Inhibition of tyrosine phosphorylation in the rat hepatic lectin 1 subunit of the rat asialoglycoprotein receptor prevents ATP-dependent receptor inactivation in permeabilized hepatocytes

Abstract

We previously reconstituted the ATP-dependent inactivation of asialoglycoprotein receptors (ASGPRs) in digitonin-permeabilized hepatocytes (Medh, J. D., and Weigel, P. H. (1991) J. Biol. Chem. 266, 8771-8778). Here we report that rat hepatic lectin 1 (RHL1) is the only ASGPR subunit that becomes radiolabeled when permeabilized washed hepatocytes are incubated at 4 degrees C in the presence of [gamma-32P]ATP; RHL2 and RHL3 are not radiolabeled. Phosphorylation of RHL1 was rapid (t1/2 appoximately 4 min) and complete within 30 min. Inclusion of 20 mM EDTA inhibited phosphorylation of RHL1 completely. Phosphoamino acid analysis identified Tyr(P) as the predominant (> 90%) radiolabeled phosphoamino acid. Addition of vanadate enhanced phosphorylation of Tyr in RHL1 4-fold. Phosphorylation of RHL1 occurred to the same extent in hepatocytes permeabilized with either 0.006% (w/v) or 0.055% digitonin and in the presence or the absence of ligand (50 micrograms/ml asialo-orosomucoid; ASOR) and/or 10 mM CaCl2. Sequential purification of active ASGPRs (using ASOR-Sepharose) and inactive ASGPRs from the ASOR-Sepharose flow-through (using anti-ASGPR antibody-Sepharose) demonstrated that radiolabeled RHL1 was present almost exclusively in active ASGPR oligomers. When permeabilized hepatocytes radiolabeled with [gamma-32P]ATP at 4 degrees C were warmed to 37 degrees C, a temperature at which ATP-dependent ASGPR inactivation occurs, RHL1 was dephosphorylated rapidly (t1/2 approximately 4 min) and completely within approximately 30 min. Western blot analysis using a monoclonal anti-Tyr(P) antibody showed that the steady-state level of endogenous Tyr(P) in RHL1 doubled as a result of ATP treatment at 4 degrees C and then decreased to undetectable levels upon warming to 37 degrees C. The protein-tyrosine kinase inhibitor tyrphostin 51 inhibited phosphorylation of RHL1 at 4 degrees C and also prevented ATP-dependent ASGPR inactivation at 37 degrees C. We conclude that phosphorylation of Tyr in RHL1 of active ASGPRs is a prerequisite for ATP-dependent ASGPR inactivation.