An S752D activation loop mutation dynamically primes Muscle-Specific Kinase for activation

Area of Science:

• Molecular Biology
• Biochemistry
• Neuroscience

Background:

• Muscle-specific kinase (MuSK) is crucial for neuromuscular junction (NMJ) formation and function.
• MuSK's autoinhibited state, stabilized by activation loop (A-loop) tyrosines, presents a barrier to understanding its regulation.
• Previous work suggested A-loop serine phosphorylation might prime MuSK for activation.

Purpose of the Study:

• To investigate the role of A-loop serine phosphorylation in MuSK activation.
• To elucidate the structural and dynamic changes associated with MuSK autoinhibition relief.
• To provide a mechanistic framework for MuSK regulation at the NMJ.

Main Methods:

• X-ray crystallography to determine MuSK structures.
• Biochemical assays to measure enzyme activity and binding affinity.
• Hydrogen-deuterium exchange and mass spectrometry (HDX-MS) to assess protein dynamics.

Main Results:

• A phosphomimetic S752D mutation disrupted autoinhibitory interactions, increasing ATP binding and catalytic turnover.
• HDX-MS revealed the S752D mutation enhances A-loop flexibility, relieving autoinhibition.
• The S752D mutation stabilized the juxtamembrane NPXY motif, a key site for Dok7 interaction.

Conclusions:

• A-loop serine phosphorylation acts as a priming mechanism for MuSK activation.
• Dynamic transitions in the A-loop and JM region are critical for relieving MuSK autoinhibition.
• These findings offer insights into the molecular mechanisms governing MuSK signaling at the NMJ.