Interaction of Daptomycin and Calcium with Lipid Bilayers

Daptomycin is a cyclic lipopeptide antibiotic recently approved for clinical treatment of gram positive bacterial infections. It displays strong bactericidal behavior in the presence of excess calcium ions and in biomembranes containing negatively charged PG headgroups. Yet, the molecular mechanism of daptomycin, beyond the suggestion of ion channel formation and cellular depolarization, remains unknown. Recent data has shown results inconsistent with ion channel formation, prompting a more detailed investigation of Daptomycin and calcium interactions with lipid bilayers at higher structural resolutions. In order to fully characterize the mechanism of Daptomycin, it is necessary to understand the role of individual components that are integral to Daptomycin's mode of action in the lipid bilayer. By using lamellar diffraction of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-sn-glycero-3-phosphoglycerol (DOPG) multilayers, we examined the individual and combined effects of Daptomycin and calcium ions on bilayer structure. We found that calcium ions alone result in a slight thickening effect of the bilayer, partially supporting molecular simulations of calcium ions and PG bilayers that propose a calcium ordering effect at physiological calcium concentrations. Inclusion of daptomycin alone produced a bilayer thinning effect, suggesting that Daptomycin is capable of inserting into the headgroup region of the lipid bilayer during binding. Lastly, samples that contained both Daptomycin and calcium ions showed a bilayer thickening effect. These results suggest that Daptomycin binds to the membrane in the absence of calcium ions but induces a structural thickening of the bilayer in the presence of calcium ions. The apparent thickening effect of Daptomycin is unlike other membrane-active antibiotics, such as melittin or LL-37, which induce bilayer thinning. These studies may form the basis for more detailed investigations to characterize the molecular mechanisms of Daptomycin and other cyclic lipopeptides.