possible structural basis for the two splice variants.
Summar~Class II major histocompatibility complex (MFIC) molecules present peptides derived from processed antigen to antigen-specific CD4-positive T cells. In addition, class II molecules bind with high affinity another class of antigens, termed superantigens. T cell stimulation by superantigens depends almost exclusively on the V3 segment expressed by the T ceil receptor (TCR). Mapping of the superantigen binding site on class II molecules should provide valuable information on how MHC and TCR molecules interact. Recombinant mouse I-A class II molecules expressed on transfected L cells were analyzed for their ability to bind the toxic shock syndrome toxin 1. Polymorphic residues in the ol helices of both the ol and B chains of I-A contributed to quantitative toxin binding, suggesting that the toxin binds to either a combinatorial or a conformational site on class II MHC molecules.
Prior studies have revealed the presence of chymotrypsinlike protease in peripheral organs, although no definitive evidence for the synthesis of this enzyme in tissue other than the pancreas is available. In an attempt to detect chymotrypsinogen mRNA in peripheral organs, a fragment of the pancreatic chymotrypsin mRNA from rat was amplified using PCR. The sequence was identified as a portion of the rat chymotrypsin B gene overlapping exon 5 through exon 7. It was subcloned into the pGEM-4Z vector and used as a template for the vitro transcription of an antisense riboprobe. Using ribonuclease protection and Northern blot analyses, chymotrypsin mRNA was detected in the rat pancreas, stomach, duodenum, ovary, and spleen. Monoclonal and polyclonal antisera against chymotrypsin detected chymotrypsinlike immunoreactivity in rat and human pancreas, rat stomach, duodenum and jejunum. Electrophoresis and immunoblotting revealed chymotrypsin-chymotrypsinogen bands (25-29 kDa) in the stomach and duodenum. Synthesis of a potent protease such as chymotrypsin in tissue other than pancreas is significant, suggesting a potential physiological and/or pathological role in these tissues.
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