Ordered phosphorylation of p42mapk by MAP kinase kinase

Area of Science:

• Molecular Biology
• Enzymology
• Signal Transduction

Background:

• p42 mitogen-activated protein kinase (MAPK) is a key signaling molecule.
• The phosphorylation status of p42MAPK regulates its activity and interactions.
• Understanding the sequential phosphorylation events is crucial for deciphering MAPK pathway function.

Purpose of the Study:

• To describe the preparation of milligram quantities of [32P]p42MAPK phosphorylated at specific sites (Tyr185 or Tyr185/Thr183).
• To investigate the substrate properties of differentially phosphorylated p42MAPK for protein phosphatases.
• To elucidate the order and functional significance of Tyr185 and Thr183 phosphorylation in p42MAPK activation.

Main Methods:

• Radiolabeling of p42MAPK with [32P]ATP.
• Controlled phosphorylation of p42MAPK using limiting ATP conditions to favor Tyr185 phosphorylation.
• Enzyme kinetic assays to determine the Michaelis constant (Km) for MAP kinase kinase (MAPKK) with different p42MAPK phosphorylation states.

Main Results:

• Milligram amounts of [32P]p42MAPK, phosphorylated at Tyr185 or diphosphorylated at Tyr185/Thr183, were successfully prepared.
• Tyr185-phosphorylated p42MAPK accumulated under conditions of limiting ATP, while Thr183 phosphorylation required further activation.
• Tyr185-phosphorylated p42MAPK showed a significantly lower apparent Km (46.6 nM) for MAPKK compared to the dephospho form (476 nM), indicating a 10-fold increase in affinity.

Conclusions:

• The phosphorylation of Tyr185 on p42MAPK precedes and is a prerequisite for Thr183 phosphorylation.
• This sequential phosphorylation dramatically enhances the affinity of p42MAPK for its upstream activator, MAPKK.
• The findings provide critical insights into the regulatory mechanism of MAPK pathway activation and substrate recruitment.