Activation of protein kinase C or cAMP-dependent protein kinase increases phosphorylation of the c-erbA-encoded thyroid hormone receptor and of the v-erbA-encoded protein.

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Abstract
The c‐erbA proto‐oncogene encodes a nuclear receptor for thyroid hormone (T3), which is believed to stimulate transcription from specific target promoters upon binding to cis‐acting DNA sequence elements. The v‐erbA oncogene of avian erythroblastosis virus (AEV) encodes a ligand‐independent version of this nuclear receptor. The v‐erbA product inhibits terminal differentiation of avian erythroblasts, presumably by affecting the transcription of specific genes. We show here that the c‐erbA‐encoded nuclear receptor (p46c‐erbA) is phosphorylated on serine residues on two distinct sites. One of these sites, defined by the limit tryptic phosphopeptide 28SSQCLVK, is retained on the v‐erbA‐encoded P75gag‐v‐erbA protein. This site is located in the amino‐terminal domain of these molecules, 21 amino acids upstream of the DNA‐binding region. Phosphorylation of this site in both p46c‐erbA and P75gag‐v‐erbA is enhanced 10‐fold following treatment of cells with activators of either protein kinase C or cAMP‐dependent protein kinase. Since cAMP‐dependent protein kinase phosphorylates both p46c‐erbA and P75gag‐v‐erbA in vitro at the same site as that observed in vivo, at least part of the cAMP‐dependent phosphorylation of erbA molecules in cells could result from direct phosphorylation by this enzyme. The possible role phosphorylation may play in the function of the erbA‐encoded transcriptional factors is discussed.