Association of protein kinase C with phospholipid monolayers: Two-stage irreversible binding

The association of protein kinase C (PKC) with phospholipid (PL) monolayers spread at the air-water interface was examined. PKC-PL binding induced surface pressure changes that were dependent on the amount of PKC, the phospholipid composition of the monolayers, the presence of Ca²⁺, and the initial surface pressure of the monolayer (π₀). Examination of surface pressure increases induced by PKC as a function of phospholipid surface pressure, π₀, revealed that PKC-phosphatidylserine (PS) association had a critical pressure of 43 dyn/cm. Above this surface pressure, PKC cannot cause further surface pressure changes. This high critical pressure indicated that PKC should be able to penetrate many biological membranes which appear to have surface pressures of about 30 dyn/cm. PKC-induced surface pressure changes were Ca²⁺ dependent only for PL monolayers spread at a π₀ greater than 26 dyn/cm. PKC alone (in the absence of PL) formed a film at the air-water interface with a surface pressure of about 26 dyn/cm. Calcium-dependent binding was studied at the higher surface pressures which effectively excluded PKC from the air-water interface. Subphase depletion measurements suggested that association of PKC with PS monolayers consisted of two stages: a rapid Ca²⁺-dependent interaction followed by a slower process that resulted in irreversible binding of PKC to the monolayer. The second stage appeared to involve penetration of PKC into the hydrocarbon region of the phospholipid. The commonly used in vitro substrates for PKC, histone and protamine sulfate, also associated with and penetrated PS monolayers with critical pressures of 50 and 60 dyn/cm, respectively. Despite simultaneous interaction of PKC and histone with the phospholipid monolayers, phosphorylation was minimal, even in the presence of phorbol esters. Consequently, PKC was not able to phosphorylate histones on a planar, nonaggregated membrane surface. In contrast, protamine sulfate disrupted the monolayer, dissociated PKC from the interface, and caused aggregation in the subphase. PKC was highly active in the latter circumstance. These results suggested that PKC was unable to phosphorylate these substrates when bound to planar, nonaggregated membrane surfaces or that PKC had high substrate specificity under these conditions.