Internalization of glycosyl‐phosphatidylinositol (GPI)‐anchored lymphocyte proteins II. GPI‐anchored and transmembrane molecules internalize through distinct pathways

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Abstract
Ly‐6A.2 (T cell‐activating protein, TAP) and Thy‐1 are glycosyl‐phosphatidylinositol (GPI)‐anchored proteins expressed on the surface of murine T lymphocytes. We have found that Ly‐6A.2 (TAP) and Thy‐1 are internalized by T cells. In the present study we have investigated whether these GPI‐anchored proteins enter cells by endocytosis through coated pits. Two lines of evidence argue against the involvement of coated pits in the internalization of Ly‐6A.2 (TAP) and Thy‐1. First, drugs that effectively blocked the endocytosis of transferrin receptor and H‐2 class I molecules, (which are known to be internalized via coated pits) did not inhibit the internalization of the GPI‐anchored proteins. Second, in ultrastructural analyses, Ly‐6A2 (TAP) and Thy‐1, in contrast to the transferrin receptor, were rarely found in coated pits or vesicles. These observations suggest that the GPI‐anchored proteins on T lymphocytes are internalized by a distinct pathway that does not involve endocytosis through coated pits.