Structural Characterization of the C2 Domains of Classical Isozymes of Protein Kinase C and Novel Protein Kinase Cε by using Infrared Spectroscopy

Abstract

The amide I regions in the original infrared spectra of PKCα-C2 in the Ca²⁺-free and Ca²⁺-bound states are both consistent with a predominantly β-sheet secondary structure. Spectroscopic studies of the thermal denaturation revealed that for the PKCα-C2 domain alone the secondary structure abruptly changed at 50°C. While in the presence of Ca²⁺, the thermal stability of the protein increased considerably. Phosphatidic acid binding to the PKCα-C2 domain was characterized, and the lipid–protein binding becoming Ca²⁺-independent when 100 mol% phosphatidic acid vesicles was used. The effect of lipid binding on secondary structure and thermal stability was also studied. In addition, the secondary structure of the C2 domain from the novel PKCε was also determined by IR spectroscopy and β-sheet was seen to be the major structural component. Spectroscopic studies of the thermal denaturation in D₂O showed a broadening in the amide I^′ band starting at 45°C. Phosphatidic acid containing vesicles were used to characterize the effect of lipid binding on the secondary structure. It was observed through thermal stability experiments that the secondary structure did not change upon lipid binding and the protein stability was very high with no significant changes occurring in the secondary structure after heating.