Casein kinase I phosphorylates the 25-kDa mRNA cap-binding protein.

Haas DW, Hagedorn CH
Arch Biochem Biophys. 1991 284 (1): 84-9

PMID: 1989504 · DOI:10.1016/0003-9861(91)90267-m

The 25-kDa mRNA cap-binding protein (eIF-4E) exists in both phosphorylated and dephosphorylated forms in eukaryotic cells. Phosphorylated eIF-4E appears to be preferentially associated with 48 S initiation complexes and with the 220-kDa subunit of eIF-4F. In addition, dephosphorylation of eIF-4E has been observed during heat shock and mitosis which are accompanied by decreased protein synthesis. However, the control of eIF-4E phosphorylation and its regulatory role remain poorly understood. Using eIF-4E as a substrate we have identified and purified from rabbit reticulocytes a protein kinase that phosphorylates eIF-4E in vitro. This enzyme phosphorylated eIF-4E on both serine and threonine residues with an apparent Km of 3.7 microM. The molecular mass of the enzyme and specificity for substrates other than eIF-4E suggested that this enzyme was a species of casein kinase I. This was confirmed by comparing the phosphopeptide map of the purified reticulocyte enzyme with that of rabbit skeletal muscle casein kinase I and by comparing phosphopeptide maps of eIF-4E phosphorylated in vitro by each enzyme. We conclude that casein kinase I phosphorylates eIF-4E in vitro and suggest that eIF-4E may be phosphorylated by casein kinase I in intact cells under some physiologic conditions.

MeSH Terms (12)

Animals Casein Kinases Eukaryotic Initiation Factor-4E Humans In Vitro Techniques Peptide Initiation Factors Phosphorylation Phosphoserine Phosphothreonine Protein Kinases Rabbits RNA Caps

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