Human apolipoprotein A-I (apoA-I) is a 28 kDa protein and a major component of high-density lipoproteins, mediating several essential metabolic functions related to heart disease. In the present study the potential protective role against bacterial pathogens was explored. ApoA-I suppressed bacterial growth of Escherichia coli and Klebsiella pneumoniae. The protein was able to bind lipopolysaccharides and showed a strong preference for bilayer vesicles made of phosphatidylglycerol over phosphatidylcholine. Lysine side chains of apoA-I were acetylated to evaluate the importance of electrostatic forces in the binding interaction with both membrane components. Electrophoresis properties, dot blot analysis, circular dichroism, and fluorescence spectroscopy to probe for changes in protein structure indicated that the acetylated protein displayed a strongly reduced LPS and PG binding. A mutant containing only the N-terminal domain of apoA-I also showed a reduced ability to interact with the membrane components, although to a lesser extent. These results indicate the potential for apoA-I to function as an antimicrobial protein and exerts this function through lysine residues.
Human apolipoprotein A‐I (apoA‐I) is a 28 kDa protein and a major component of high‐density lipoproteins, mediating several essential metabolic functions related to heart disease. Recently, a novel protective role against bacterial pathogens has emerged. ApoA‐I suppressed bacterial growth of log phase Escherichia coli and Klebsiella pneumoniae, resulting in a reduced colony count. We suggest that apoA‐I interacts with gram‐negative bacteria in two ways: 1) by associating with lipopolysaccharides (LPS) which are the major constituent of the outer bacterial membrane, thereby reducing endotoxic effects, and 2) destabilizing the negatively charged phospholipid bilayer of the inner bacterial membrane, resulting in cell lysis. To analyze the role of lysine in LPS and phospholipid binding, lysine side chains were acetylated using acetic anhydride in saturated sodium acetate. Electrophoresis analysis and 1‐anilino‐naphthalene‐8‐sulfonate fluorescence of apoA‐I in the absence or presence of LPS indicated decreased binding upon acetylation. ApoA‐I was able to lyse phosphatidylglycerol vesicles; however, acetylated apoA‐I showed a marked decrease in activity. These results indicate the potential for apoA‐I to function as an antimicrobial protein and the importance of lysine residues.
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