A novel calcium-dependent phosphorylation of a ribosome-associated protein

The degree of phosphorylation of ribosome-associated proteins of intact GH3 pituitary tumor cells before and after Ca2+ depletion was examined. A 26-kDa ribosome-associated protein was found to dephosphorylate upon Ca2+ depletion of the cells and to re-phosphorylate upon restoration of the cation. This protein was distinct from eukaryotic initiation factor 4E on the basis of subcellular distribution, antigenicity, and susceptibility to dephosphorylation in Ca2+-depleted cells. Temporal correlation was observed between the phosphorylation/dephosphorylation state of the 26-kDa protein, and the stimulation/inhibition of protein synthesis by Ca2+ repletion/depletion. Phosphorylation of the 26-kDa protein was rapidly abolished in Ca2+-repleted cells by the ionophore, A23187, which elevates cytosolic Ca2+ while reducing sequestered cation. Dephosphorylation of the protein and inhibition of protein synthesis occurred at comparable times after addition of ionophore. The novelty of a Ca2+-dependent phosphorylation supported by sequestered rather than cytosolic free Ca2+ is discussed.