Precisely ordered phosphorylation reactions in the p38 mitogen-activated protein (MAP) kinase cascade

Area of Science:

• Cellular signaling pathways
• Molecular biology
• Biochemistry

Background:

• MAP kinase (MAPK) cascades are crucial for eukaryotic responses to external stimuli and stress.
• These cascades involve sequential phosphorylation events catalyzed by MAP3Ks and MAP2Ks.
• The precise order of these phosphorylation reactions, particularly within specific modules like p38 MAPK, remains an area of investigation.

Purpose of the Study:

• To determine whether the double phosphorylation reactions in the p38 MAP kinase module occur sequentially or randomly.
• To elucidate the specific order of phosphorylation events for MEK6 and p38α.
• To investigate the phosphorylation sequence of the p38 MAPK substrate ATF2.

Main Methods:

• Utilized mass spectrometry to track phosphorylation events in the p38 MAP kinase module.
• Analyzed the phosphorylation of MEK6 by MAP3Ks (TAO2 and ASK1) and the subsequent phosphorylation of p38α by MEK6.
• Applied kinetic modeling to progress curves of intermediate formation.

Main Results:

• Demonstrated that MEK6 is phosphorylated in a precise order: first on Thr-211, then on Ser-207, by both TAO2 and ASK1.
• Confirmed that p38α is phosphorylated sequentially: first on Tyr-182, then on Thr-180.
• Observed a precise phosphorylation sequence for the transcription factor ATF2, a substrate of p38α.
• Kinetic models supported the ordered reaction mechanism and revealed processivity in MEK6 phosphorylation by ASK1.

Conclusions:

• The double phosphorylation reactions within the p38 MAP kinase module are precisely ordered, not random.
• The observed order is influenced by both substrate binding and catalytic rates.
• This ordered phosphorylation is a conserved mechanism, also seen in other MAPK pathways like ERK2.
• Findings provide a deeper understanding of the regulation and fidelity of MAP kinase signaling.