An N-terminal three-helix fragment of the exchangeable insect apolipoprotein apolipophorin III conserves the lipid binding properties of wild-type protein.

Apolipophorin III (apoLp-III) from the greater wax moth Galleria mellonella is an exchangeable insect apolipoprotein that consists of five amphipathic alpha-helices, sharing high sequence identity with apoLp-III from the sphinx moth Manduca sexta whose structure is available. To define the minimal requirement for apoLp-III structural stability and function, a C-terminal truncated apoLp-III encompassing residues 1-91 of this 163 amino acid protein was designed. Far-UV circular dichroism spectroscopy revealed apoLp-III(1-91) has 50% alpha-helix secondary structure content in buffer (wild-type apoLp-III 86%), increasing to essentially 100% upon interactions with dimyristoylphosphatidylcholine (DMPC). Guanidine hydrochloride denaturation studies revealed similar stability properties for wild-type apoLp-III and apoLp-III(1-91). Resistance to denaturation for both proteins increased substantially upon association with phospholipid. In the absence of lipid, wild-type apoLp-III was monomeric whereas apoLp-III(1-91) partly formed dimers and trimers. Discoidal apoLp-III(1-91)-DMPC complexes were smaller in diameter (13.5 nm) compared to wild-type apoLp-III (17.7 nm), and more molecules of apoLp-III(1-91) associated with the complexes. Lipid interaction revealed that apoLp-III(1-91) binds to modified spherical lipoprotein surfaces and efficiently transforms phospholipid vesicles into discoidal complexes. Thus, the first three helices of G. mellonella apoLp-III contain the basic features required for maintenance of the structural integrity of the entire protein.