Importance of central alpha-helices of human apolipoprotein A-I in the maturation of high-density lipoproteins.

We have studied the role of amphipathic alpha-helices in the ability of apoA-I to promote cholesterol efflux from human skin fibroblasts and activate lecithin:cholesterol acyltransferase (LCAT). Three apoA-I mutants were designed, each by deletion of a pair of predicted adjacent central alpha-helices [Delta(100-143), Delta(122-165), Delta(144-186)], and expressed in Escherichia coli. This strategy was used to minimize disruption of the predicted secondary structure of the resulting protein. These three central deletion mutants have been previously shown to be expressed as stable folded proteins but to exhibit altered phospholipid-binding properties. When recombined with phospholipids to form homogeneous LpA-I containing equivalent amounts of POPC and tested for their ability to promote diffusional cholesterol efflux from normal [3H]cholesterol-labeled fibroblasts, each mutant and the wild-type recombinant protein (Rec.-apoA-I) promoted cholesterol efflux with very similar rates at all the concentrations tested. These experiments showed that all LpA-I could acquire cellular cholesterol with similar affinity and binding capacity. However, when the cell-incubated LpA-I were incubated with purified LCAT, two mutants, Delta(122-165) and Delta(144-186), appeared incapable of activating the enzyme. To directly determine their ability to activate LCAT, each mutant and the control were recombined with equivalent amounts of cholesterol and phospholipid and incubated with the purified enzyme. The results show that whereas deletion of residues 100-143 has little effect on LCAT activation, deletion of residues 122-165 or 144-186 results in an inability of the mutants to promote cholesterol esterification. In conclusion, our results show that no specific sequence in the central domain of apoA-I is required for efficient diffusional cholesterol efflux from normal fibroblasts; however, residues 144-186 appear critical for optimum LCAT activation and cholesteryl ester accumulation. Since deletion of residues 144-186 also perturbs phospholipid association and prevents the formation of large LpA-I particles [Frank, P. G., Bergeron, J., Emmanuel, F., Lavigne, J. P., Sparks, D. L., Denèfle, P., Rassart, E., and Marcel, Y. L. (1997) Biochemistry 36, 1798-1806], the data show that this pair of alpha-helices plays an important role in the maturation of HDL. Sequence analysis of these apoA-I helices further identifies specific residues that appear essential to this activity.