Increased Activation of Protein Kinase C with Cubic Phase Lipid Compared with Liposomes
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Protein kinase C (PKC) activation is measured using liposomes containing phosphatidylserine. Certain lipids display a wide range of polymorphism, depending on conditions. They can give rise to nonlamellar phases, such as hexagonal or cubic phases, as well as to lamellar phases. In this paper, we studied the activity and membrane binding of PKC in lipid bicontinuous cubic phases and hexagonal phases. The cubic phase lipid systems were (1) monoolein with 1-palmitoyl-2-oleoyl-3-phosphatidylserine (MO/PS) and (2) dielaidoylphosphatidylethanolamine/alamethicin (DEPE/alamethicin). Under fully hydrated conditions, both of the above lipid mixtures are bicontinuous cubic phases with a space group of Pn3m within certain concentration ratios and temperature ranges. Dioleoylphosphatidylethanolamine (DOPE) with up to 10 mol % PS exists in the hexagonal phase at room temperature. These cubic and hexagonal phases were able to support the PKC-catalyzed phosphorylation of histone. The amount of PKC bound to the MO/PS cubic phase showed little increase between 5 and 10 mol % PS. For both of the cubic phase systems studied, only a minor fraction of the PKC was bound to the membrane. This indicates that the specific activity of the enzyme bound to cubic phase membranes is much greater than that bound to phospholipid in the lamellar phase. Addition of up to 50 mol % MO to lipid in the lamellar phase had relatively small effects on the activity of PKC. The increase in PKC activity correlated well with an increase in PKC binding, resulting in little change in the specific activity of the membrane-bound form. These findings may be physiologically relevant due to the apparent presence of the cubic phase in certain biological structures. Also, these phases have little or no curvature strain, a property which has been shown to correlate with activation of PKC. Therefore, other factors, such as a curved morphology and/or interfacial polarity, must be responsible for the activation of PKC in these lipid systems.
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