The asialoglycoprotein (ASGP) receptor undergoes constitutive endocytosis through the coated pit/coated vesicle pathway in hepatocytes. Studies on HepG2 cells have shown that the receptor is phosphorylated at serine under control conditions and following protein kinase C stimulation. This study examined whether the ASGP receptor could also serve as a substrate for a tyrosine kinase in HepG2 cells. 32P labeling was performed in membrane preparations, in permeabilized cells at 4°C, and in intact cells at 37°C. The phosphorylated ASGP receptor was isolated by immunoprecipitation, hydrolyzed in 6 N HCl at 110°C, and analyzed by two-dimensional high voltage electrophoresis. The receptor isolated from a membrane preparation incubated in vitro with [γ-32P]ATP incorporated radiolabel predominantly (> 90%) into phosphotyrosine. ASGP receptor phosphorylation at both tyrosine and serine was detected in intact cells incubated with phosphatase inhibitors for 60 min at 37°C. The presence of both phenylarsine oxide (20 μM) and sodium orthovanadate (200 μM) was required for tyrosine phosphorylation. Use of these inhibitors together resulted in a 16.4-fold increase in phosphorylation of the immunoprecipitated ASGP receptor, whereas phosphorylation of total HepG2 membrane proteins was not significantly augmented by this procedure. Selective proteolytic digestion of ASGP receptors is isolated vesicles demonstrated that the phosphorylation site identified in these studies is located at tyrosine 5 in the cytoplasmic tail.
|Original language||English (US)|
|Number of pages||6|
|Journal||Journal of Biological Chemistry|
|State||Published - Mar 23 1990|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology