A mechanism for the partial insertion of protein kinase C into membranes

We propose that the principle driving force allowing protein kinase C (PKC) to insert partway into membranes is the transient creation of an interior hydrophilic phase within the membrane. We further suggest that this phase is composed of non-bilayer-forming elements, such as diacylglycerol or phorbol esters. We used the combination of fluorescence resonance energy transfer (using fluorescently labeled phospholipid molecules and the endogenous tryptophan residues of PKC) and fluorescence quenching by the water-soluble reagent potassium iodide. The experimental system used micelles and purified PKC. Our model accounts for both the established kinetic data on PKC as well as the physical requirements of protein-membrane interaction. Moreover, it establishes PKC as the first example of a partially embedded membrane protein, and provides a mechanism to account for its activation.