Contributions to maxima in protein kinase C activation

In many lipid systems, the activity of protein kinase C (PKC) exhibits a pe ak followed by a decline as the mol % of one component is increased. In the se systems, an increase in one lipid component is always at the expense of another or accompanied by a change in total lipid concentration. Here we re port that in saturated phosphatidylserine (PS)/phosphatidylcholine (PC)/dia cylglycerol (DAG) mixtures, increasing PS or DAG at the expense of PC revea led an optimal mol % PS, dependent on mol % DAG, with higher mol % PS dimin ishing activity. The decrease at high mol % PS is probably not attributable simply to more gel-phase lipid due to the higher melting temperature of sa turated PS versus PC because a similar peak in activity occurred in unsatur ated lipid systems. Increasing the total lipid concentration at suboptimal mol % PS provided the same activity as higher mol % PS at lower total lipid concentration. However, at optimal mol % PS, activity increased and then d ecreased as a function of total lipid concentration, PKC autophosphorylatio n also exhibited an optimum as a function of mol % PS, and increasing the P RC concentration increased the mol % PS at which activity decreased, both f or autophosphorylation and for heterologous phosphorylation. Formation of t wo-dimensional crystals of PKC on lipid monolayers also exhibited a peak. a s a function of mol % PS, and the unit cell size of the crystals formed shi fts from 50 x 50 Angstrom at low mol % PS to 75 x 75 Angstrom at higher PS. Collectively, these data suggest the existence of optimal lipid compositio ns for PKC activation, with increased quantity of these domains serving to dilute out enzyme-substrate aggregates and/or enzyme-enzyme aggregates on t he lipid surface.