Interaction of locust apolipophorin III with lipoproteins and phospholipidvesicles: effect of glycosylation

Apolipophorin III (apoLp-III) from Locusta migratoria is an exchangeable ap olipoprotein that binds reversibly to lipoprotein surfaces, The native prot ein is glycosylated at Asn-18 and Asn-85, Variable attachment of five disti nct oligosaccharide moieties at the two glycosylation sites results in mole cular weight heterogeneity, as seen by mass spectrometry. The main mass pea k of 20,488 Da decreases to 17,583 Da after removal of carbohydrate, indica ting that apoLp-III carbohydrate mass is similar to 14% by weight. Deglycos ylated apoLp-III induced clearance of dimyristoylphosphatidylcholine and di myristoylphosphatidylglycerol vesicles at a faster rate than glycosylated a poLp-III. However, in lipoprotein binding: assays, in which apoLp-III inter acts with surface-localized diacylglycerol, only minor differences in bindi ng were observed, The fluorescence properties of 1-anilinonaphthalene-8-sul fonate were unaffected by the glycosylation state of apoLp-III, indicating that no changes in the relative amount of exposed hydrophobic surface occur red as a result of carbohydrate removal. We propose that glycosyl moieties affect the ability of apoLp-III to transform phospholipid bilayer vesicles into disc-like complexes by steric hindrance. This is due to the requiremen t that apoLp-III penetrate the bilayer substrate prior to conformational op ening of the helix bundle. On the other hand, the glycosyl moieties do not affect lipoprotein binding interactions as it does not involve deep protein penetration into the lipid milieu, Rather, lipoprotein binding is based on oriented protein contact with. the Lipid surface followed by opening of th e helix bundle, which allows formation of a stable interaction with surface exposed hydrophobic sites.