Examination of the structure/function relationship in the exchangeable apolipoprotein, apolipophorin-III

Exchangeable apolipoproteins are proteins that reversibly bind lipoprotein particles to facilitate their transport in vivo. The structure/function rel ationship of apolipophorin-III (apo-III), the only insect exchangeable apol ipoprotein, has been investigated by Examining the association of this prot ein with lipid vesicles. The importance of a conserved leucine residue, rep orted to be essential for apo-III binding to lipids, has been evaluated thr ough site-directed mutagenesis. A unique cysteine replaces the conserved le ucine at position 30 in recombinant apo-III (L30C protein). This substituti on results in the covalent dimerization of the apo-III mutant via a disulfi de bond. The cysteine mutation causes no difference in surface hydrophobici ty of the L30C proteins when compared to the wild type apo-III. Wild type a po-III, L30C monomer, and L30C dimer associate with dimyristoylphosphatidyl choline (DMAC) vesicles in a similar manner, resulting in a reduction of tu rbidity of a phospholipid vesicle suspension. Analysis with transmission el ectron microscopy (TEM) reveals disk-like complexes identical to those prev iously reported,with the wild type ape-ill. Because the mutation of the con served leucine seems to affect the solution behavior and surface hydrophobi city of apo-III, this residue is likely to be exposed to the aqueous enviro nment. However, the similar behaviors of the wild type protein, the L30C mo nomer, and L30C dimer with respect to the binding of phospholipid vesicles suggest that this residue is nor absolutely required for the protein bindin g to hydrophobic or amphiphilic interfaces. (C) 1999 John Wiley & Sons, Inc .