Global consequences of activation loop phosphorylation on protein kinase A

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Protein kinase activity is commonly regulated by phosphorylation of the activation loop.
• The catalytic subunit of cAMP-dependent protein kinase (PKA C-subunit) autophosphorylates at Thr(197) in its activation loop.
• While Arg(194) mutation blocks autophosphorylation, Thr(197) can be phosphorylated by phosphoinositide-dependent protein kinase 1 (PDK1) in vitro.

Purpose of the Study:

• To investigate the structural and functional effects of Thr(197) phosphorylation on the PKA C-subunit.
• To compare the wild-type PKA C-subunit with an R194A mutant in the presence and absence of activation loop phosphorylation.

Main Methods:

• Enzyme kinetics (K(m) determination for kemptide and ATP).
• Biophysical techniques (fluorescence, circular dichroism) to assess enzyme stability.
• Peptide binding assays with a PKA inhibitor peptide (IP20).
• Deuterium exchange mass spectrometry (DXMS) to probe structural dynamics.

Main Results:

• Phosphorylation of Thr(197) significantly decreased K(m) for both kemptide and ATP.
• Enzyme stability increased upon Thr(197) phosphorylation, as indicated by fluorescence and CD spectroscopy.
• Binding affinity between the PKA C-subunit and IP20 peptide was enhanced by phosphorylation.
• DXMS revealed reduced amide hydrogen exchange rates in phosphorylated PKA C-subunit, indicating increased structural rigidity.
• Major dynamic changes were observed in the activation segment, p+1 loop/APE regions, and alphaH-alphaI loop motifs.

Conclusions:

• Activation loop phosphorylation of PKA C-subunit enhances catalytic efficiency and enzyme stability.
• Phosphorylation induces conformational changes that affect substrate and inhibitor binding.
• Structural dynamics are reduced upon phosphorylation, suggesting a more rigid enzyme conformation.
• A phosphorylation-induced salt bridge between Glu(208) and Arg(280) is predicted, contributing to enzyme stabilization.