Casein kinase I phosphorylates the 25-kDa mRNA cap-binding protein.
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.
Haas
D W
DW
Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN 37232-2279.
Hagedorn
C H
CH
D000818:Animals; D047388:Casein Kinases; D039561:Eukaryotic Initiation Factor-4E; D006801:Humans; D066298:In Vitro Techniques; D010448:Peptide Initiation Factors / Q000378:metabolism*; D010766:Phosphorylation; D010768:Phosphoserine / Q000378:metabolism; D010769:Phosphothreonine / Q000378:metabolism; D011494:Protein Kinases / Q000302:isolation & purification / Q000378:metabolism*; D012315:RNA Caps / Q000378:metabolism; D011817:Rabbits
D039561:Eukaryotic Initiation Factor-4E; D010448:Peptide Initiation Factors; D012315:RNA Caps; D010769:Phosphothreonine; D010768:Phosphoserine; D011494:Protein Kinases; D047388:Casein Kinases