Phosphoprotein phosphatase 2A dephosphorylates eIF-4E and does not alter binding to the mRNA cap

Area of Science:

• Molecular Biology
• Cellular Biology
• Biochemistry

Background:

• The phosphorylation of eukaryotic initiation factor-4E (eIF-4E), a 25 kDa mRNA cap-binding protein, is crucial for regulating protein synthesis.
• eIF-4E phosphorylation is modulated during physiological processes like cell growth and pathological conditions such as adenovirus infection.

Purpose of the Study:

• To investigate the regulation of eIF-4E phosphorylation and dephosphorylation in Hep G2 cells.
• To compare the efficacy of different agents in inducing eIF-4E phosphorylation and identify the primary phosphatase involved in its dephosphorylation.

Main Methods:

• Hep G2 cells were treated with okadaic acid and phorbol 12-myristate 13-acetate to induce eIF-4E phosphorylation.
• eIF-4E was isolated and dephosphorylated using various phosphatases, including phosphoprotein phosphatase 2A, alkaline phosphatase, and acid phosphatase.
• The mRNA cap-binding ability of purified eIF-4E was assessed after dephosphorylation.

Main Results:

• Okadaic acid was more potent than phorbol 12-myristate 13-acetate in increasing the phosphorylated fraction of eIF-4E.
• Phosphoprotein phosphatase 2A effectively dephosphorylated eIF-4E from treated cells, while alkaline and acid phosphatases showed minimal effect.
• The mRNA cap-binding capacity of eIF-4E was not altered by dephosphorylation mediated by phosphoprotein phosphatase 2A.

Conclusions:

• The observed effects of eIF-4E phosphorylation on protein synthesis may require alternative explanations beyond direct modulation of mRNA binding.
• In vivo phosphorylation of eIF-4E is not exclusively catalyzed by protein kinase C, suggesting involvement of other kinases.