C2 domains of protein kinase C isoforms α, β, and γ: Activation parameters and calcium stoichiometries of the membrane-bound state

The independently folding C2 domain motif serves as a Ca²⁺-dependent membrane docking trigger in a large number of Ca²⁺ signaling pathways. A comparison was initiated between three closely related C2 domains from the conventional protein kinase C subfamily (cPKC, isoforms α, β, and γ). The results reveal that these C2 domain isoforms exhibit some similarities but are specialized in important ways, including different Ca²⁺ stoichiometries. In the absence of membranes, Ca²⁺ affinities of the isolated C2 domains are similar (2-fold difference) while Hill coefficients reveal cooperative Ca²⁺ binding for the PKCβ C2 domain but not for the PKCα or PKCγ C2 domain (H = 2.3 ± 0.1 for PKCβ, 0.9 ± 0.1 for PKCα, and 0.9 ± 0.1 for PKCγ). When phosphatidylserine-containing membranes are present, Ca²⁺ affinities range from the sub-micromolar to the micromolar (7-fold difference) ([Ca²⁺]_1/2 = 0.7 ± 0.1 μM for PKCγ, 1.4 ± 0.1 μM for PKCα, and 5.0 ± 0.2 μM for PKCβ), and cooperative Ca²⁺ binding is observed for all three C2 domains (Hill coefficients equal 1.8 ± 0.1 for PKCα, 1.3 ± 0.1 for PKCα, and 1.4 ± 0.1 for PKCγ). The large effects of membranes are consistent with a coupled Ca²⁺ and membrane binding equilibrium, and with a direct role of the phospholipid in stabilizing bound Ca²⁺. The net negative charge of the phospholipid is more important to membrane affinity than its headgroup structure, although a slight preference for phosphatidylserine is observed over other anionic phospholipids. The Ca²⁺ stoichiometries of the membrane-bound C2 domains are detectably different. PKCβ and PKCγ each bind three Ca²⁺ ions in the membrane-associated state; membrane-bound PKCα binds two Ca²⁺ ions, and a third binds weakly or not at all under physiological conditions. Overall, the results indicate that conventional PKC C2 domains first bind a subset of the final Ca²⁺ ions in solution, and then associate weakly with the membrane and bind additional Ca²⁺ ions to yield a stronger membrane interaction in the fully assembled tertiary complex. The full complement of Ca²⁺ ions is needed for tight binding to the membrane. Thus, even though the three C2 domains are 64% identical, differences in Ca²⁺ affinity, stoichiometry, and cooperativity are observed, demonstrating that these closely related C2 domains are specialized for their individual functions and contexts.