Apolipophorin III interaction with phosphatidylglycerol and lipopolysaccharide: A potential mechanism for antimicrobial activity.

Apolipophorin III (apoLp-III) is a model insect apolipoprotein to study structure-function relationships of exchangeable apolipoproteins. The protein associates with lipoproteins to aid in the transport of neutral lipids, and also interacts with the bacterial membrane. To better understand a potential role as an antimicrobial protein, the binding interaction of apoLp-III from Locust migratoria and Galleria mellonella with phosphatidylglycerol and lipopolysaccharides was analyzed. ApoLp-III from either species induced a robust release of calcein from phosphatidylglycerol vesicles, but was ineffective for phosphatidylcholine vesicles with comparable side-chain architecture. Acetylation of L. migratoria apoLp-III lysine residues greatly reduced the calcein release from phosphatidylglycerol vesicles, indicating a critical role of lysine side-chains in phosphatidylglycerol vesicles interaction. Isothermal calorimetry provided K values of 0.26 μM (L. migratoria) and 0.50 μM (G. mellonella) for binding to dimyristoylphosphatidylglycerol vesicles, which is an order of magnitude stronger compared to zwitterionic vesicles. A strong preference of apoLp-III for dimyristoylphosphatidylglycerol vesicles was also observed with differential scanning calorimetry with a concentration dependent shift in the lipid phase transition temperature. Native PAGE analysis showed that LPS binding was significantly weaker for L. migratoria apoLp-III compared to G. mellonella apoLp-III. This difference was confirmed by fluorescence titration analysis of L. migratoria apoLp-III, which also indicated that acetylation of the apolipoprotein did not affect LPS binding. Taken together, the results indicate that apoLp-III phosphatidylglycerol interaction may follow a detergent model with an important electrostatic binding component. Since lipopolysaccharide binding was not affected by neutralization of apoLp-III lysine-side chains, the binding interaction may be distinctly different from that of phosphatidylglycerol.