Apolipophorin III: role model apolipoprotein.

It has been one-quarter century since the identification of apolipophorin III (apoLp-III) as an important component of insect hemolymph lipid transport processes. Original studies of flight-related lipid transport that led to the discovery of apoLp-III have been followed by detailed studies of its structure and function relations, species distribution as well as its physiological roles beyond lipid transport. The non-exchangeable apoLp-I and -II, which are derived from a common precursor, are structural protein components of the multifunctional lipophorin particle. ApoLp-I/II have been identified as members of a broad lipid-binding protein family based on sequence similarities with their vertebrate counterparts. By contrast, apoLp-III can be found as a lipid-free hemolymph protein that associates with lipophorin during hormone-induced lipid mobilization. Based on structural characterization, apoLp-III belongs to a large family of exchangeable apolipoproteins characterized by segments of amphipathic alpha-helix. The remarkable structural adaptability of apoLp-III can be ascribed to its globular amphipathic alpha-helix bundle conformation wherein hydrophobic lipid-binding regions are stabilized in the absence of lipid by helix-helix interactions. Upon exposure to potential lipid surface-binding sites, the globular helix bundle opens to expose its hydrophobic interior permitting substitution of helix-helix contact in the bundle for helix-lipid interactions. Novel functions of apoLp-III beyond lipid transport have been identified recently. The expanding role of apoLp-III in innate immunity promises to offer exciting research opportunities in the future.