| Summary: | The mouse monoclonal antibody MRC OX-2 recognises a rat cell surface glycoprotein of M<sub>R</sub> about 45,000 which is expressed on brain and lymphoid tissues. The OX-2 antigen is of interest because its tissue distribution and biochemical properties are similar to Thy-1, a cell surface antigen which is known to have a structure resembling an immunoglobulin domain. In this thesis, the structure of OX-2 antigen is determined to evaluate whether it shares sequence homology with Thy-1 and immunoglobulins. OX-2 antigen was purified from rat brain by monoclonal antibody affinity chromatography, and several tryptic peptides were isolated and sequenced. An oligonucleotide probe was designed from the peptide sequence and used to isolate OX-2 cDNA clones from a thymocyte cDNA library. Several cDNA clones were analysed to derive the complete sequence of OX-2 antigen. The mature protein is 248 amino acids long, comprising of two extracellular domains that show homology with immunoglobulin, followed by a transmembrane segment and a short cytoplasmic region. The N-terminal domain fits best with V-regions while the second domain is like an Ig C-domain. Thus the structure is similar to an Ig light chain or one chain of the T cell receptor. Using Southern blot analysis, no rearrangement of the OX-2 gene could be detected in rat tissues which express the protein. Homologues of the OX-2 gene were indentified in Southern blots of mouse and human DNA, and the human OX-2 gene was isolated from a genomic library. Partial characterisation of the human gene showed that domain I is encoded by a single exon, the sequence of which is highly conserved between rat and human. Preliminary S1 nuclease mapping of rat OX-2 mRNA indicates that not all transcripts are capable of encoding the normal OX-2 protein. Abnormal transcripts which differ from the normal message at exon splice sites are abundant in RNA from lymphoid tissues, but absent from brain.
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