Identification and characterization of a divalent cation-dependent glycosaminoglycan-binding protein from rat liver endothelium

Rat liver endothelial cells (LECs) express a membrane-associated Ca2+-dependent hyaluronan-binding activity (CaHA-BP) which is distinct from the Ca2+-independent, endocytic LEC HA receptor (Yannariello-Brown et al., J. Cell Biochem., 48, 73-80, 1992). The CaHA-BP is specific for a subset of glycosaminoglycans, since Ca2+-dependent binding of 125I-HA (approximately 80 kDa) to LECs was competed with a 100-fold excess (w/w) of HA, chondroitin sulfate, and heparin, but not with chondroitin. The CaHA-BP activity on intact LECs was pH-dependent. Optimal binding occurred at pH 6.0; no binding was detected at pH values <or= 5 or >or= 9. 125I-HA, pre-bound in the presence of Ca2+, could also be dissociated with an acidic buffer (pH 5.0), as well as the divalent cation chelators EDTA and EGTA. 125I-HA binding was stimulated by divalent cations other than Ca2+, such as Mg2+, Mn2+, and Ba2+; with the exception of Zn2+. A photoaffinity crosslinking reagent (125I-ASD-HA) was used to identify specifically crosslinked polypeptides on LECs. In the absence of Ca2+, and in the presence of EGTA, only bands at 175/166 kDa were consistently crosslinked. These bands have been previously identified as the LEC Ca2+-independent endocytic HA receptor (Yannariello-Brown et al., J. Biol. Chem., 267, 20451-20455, 1992). In the presence of Ca2+, crosslinking was consistently seen to a 68 kDa polypeptide. Crosslinking was competed with a 100-fold excess (w/w) of HA. These and other data suggest that a 68 kDa protein is the most likely candidate for the CaHA-BP in LECs.