When More Is Less: Dual Phosphorylation Protects Signaling Off State against Overexpression

Area of Science:

• Cellular signaling
• Molecular biology
• Biophysics

Background:

• Signaling pathways rely on kinases activated by multisite phosphorylation.
• Mitogen-activated protein kinase (MAPK) kinase (Mek) activates Erk through two phosphorylations.
• Understanding how kinase activity relates to protein abundance is crucial for signaling pathway regulation.

Purpose of the Study:

• To investigate the relationship between Erk protein abundance and its activation levels using kinetic models.
• To elucidate the mechanisms underlying the observed response of Erk activity to varying Erk concentrations.

Main Methods:

• Development and analysis of mathematical kinetic models.
• Simulation of phosphorylation cycles and enzyme kinetics.
• Analysis of steady-state and dynamic behaviors of the signaling pathway.

Main Results:

• A counterintuitive bell-shaped response of Erk activity to increasing Erk abundance was observed.
• Signaling remains robust to changes in Erk levels due to competition for Mek.
• The unmodified and singly phosphorylated Erk forms compete for the activating kinase Mek.

Conclusions:

• Phosphorylation cycles possess intrinsic robustness mechanisms.
• This robustness protects signaling pathways from noise and aberrant activation, such as from gene expression fluctuations or kinase overexpression.
• This mechanism is relevant for understanding diseases like cancer involving altered gene expression.