Membrane binding kinetics of protein kinase C betaII mediated by the C2 domain

Conventional isoforms of protein kinase C (PKC) are activated when their two membrane-targeting modules, the C1 and C2 domains, bind the second messengers diacylglycerol (DG) and Ca2+, respectively. This study investigates the mechanism of Ca2+-induced binding of PKC betaII to anionic membranes mediated by the C2 domain. Stopped-flow fluorescence spectroscopy reveals that Ca2+-induced binding of the isolated C2 domain to anionic vesicles proceeds via at least two steps: (1) rapid binding of two or more Ca2+ ions to the free domain with relatively low affinity and (2) diffusion-controlled association of the Ca2+-occupied domain with vesicles. Ca2+ increases the affinity of the C2 domain for anionic membranes by both decreasing the dissociation rate constant (k(off)) and increasing the association rate constant (k(on)) for membrane binding. For binding to vesicles containing 40 mol % anionic lipid in the presence of 200 microM Ca2+, k(off) and k(on) are 8.9 s(-1) and 1.2 x 10(10) M(-1) x s(-1), respectively. The k(off) value increases to 150 s(-1) when free Ca2+ levels are rapidly reduced, decreasing the average lifetime of the membrane-bound C2 domain (tau = k(off)(-1)) from 110 ms in the presence of Ca2+ to 6.7 ms when Ca2+ is rapidly removed. Experiments addressing the role of electrostatic interactions reveal that they stabilize either the initial C2 domain-membrane encounter complex or the high-affinity membrane-bound complex. Specifically, lowering the phosphatidylserine mole fraction or including MgCl2 in the binding reaction decreases the affinity of the C2 domain for anionic vesicles by both reducing k(on) and increasing k(off) measured in the presence of 200 microM Ca2+. These species do not affect the k(off) value when Ca2+ is rapidly removed. Studies with PKC betaII reveal that Ca2+-induced binding to membranes by the full-length protein proceeds minimally via two kinetically resolvable steps: (1) a rapid bimolecular association of the enzyme with vesicles near the diffusion-controlled limit and, most likely, (2) subsequent conformational changes of the membrane-bound enzyme. As is the case for the C2 domain, k(off) for full-length PKC betaII increases when Ca2+ is rapidly removed, reducing tau from 11 s in the presence of Ca2+ to 48 ms in its absence. Thus, both the C2 domain and the slow conformational change prolong the lifetime of the PKC betaII-membrane ternary complex in the presence of Ca2+, with rapid membrane release triggered by removal of Ca2+. These results provide a molecular basis for cofactor regulation of PKC whereby the C2 domain searches three-dimensional space at the diffusion-controlled limit to target PKC to relatively common anionic phospholipids, whereupon a two-dimensional search is initiated by the C1 domain for the more rare, membrane-partitioned DG.