cDNA clones, whose fusion proteins were recognised by an anti‐(T3 gamma chain) serum, were isolated from a lambda gt11 expression library prepared from the human T leukaemia cell line J6. The clones encoded a unique sequence related to that of the T3 delta chain, and hybridised to two mRNA transcripts of 0.8 and 3.5 kb in size, whose expression was restricted to T lymphocytes. The 182 amino acid sequence deduced from the cDNA revealed a typical signal peptide, a predominantly hydrophilic 89 amino residue domain with two N‐glycosylation sites, a hydrophobic domain with a centrally located glutamic acid residue and a 44‐residue domain with at least one potential serine phosphorylation site for protein kinase C. Given this arrangement the T3 gamma polypeptide most probably has a transmembrane orientation with the N‐terminal domain exposed on the cell surface. The amino acid and nucleotide sequences showed marked homology with those of the T3 delta chain, suggesting that the respective genes arose by duplication about 200 million years ago. The intracellular and membrane‐proximal half of the extracellular domains were especially well conserved.

Monoclonal antibodies OKT11 (gamma 1) and OKT11A (gamma 2) are described and appear to have similar binding specificities. They bind, in immunofluorescence, with greater than 95% of infant thymocytes, staining both cortical and medullary cells, 65-80% of blood lymphocytes and selectively stain the T cell-dependent paracortical areas of tonsil. A small proportion (9-12%) of bone marrow lymphocytes stain, but this population excludes the terminal transferase-positive cells. Both the gamma 1 and gamma 2 antibodies stain the surface membrane Ig-negative lymphocytes in blood and tonsil and are to block sheep E rosette formation (to normal or leukemic T cells). In contrast, other monoclonal anti-T reagents tested (OKT1, OKT3, OKT4, OKT6, OKT8, OKT9, OKT10) did not block E rosette formation. E rosette formation and OKT11 bindings are coincident on T-ALL cell lines and both are trypsin-sensitive. In a series of 145 leukemias and 26 leukemic cell lines investigated, only leukemias with a T cell phenotype including E rosette positivity were reactive with OKT11 and OKT11A. OKT11A binds to a polypeptide of approximately 50 000 molecular weight on thymic lymphocytes. This structure may carry the recognition site for sheep erythrocytes. These antibodies provide additional useful markers for T cell analysis and are of potential therapeutic value.

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N-terminal sequencing of the 55- and 50-kDa polypeptides affinity purified on a phosphotyrosine monoclonal antibody column from activated Jurkat T cells identified alpha and beta tubulin. Two-dimensional gel analysis indicated that alpha tubulin was directly phosphorylated on tyrosine. beta Tubulin was not detectably tyrosine phosphorylated but was precipitated by anti-phosphotyrosine (PTyr) antibody by virtue of its association with the alpha subunit as a heterodimer. Phosphotyrosyl alpha tubulin was not incorporated into intact microtubules and was all in the unpolymerized soluble fraction. These results suggest that tyrosine phosphorylation of alpha tubulin may inhibit the ability of this subunit to polymerize into microtubules. Stimulation of Jurkat T cells via T cell receptor increased the amount of tubulin precipitated by the anti-PTyr antibody. These data raise the possibility that the polymerization of tubulin heterodimers may be regulated by phosphorylation on tyrosine during T cell activation.

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