Phosphorylation of a membrane receptor for glycoproteins. Possible transmembrane orientation of the chicken hepatic lectin.

The chicken hepatic lectin, a receptor for partially deglycosylated serum glycoproteins, has been identified as a phosphoprotein. Phosphorylation was detected by incorporation of 32P into the protein in cultured hepatocytes and by two-dimensional gel analysis of protein purified from liver tissue. In addition, forms of the receptor containing one, two, and three sialic acid residues have been detected, with the disialylated form predominating. The site of phosphorylation has been identified as Ser7 in the complete amino acid sequence of the receptor (Drickamer, K. (1981) J. Biol. Chem. 256, 5827-5839). The presence of a protein kinase target site near the NH2-terminal of the receptor, a stretch of 25 uncharged, hydrophobic residues in positions 24 through 48, and a site of glycosylation at position 67 suggests that the chicken hepatic lectin is probably a transmembrane protein, oriented with COOH-terminal outside the cell and NH2-terminal in the cytoplasm.