The glycophospholipid anchor of Thy-1. Biosynthetic labeling experiments with wild-type and class E Thy-1 negative lymphomas

The Thy-1 antigen of the surface of lymphocytes and neurons is anchored to the plasma membrane via a glycophospholipid moiety. In contrast, the Thy-1 synthesized by the class E Thy-1 negative mutant lymphoma is secreted as a hydrophilic species. The present investigation uses the approach of biosynthetic labeling to investigate further the structure of the intracellular Thy-1 of wild-type cells and the secreted Thy-1 of these mutant cells. In the wild-type cells, Thy-1 can be labeled with [³H]mannose, [³H]galactose, [³H]fucose, [³H]ethanolamine, and [³H]palmitic acid. In the latter two cases the label is recovered almost exclusively in a detergent-binding Pronase fragment of the protein. The incorporated label is in the form of [³H]ethanolamine, or [³H]palmitate and stearate, respectively. Reductive methylation of biosynthetically labeled Thy-1 and a nonradioactive sample of Thy-1 shows that [³H]ethanolamine is incorporated equally into two residues of ethanolamine, only one of which has a free amino group. A single residue of glucosamine with a free amino group is also detected. Each of the sugar precursors is incorporated with extensive conservation of chemical identity. In the class E cells, each of the labeled sugars but neither [³H]ethanolamine nor [³H]palmitate is incorporated into Thy-1. The anchor moiety therefore appears to be entirely missing, although N-linked oligosaccharide processing is essentially normal. We postulate that the anchor deficiency in the mutant cells results from a biosynthetic lesion.