Phosphorylation of the Secreted, Free α Subunit of Human Chorionic Gonadotropin

• Published in: Proceedings of the National Academy of Sciences - PNAS 1986-12, Vol.83 (24), p.9493-9496
• Main Authors: Saccuzzo, Jean E., Krzesicki, Raymond F., Perini, Fulvio, Ruddon, Raymond W.
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Cell Line; Cell lines; Cell membranes; Cell physiology; Cells; Chorionic Gonadotropin - metabolism; Dimers; Enzymes; Fundamental and applied biological sciences. Psychology; Gonadotropins; Hepatocytes; Hormonal regulation; Humans; Macromolecular Substances; Molecular and cellular biology; Phorbols; Phosphoproteins - metabolism; Phosphorylation; Post translational modification; Protein Processing, Post-Translational; Tetradecanoylphorbol Acetate - pharmacology
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.83.24.9493

Phosphorylation of secretory proteins is an uncommon event. In this manuscript, the phosphorylation of human chorionic gonadotropin, a glycoprotein hormone secreted by the JAR choriocarcinoma cell line, is described. Labeling of JAR cells with 32PO4 indicates that both the intracellular and the secreted forms of the free α subunit are phosphorylated. Although the secreted α β dimer also incorporates 32PO4, there is little detectable phosphorylation of the intracellular precursors of α β dimer, suggesting that dimer phosphorylation occurs as a late event in post-translational processing. In addition, phorbol 12-myristate 13-acetate markedly stimulates the phosphorylation of both intracellular and secreted forms of free α subunit and to a lesser extent of secreted α β dimer. In vitro assays, using homogenates of JAR cells as a source of protein kinase activity, indicate that the uncombined α subunit is preferentially phosphorylated. The phosphorylation sites are on serine and threonine residues in the α subunit.