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The H-2 histocompatibility complex of the mouse is a multigene family, some members of which are essential for the immune response to foreign antigens. The structure and organization of these genes have been established by molecular cloning, and their regulation and function is being defined by expression of the cloned genes.

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Abstract. Ly-6E, a glycosyl phosphatidylinositol (GPI)-anchored murine alloantigen that can activate T cells upon antibody cross-linking, has been converted into an integral membrane protein by gene fusion. This fusion product, designated Ly-6ED b, was characterized in transiently transfected COS cells and demonstrated to be an integral cell surface membrane protein. Furthermore, the fusion antigen can be expressed on the surface of the BW5147 class "E" mutant cell line, which only expresses integral membrane proteins but not GPI-anchored proteins. The capability of this fusion antigen to activate T cells was examined by gene transfer studies in D10G4.1, a type 2 T cell helper clone. When transfected into D10 cells, the GPIanchored Ly-6E antigen, as well as the endogenous GPI-anchored Ly-6A antigen, can initiate T cell activation upon antibody cross-linking. In contrast, the transmembrane anchored Ly-6ED b antigen was unable to mediate T cell activation. Our results demonstrate that the GPI-anchor is critical to Ly-6A/E-mediated T cell activation.T HE murine Ly-6A/E antigen is a member of the Ly-6 multigene family which consists of approximately 20 genes located within a region of ~500 kb on chromosome 15 (14, 15; Philbrick, W. M., and A. L. M. Bothwell, unpublished results). The known proteins encoded in this locus are associated with cells of the hematopoietic lineage and their expression is developmentally regulated in a cell typespecific manner (11,23,28). Recently, it has been shown that Ly-tA/E belongs to a class of surface antigens which are anchored to the plasma membrane through a glycosylphosphatidylinositol (GPI) ~ moiety (23). This GPI tail is rapidly added to the nascent protein through a posttranslational modification (17,18,25).Physiological triggering of T cells is mediated through an antigen-specific T cell receptor/CD3 complex. However, cross-linking of the Ly-6A/E antigen on the cell surface has also been shown to activate T cells, resulting in its designation as "T cell activation protein" (TAP) (21, 23). Two other unrelated T cell surface antigens, Thy-1 and Qa-2, are also anchored to the cell surface by a GPI tail, and like Ly-6, can activate T cells via antibody cross-linking (12, 24). This recurrent relationship between triggering capability and attachment structure has led to the suggestion that the GPI anchor may be involved in the signal transduction (4,22,23).To determine whether the GPI anchor of the Ly-6A/E molecule is essential for its role in T cell activation, we have undertaken gene transfer studies in D10, an antigen-specific 1. Abbreviations used in this paper: GPI, glycosyl phosphatidylinositol; TAP, T cell activation protein.type 2 T cell helper clone. This clone expresses high levels of surface Ly-6A antigen, which can be distinguished from its allelic form, Ly-6E by appropriate mAbs (see Results). D10 offers the advantages of a cloned cell line, and yet resembles isolated T cells more closely than either tumor lines or T cell hybridomas with regard to the physiological con...