Role of the Arg123–Tyr166 Paired Helix of Apolipoprotein A-I in Lecithin:Cholesterol Acyltransferase Activation

The Arg123-Tyr166 central and Ala190-Gln243 carboxyl-terminal pairs of helices of apoA-I were substituted with the pair of helices of apoA-II, resulting in the apoA-I(Delta(Arg123-Tyr166), nablaA-II(Ser12-Ala75)) and apoA-I(Delta(Ala190-Gln243), nablaA-II(Ser12-Gln77)) chimeras, respectively. The structures of these chimeras in aqueous solution and in reconstituted high density lipoproteins (rHDL) and the lecithin:cholesterol acyltransferase (LCAT) activation properties of the rHDL were studied. Recombinant human apoA-I and the chimeras were expressed in Escherichia coli and purified from the periplasmic space. Binding of the apolipoproteins with palmitoyloleoylphosphatidylcholine was associated with a similar shift of Trp fluorescence maxima from 337 to 332 nm, from 339 to 334 nm, and from 337 to 333 nm, respectively. All rHDL had a Stokes radius of 4.8 nm and contained 2 apolipoprotein molecules/particle. Circular dichroism measurements revealed eight alpha-helices per apoA-I and per chimera molecule. The catalytic efficiencies of LCAT activation were 1.5 +/- 0.33 (mean +/- S.D.; n = 3), 0.054 +/- 0.009 (p < 0.001 versus apoA-I), and 1.3 +/- 0.32 (p = not significant versus apoA-I) nmol of cholesteryl ester/h/microM, respectively. The lower LCAT activity of the central domain chimera was due to a 27-fold reduced Vmax with unaltered Km. Binding of radiolabeled LCAT to rHDL of apoA-I and apoA-I(Delta(Arg123-Tyr166), nablaA-II(Ser12-Ala75)) was very similar. In conclusion, although substitution of the Arg123-Tyr166 central or Ala190-Gln243 carboxyl-terminal pair of helices of apoA-I with the pair of helices of apoA-II yields chimeras with structure similar to that of native apoA-I, exchange of the central domain (but not the carboxyl-terminal domain) of apoA-I reduces the rate of LCAT activity that is independent of binding to rHDL.