Structural determination of lipid-bound ApoA-I using fluorescence resonance energy transfer

Based on the x-ray crystal structure of lipid-free Delta 43 apoA-1, two monomers of apoA-1 were suggested to bind to a phospholipid bilayer in an antiparallel paired dimer, or belt orientation. This hypothesis challenges the currently held model in which each of the two apoA-I monomers fold as antiparallel alpha -helices or pick-et fence orientation When apoA-I is bound to a phospholipid disc, the first model predicts that the glutamine at position 132 on one apoA-I molecule lies within 16 Angstrom of glutamine 132 in the second monomer, whereas, the second model predicts glutamines at position 132 to be 104 Angstrom apart. To distinguish between these models, glutamine at position 132 was mutated to cysteine in wild-type apoA-I to produce Q132C apoA-I, which were labeled with thiol-reactive fluorescent probes. Q132C apoA-I was labeled with either fluorescein (donor probe) or tetramethylrhodamine (acceptor probe) and then used to make recombinant phospholipid discs (recombinant high density lipoprotein (rHDL)). The rHDL containing donor- and acceptor-labeled Q132C apoA-I were of similar size, composition, and lecithin:cholesterol acyltransferase reactivity when compared to rHDL- containing human plasma apoA-I. Analysis of donor probe fluorescence showed highly efficient quenching in rHDL containing one donor- and one acceptor-labeled Q132C apoA-I. rHDL containing only acceptor probe-labeled Q132C apoA-I showed rhodamine selfquenching. Both of these observations demonstrate that position 132 in two lipid-bound apoA-I monomers were in close proximity, supporting the ''belt conformation hypothesis for apoA-I on rHDL.