The affinities of Ca
2+ and anionic lipid vesicles from the C2
domains of classical proteinkinase C subfamily (
![](/images/gifchars/alpha.gif)
,
![](/images/gifchars/beta2.gif)
II, and
![](/images/gifchars/gamma.gif)
) were studied using isothermal titration calorimetry (ITC). In addition,the thermal stability of these C2
domains in the presence of different ligand concentrations was analyzedusing differential scanning calorimetry (DSC). These three closely related C2
domains bind Ca
2+ in asimilar way, demonstrating the presence of two sets of sites. The first set of sites binds one Ca
2+ ionexothermically with similar high affinity for the three proteins (
Kd around 1
![](/images/entities/mgr.gif)
M), while the second set ofsites binds endothermically approximately two Ca
2+ ions with lower affinity, which varies for each C2
domain: 22.2
![](/images/entities/mgr.gif)
M for the PKC
![](/images/gifchars/alpha.gif)
-C2
domain, 17.2
![](/images/entities/mgr.gif)
M for the PKC
![](/images/gifchars/beta2.gif)
II-C2
domain, and 4.3
![](/images/entities/mgr.gif)
M for thePKC
![](/images/gifchars/gamma.gif)
-C2
domain. In the absence of Ca
2+, the three C2
domains showed a weak interaction with vesiclescontaining anionic phospholipids. However, in the presence of a saturating Ca
2+ concentration, the C2
domains increased their affinities for the anionic lipid vesicles. In all cases, the C2
domains bound thevesicles exothermically and with similar affinities. A DSC thermal stability study of the C2
domains inthe presence of Ca
2+ and anionic lipids provided further information about this protein-ligand interaction.The presence of increasing Ca
2+ concentrations was matched by an increase in the
Tm in all cases, whichwas even greater in the presence of anionic lipid vesicles. The extent of the change in
Tm differed foreach C2
domain, reflecting the differing effect of the ligands bound during the protein stabilization.Denaturation of the C2
domains was irreversible both in the absence and in the presence of ligands,although the thermograms were not kinetically controlled. The dependence of the
Tm on the Ca
2+concentration indicates that the protein stabilization observed by DSC primarily reflects the saturation bythe cation of the low-affinity set of sites.