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Phosphoryl transfer step in the C-terminal Src kinase controls Src recognition.

Scot A Lieser1, Caitlin Shindler, Brandon E Aubol

  • 1Department of Pharmacology, University of California at San Diego, La Jolla, California 92093-0728, USA.

The Journal of Biological Chemistry
|December 30, 2004
PubMed
Summary

The C-terminal Src kinase (Csk) down-regulates Src family kinases. Csk recognizes Src not by strong binding, but by a rapid phosphorylation step, acting as a chemical clamp for substrate recognition.

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Cell Signaling

Background:

  • Src family kinases are crucial for cellular growth and differentiation.
  • C-terminal Src kinase (Csk) phosphorylates and down-regulates Src family kinases.
  • The precise mechanisms of Csk-Src substrate recognition remain largely unknown.

Purpose of the Study:

  • To investigate the chemical and physical factors governing Csk's recognition of its substrate, Src.
  • To elucidate the kinetic parameters that modulate Src substrate affinity and turnover by Csk.

Main Methods:

  • Utilized rapid quench flow experiments to analyze Src phosphorylation kinetics.
  • Employed stopped-flow fluorescence and catalytic trapping experiments to assess ADP release.
  • Conducted single turnover kinetic and equilibrium sedimentation experiments to determine binding affinities (K(m) and K(d)).

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Main Results:

  • Src phosphorylation by Csk occurs in a biphasic manner, indicating a rapid and favorable phosphoryl transfer step.
  • Turnover is not limited by ADP release, distinguishing it from other kinase-substrate interactions.
  • The dissociation constant (K(d)) for Src is significantly larger than the Michaelis constant (K(m)), suggesting weak initial binding.

Conclusions:

  • Csk achieves efficient recognition and phosphorylation of Src not through strong protein-protein interactions.
  • A rapid phosphoryl transfer step acts as a 'chemical clamp,' facilitating substrate recognition and turnover.
  • This mechanism highlights a unique mode of kinase-substrate interaction crucial for regulating Src signaling pathways.