Abstract
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 Kd 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.
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