Role of Lysine Residues of Locusta migratoria Apolipophorin III in Structure and Function

摘要

Locusta migratoria apolipophorin III is an 18 kDa exchangeable apolipoprotein present in the hemolymph of locusts. In addition to its known role in diacylglycerol transport, the protein has antimicrobial properties targeting bacterial membranes. Apolipophorin III contains eight lysine residues, which interact with negatively charged membrane phospholipids. To better understand the importance of these lysines, a site-directed mutagenesis approach was employed. A series of lysine to glutamine variants were generated targeting lysines clusters in helix 2 and 5. The variants showed decreased helical structure and stability in particular when multiple residues were substituted. Ionic interactions with negatively charged phosphatidylglycerol (PG) were weakened in particular when seven of the eight lysines were substituted. In contrast, binding to phosphatidylcholine and lipoprotein was significantly improved when all lysines were substituted. This suggested that lysine residues are critical for protein structure and PG binding, but not for phosphatidylcholine and lipoprotein binding interaction.