Formation of a Tight Complex between Amphidinol 3 and Sterols in Lipid Bilayers Revealed by Short-Range Energy Transfer

The exploration of molecular recognition in lipid bilayers is still extremely difficult. In this report, we leveraged short-range energy transfer (ET) that enabled detection of close contacts within 1 nm distance, and applied it to the interaction of natural products with sterols in lipid bilayers. Amphidinol 3 (AM3), a polyhydroxy-polyene metabolite from a marine dinoflagellate, possesses potent antifungal activity by forming membrane pores in a sterol-dependent manner. Although AM3 was shown to interact directly with membrane sterols, the mode of interaction is yet to be fully elucidated. Herein, we found that AM3 and cholestatrienol (CTL), a fluorescent sterol, can be an ET pair because the emission spectrum of the former overlaps with the excitation of the latter. We further confirmed that CTL exerts the sterol-dependent pore formation of AM3 as in the case of cholesterol. Then, titration using intermolecular ET in bilayers revealed that AM3 and CTL form a 1:1 complex with a dissociation constant of 1.4 x 10(-5) M. The distance between the AM3 polyene and CTL triene was estimated to be less than 1 nm. Based on this information, we proposed a tight binding model between AM3 and the sterol in lipid bilayers.