Glycohydrolases β-hexosaminidase and β-galactosidase are associated with lipid microdomains of Jurkat T-lymphocytes

Growing evidence suggests the presence of active lysosomal enzymes in extra-lysosomal compartments, such as the plasma membrane. Although in the past little attention was paid to glycohydrolases acting on cellular compartments different from lysosomes, there is now increasing interest on plasma membraneassociated glycohydrolases because they should be involved, together with glycosyltransferases, in glycosphingolipids oligosaccharide modification processes regulating cell-to-cell and/or celleenvironment interactions in both physiological and pathological conditions. Starting from the previous evidence of the presence of b-hexosaminidase and b-galactosidase at the plasma membrane of cultured fibroblasts, we here investigated the association of these glycohydrolases with lipid microdomains of Jurkat T-lymphocytes. Monosialoganglioside GM3 represents the major glycosphingolipid constituent of T-cell plasma membrane and its amount largely increases after T-cell stimulation. b-hexosaminidase and b-galactosidase cleave specific b-linked terminal residues from a wide range of glycoconjugates and in particular are involved in the stepwise degradation of GM1 to GM3 ganglioside. Here we demonstrated that fully processed plasma membrane-associated b-hexosaminidase and b-galactosidase co-distribute with the lipid microdomain markers and co-immunoprecipitate with the signalling protein lck in Jurkat T-cell. Furthermore, Jurkat cell stimulation up-regulates the expression and activity of lysosomal b-hexosaminidase and b-galactosidase and increases their targeting to lipid microdomains. The nonrandom distribution of plasma membrane-associated b-hexosaminidase and b-galactosidase and their localization within lipid microdomains, suggest a role of these enzymes in the local reorganization of glycosphingolipid-based signalling units