Functional and structural properties of lipid-associated apolipoprotein J (clusterin)

Apolipoprotein J (apoJ, clusterin) is a multifunctional protein normally as sociated with lipids in plasma and cerebrospinal fluid, and secreted as lip oparticles by hepatocytes and astrocytes. To investigate whether the struct ural and functional properties of apoJ are modulated upon binding to lipids , we prepared apoJ high-density lipoprotein (HDL)-like particles employing either synthetic or plasma HDL-derived lipids. The majority of the resultin g lipoparticles contained one molecule of apoJ per particle and exhibited t he same alpha 2 electrophoretic mobility characteristic of apoJ-containing plasma HDL. Particle size seemed to be dependent on the presence of cholest erol in the lipid mixture and ranged from diameters of 10 nm in the presenc e of cholesterol to 20nm in the absence of cholesterol. CD analysis and Fou rier-transform infrared spectroscopy revealed similar changes in the apoJ s econdary structure induced by its incorporation into lipoparticles, namely a decrease in alpha-helix content and an increase in beta-turn structures. Two functional assays, the binding interaction with Alzheimer's amyloid bet a peptides and the inhibitory activity of the complement membrane-attack co mplex, did not detect any changes in apoJ activity following its lipidation (P > 0.05). On the contrary, the binding affinity to the cellular receptor megalin was enhanced significantly (P < 0.01) after the association with l ipids; the K-d value decreased from 78.8 +/- 10.7 nM for the delipidated fo rm to 37.0 +/- 7.3 nM for apoJ-HDL. Although it is not known whether the st ructural changes observed are directly responsible for the higher receptor- binding affinity, the data suggest that the complement inhibition and amylo id beta-binding motifs are located in areas of the molecule different from those involved in the apoJ-megalin interaction.