A structure for calmodulin-activated cyclic nucleotide phosphodiesterase deduced from proteolysis studies

Area of Science:

• Biochemistry
• Molecular Biology
• Enzymology

Background:

• Phosphodiesterase (PDE) enzymes play crucial roles in cellular signaling.
• PDE activity is regulated by various factors, including calmodulin and proteolytic cleavage.
• Understanding PDE conformation and activation mechanisms is key to deciphering cyclic nucleotide signaling.

Purpose of the Study:

• To elucidate the conformational states of unproteolyzed PDE.
• To investigate the role of calmodulin in PDE activation.
• To examine the impact of proteolytic cleavage on PDE conformation and activity.

Main Methods:

• Biochemical assays to study enzyme kinetics and conformation.
• Analysis of protein structure and interactions.
• Investigating the effects of calmodulin binding and proteolytic cleavage.

Main Results:

• Unproteolyzed PDE exists in two equilibrium conformations: a predominant non-functional catalytic site form and a minor functional form.
• Calmodulin preferentially binds to the functional conformation, shifting the equilibrium and activating catalysis.
• Cleavage of the calmodulin binding domain leads to irreversible acquisition of the functional conformation, independent of calmodulin.
• Proteolytic cleavage at the other terminus does not significantly alter conformational equilibrium.

Conclusions:

• A single cyclic nucleotide PDE can explain variations in enzyme activity observed in vitro.
• Intracellular calcium concentration provides absolute control over PDE activity in vivo via calmodulin.
• The findings provide a model for PDE regulation by conformation, calmodulin, and proteolysis.