THE GLYCOPHOSPHATIDYLINOSITOL ANCHOR AFFECTS THE CONFORMATION OF THY-1 PROTEIN

Thy-1 has the structure of a single variable-type immunoglobulin domai n anchored to the external face of the plasma membrane via a glycophos phatidylinositol moiety, When the lipid is removed from this anchor by either phospholipase C or D, the reactivity of the delipidated Thy-1 for a range of antibodies, including those known to be determined by a mino acid residues, is impaired. We have investigated in detail the ef fect of delipidation on the reaction with the OX7 monoclonal antibody, determined by the allelic variant residue Arg 89. Analysis of the kin etics of OX7 binding shows that delipidation affects primarily the dis sociation of antibody, increasing the dissociation rate constant k(dis s) from 0.27x10(-3) s(-1) to 2.39x10(-3) s(-1). Addition of phospholip ase to preformed antibody-antigen complex causes an immediate change f rom the slow to the faster dissociation rate, implying that delipidati on induces a conformational change in the Thy-1 protein that is suffic iently strong to dissociate bound antibody, This conformational change can be demonstrated directly by the circular dichroism spectrum of hu man Thy-1 that detects changes in the environment of Tyr residues loca ted near the antigenic epitopes. Molecular dynamics studies suggest th at, on delipidation, a conformational change occurs in the glycan chai n that affects the protein in the region of the antigenic epitopes, Th is study thus demonstrates that the glycophosphatidylinositol anchor s trongly influences the conformation of Thy-1 protein by a mechanism th at could occur generally with membrane proteins of this class.