A systematic investigation of structure/function requirements for the apolipoprotein A-I/Lecithin cholesterol acyltransferase interaction loop of high-density lipoprotein

The interaction of lecithin-cholesterol acyltransferase (LCAT) with apolipoprotein A-I (apoA-I) plays a critical role in highdensity lipoprotein (HDL) maturation.Wepreviously identified a highly solvent-exposed apoA-I loop domain (Leu159-Leu170) in nascent HDL, the so-called "solar flare" (SF) region, and proposed that it serves as an LCAT docking site (Wu, Z., Wagner, M. A., Zheng, L., Parks, J. S., Shy, J. M., 3rd, Smith, J. D., Gogonea, V., and Hazen, S. L. (2007) Nat. Struct. Mol. Biol. 14, 861-868). The stability and role of the SF domain of apoA-I in supporting HDL binding and activation of LCAT are debated. Here we show by site-directed mutagenesis that multiple residues within the SF region (Pro165, Tyr166, Ser167, and Asp168) of apoA-I are critical for both LCAT binding to HDL and LCAT catalytic efficiency. The critical role for possible hydrogen bond interaction at apoA-I Tyr166 was further supported using reconstituted HDL generated from apoA-I mutants (Tyr1663Glu or Asn), which showed preservation in both LCAT binding affinity and catalytic efficiency. Moreover, the in vivo functional significance ofNO2-Tyr166- ApoA-I, a specific post- Translational modification on apoA-I that is abundant within human atherosclerotic plaque, was further investigated by using the recombinant protein generated from E. coli containing a mutated orthogonal tRNA synthetase/tRNACUA pair enabling site-specific insertion.