Dynamics and longevity of the glycolipid-anchored membrane protein, Thy-1.

Thy-1 and a number of other proteins are anchored to the outer hemi-leaflet of membranes by a glycolipid moiety containing ethanolamine phosphate, mannose, glucosamine, and phosphatidylinositol. They nevertheless have the striking property of being able to transduce signals across the plasma membrane. We here demonstrate, for the BW5147 murine T lymphoma, that (a) greater than 90% of Thy-1 is at the cell surface, (b) Thy-1 is about one order of magnitude less concentrated in coated pits than the transferrin receptor or H-2 antigens, (c) Thy-1 undergoes at most very limited endocytosis or diacytosis, and (d) Thy-1 has an unusually slow turnover rate. Several similar observations have also been made for a second glycolipid-anchored protein, the T cell activating protein. Thus, the absence of cytoplasmic and trans-membrane domains may result in lipid-anchored proteins being confined to the cell surface and being free from constraints which affect the turnover of transmembrane proteins.

Atypical mannolipids characterize Thy-1-negative lymphoma mutants.

Essentially all eukaryotic cells, including murine lymphomas, express surface proteins, such as Thy-1, which are anchored by a phosphoinositol mannolipid. Putative mannolipid anchor precursors can be detected in these cells. Six distinct Thy-1-negative lymphoma mutants lack complete mannolipids, and three mutants synthesize atypical mannolipids. The absence of complete mannolipids can account for the lack of expression of multiple mannolipid-anchored proteins and may also account for the lack of lipid anchoring in the human disease paroxysmal nocturnal hemoglobinuria. Structural information on the mannolipids of wild-type and mutant cells indicates that anchor biosynthesis in these cells may involve both transmembrane flip-flop of intermediates and a deacylation step.