A key point mutation (V156E) affects the structure and functions of human apolipoprotein A-I

A naturally occurring point mutant of human apolipoprotein A-I (apoA-I), V1 56E, which is associated with extremely low plasma apoA-I and high density lipoprotein (HDL) levels, and coronary artery disease (Huang, W., Sasaki, J ., Matsunaga, A., Nanimatsu, Il., Moriyama, K., Hen, H. Kugi, M., Koga, T., Yamaguchi, K., and Arakawa, K. (1998) Arterioscler. Throm. Vase. Biol. 18, 389-396), was produced in an Escherichia coli expression system. The purif ied recombinant proapoA-I V156E mutant was examined in its structural and f unctional properties, both, in the lipid-free and lipid-bound states. In th e lipid-free form the mutant protein exhibited small changes in conformatio n, but was more stable, and quite resistant to self-association, compared w ith control apoA-I. The V156E mutant was able to interact with phospholipid (PL) at high PL:protein ratios (95:1, mol/mol), but was inefficient in for ming reconstituted HDL (rHDL) complexes at lower PL:protein ratios (40:1). In the lipid-bound, rHDL state, the mutant protein was somewhat more alpha- helical and formed a larger complex (110 Angstrom) than control apoA-I (97 Angstrom). Furthermore, the rHDL particles containing the V156E mutant did not rearrange to smaller particles in the presence of low density lipoprote ins, and had minimal reactivity with lecithin-cholesterol acyltransferase ( LCAT), compared with rHDL particles made with control apoA-I. These results suggest a key role for Val-156, or the adjacent central region of apoA-I i n the modulation of apoA-I conformation, stability, and self-association in solution, and in the formation of small HDL, the conformational adaptabili ty of apoA-I leading to structural rearrangements of HDL, and the activatio n of LCAT.