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T cells expressing T cell receptor (TCR) complexes that lack CD3 delta, either due to deletion of the CD3 delta gene, or by replacement of the connecting peptide of the TCR alpha chain, exhibit severely impaired positive selection and TCR-mediated activation of CD8 single-positive T cells. Because the same defects have been observed in mice expressing no CD8 beta or tailless CD8 beta, we examined whether CD3 delta serves to couple TCR.CD3 with CD8. To this end we used T cell hybridomas and transgenic mice expressing the T1 TCR, which recognizes a photoreactive derivative of the PbCS 252-260 peptide in the context of H-2K(d). We report that, in thymocytes and hybridomas expressing the T1 TCR.CD3 complex, CD8 alpha beta associates with the TCR. This association was not observed on T1 hybridomas expressing only CD8 alpha alpha or a CD3 delta(-) variant of the T1 TCR. CD3 delta was selectively co-immunoprecipitated with anti-CD8 antibodies, indicating an avid association of CD8 with CD3 delta. Because CD8 alpha beta is a raft constituent, due to this association a fraction of TCR.CD3 is raft-associated. Cross-linking of these TCR-CD8 adducts results in extensive TCR aggregate formation and intracellular calcium mobilization. Thus, CD3 delta couples TCR.CD3 with raft-associated CD8, which is required for effective activation and positive selection of CD8(+) T cells.

C2-alpha-Mannosyltryptophan was discovered in RNase 2 from human urine, representing a novel way of attaching carbohydrate to a protein. Here, we have addressed two questions related to the biosynthesis of this modification: (i) is C-mannosylation part of the normal intracellular biosynthetic route, and (ii) how general is it, i.e. which organisms perform this kind of glycosylation? To answer the first question, RNase 2, which is identical to the eosinophil-derived neurotoxin, was isolated from intracellular stores of cultured human HL-60 cells. The enzyme was C-mannosylated at Trp-7, showing that the modification occurs intracellularly, before secretion of the protein. The second question was investigated by immunological and chemical analysis of RNase 2 purified from the supernatant of transiently transformed cells from different organisms. This revealed that C-mannosylation occurs in cells from man, green monkey, pig, mouse, and hamster. The observation that pig kidney cells contain the machinery for C-mannosylation of Trp-7 of human RNase 2 but that the homologous RNase from porcine kidney is not a substrate, since it does not contain a tryptophan at position 7, strongly suggests that C-mannosylated proteins other than RNase 2 exist. Recombinant RNase 2 isolated from insect cells, plant protoplasts, and Escherichia coli was not C-mannosylated. These results not only form the basis for further studies on the biochemical aspects of C-mannosylation but also have implications for the choice of cells for production of recombinant glycoproteins.

C2-α-Mannosyltryptophan was discovered in RNase 2 from human urine, representing a novel way of attaching carbohydrate to a protein. Here, we have addressed two questions related to the biosynthesis of this modification: (i) isC-mannosylation part of the normal intracellular biosynthetic route, and (ii) how general is it, i.e.which organisms perform this kind of glycosylation? To answer the first question, RNase 2, which is identical to the eosinophil-derived neurotoxin, was isolated from intracellular stores of cultured human HL-60 cells. The enzyme was C-mannosylated at Trp-7, showing that the modification occurs intracellularly, before secretion of the protein. The second question was investigated by immunological and chemical analysis of RNase 2 purified from the supernatant of transiently transformed cells from different organisms. This revealed that C-mannosylation occurs in cells from man, green monkey, pig, mouse, and hamster. The observation that pig kidney cells contain the machinery for C-mannosylation of Trp-7 of human RNase 2 but that the homologous RNase from porcine kidney is not a substrate, since it does not contain a tryptophan at position 7, strongly suggests that C-mannosylated proteins other than RNase 2 exist. Recombinant RNase 2 isolated from insect cells, plant protoplasts, and Escherichia coliwas notC-mannosylated. These results not only form the basis for further studies on the biochemical aspects ofC-mannosylation but also have implications for the choice of cells for production of recombinant glycoproteins.