Lactoferrin binding to the rat asialoglycoprotein receptor requires the receptor's lectin properties

Lactoferrin binds to rat hepatic lectin 1 (RHL1), the major subunit of the asialoglycoprotein (ASGP) receptor, with high affinity, by a galactose-independent mechanism. To better understand the molecular basis of this novel interaction, we compared the binding of lactoferrin and asialo-orosomucoid (ASOR) to isolated rat hepatocytes and to purified ASGP receptors as a function of pH, Ca²⁺ and receptor acylation. Binding of ¹²⁵I-lactoferrin and ¹²⁵I-ASOR to isolated rat hepatocytes at 4 °C decreased sharply at pH < 6, following similar titration curves. Binding of ¹²⁵I-lactoferrin and ¹²⁵I-ASOR to hepatocytes was Ca²⁺-dependent. Binding increased progressively at ≥ 300 μM CaCl₂, in the presence of 1 mM EDTA. Monensin treatment of hepatocytes, which causes hepatocytes to accumulate inactive ASGP receptors, reduced surface binding of ¹²⁵I-lactoferrin and ¹²⁵I-ASOR by 46 and 49%, respectively, with only a 16% loss of immunodetectable receptor protein from the cell surface. Finally, deacylation of purified ASGP receptors in vitro with 1 M hydroxylamine abolished receptor lectin activity as reflected by the loss of ¹²⁵I-ASOR binding as well as the complete loss of specific ¹²⁵I-lactoferrin binding. Treatment with 1 M Tris had no effect on binding of either ligand. We conclude from these data that galactose-independent lactoferrin binding to the ASGP receptor requires the receptor's carbohydrate-recognition domain to be in an active configuration. An active configuration is promoted by neutral pH and Ca²⁺, and also requires the receptor subunits to be acylated.