CD44 SELECTIVELY ASSOCIATES WITH ACTIVE SRC FAMILY PROTEIN-TYROSINE KINASES LCK AND FYN IN GLYCOSPHINGOLIPID-RICH PLASMA-MEMBRANE DOMAINS OF HUMAN PERIPHERAL-BLOOD LYMPHOCYTES

CD44 is the major cell surface receptor for the extracellular matrix g lycosaminoglycan hyaluronan and is implicated in a variety of biologic al events that include embryonic morphogenesis, lymphocyte recirculati on, inflammation, and tumor metastasis. CD44 delivers activation signa ls to T lymphocytes, B lymphocytes, natural killer cells, polymorphonu clear leukocytes, and macrophages by stimulating protein tyrosine phos phorylation and calcium influx. The mechanism of signal transduction v ia CD44 remains undefined, although CD44 was shown to physically assoc iate with intracellular protein tyrosine kinase Lck in T lymphocytes. In the present report, we show that a significant proportion of CD44 i n human peripheral blood T lymphocytes and endothelial cells is associ ated with low-density plasma membrane fractions that represent special ized plasma membrane domains enriched in glycosphingolipids and glycos ylphosphatidylinositol (GPI)-anchored proteins. CD44 and the GPI-ancho red CD59 do not appear to directly interact in the low-density membran e fractions. In human peripheral blood T lymphocytes, 20% to 30% of th e Src family protein tyrosine kinases, Lck and Fyn, are recovered from these fractions. CD44-associated protein kinase activity was selectiv ely recovered from the low-density membrane fractions, corresponding t o glycosphingolipid-rich plasma membrane microdomains. Reprecipitation of the in vitro phosphorylated proteins showed that CD44 associates n ot only with Lck but also with Fyn kinase in these membrane domains. O ur results suggest that cellular stimulation via CD44 may proceed thro ugh the signaling machinery of glycosphingolipid-enriched plasma membr ane microdomains and, hence, depend on the functional integrity of suc h domains. (C) 1998 by The American Society of Hematology.