DISTINCT INTERACTIONS AMONG GPI-ANCHORED, TRANSMEMBRANE AND MEMBRANE-ASSOCIATED INTRACELLULAR PROTEINS, AND SPHINGOLIPIDS IN LYMPHOCYTE ANDENDOTHELIAL-CELL PLASMA-MEMBRANES

Glycosylphosphatidylinositol (GPI)-anchored glycoproteins are enriched in sphingolipid-rich plasma membrane domains, which are often isolate d as low-density membrane complexes. This association is believed to a rise from the interactions between the GPI-acyl chains and sphingolipi ds, but is not fully understood. In this study, we compared the physic al properties of GPI-anchored glycoproteins from a non-polarized (muri ne T-lymphocyte) and a polarized (human endothelial) cell by equilibri um density gradient centrifugation after extraction by detergents unde r identical conditions. Unlike those on epithelial cells, the GPI-anch ored proteins of lymphocytes (Thy-1 and the heat stable antigen CD24) were enriched in the floating fractions after extraction over a wide r ange of octylglucoside concentrations. In contrast, the floatability o f endothelial GPI-anchored CD59 was markedly diminished, not only by o ctylglucoside, but also by increasing concentrations of Triton X-100, Distribution of cholera toxin binding ganglioside GM1 in the sucrose g radient fractions closely followed that of the GPI-anchored proteins i n both lymphocytes and endothelial cells under most extraction conditi ons. Analysis of the intracellular acylated molecules revealed that a significant amount of p56(lck) was always associated with the floating GPI-rich fractions of lymphocytes when extracted by Triton X-100 or o ctylglucoside at 4 degrees C, while the behaviour of endothelial cell caveolin was comparable to that of CD59. The transmembrane glycoprotei ns CD45 in lymphocytes and MHC class I antigen in endothelial cells in teracted weakly with GPI domains, whereas endothelial CD44 and lymphoc yte CD26 displayed a strong association. These results show that: (1) the physical properties of different GPI-anchored proteins may vary si gnificantly; and (2) transmembrane and acylated intracellular proteins could be associated with GPI domains to a variable extent. These diff erences probably reflect cell type-specific interactions of GPI anchor s with the sphingolipid framework of plasma membranes, as well as extr acellular interactions of GPI-anchored glycoproteins with neighbouring cell surface molecules. (C) 1997 Elsevier Science B.V.