Apolipoprotein A-I (apoA-I) activates the plasma enzyme lecithin:cholesterol acyltransferase (LCAT), catalyzing the rapid conversion of lipoprotein cholesterol to cholesterol ester. Structural mutants of apoA-I have been used to study the details of apoA-I-LCAT-catalyzed cholesterol ester formation. Several studies have shown that the alpha-helical segments corresponding to amino acids 143-164 and 165-186 (repeats 6 and 7) are essential for LCAT activation. In the present studies, we examined how the orientation of the hydrophobic face, independent of an increase in overall hydrophobicity, affects LCAT activation. We designed, expressed, and characterized a mutant, reverse of 6 apoA-I (RO6 apoA-I), in which the primary amino acid sequence of repeat 6 (amino acids 143-164) was reversed from its normal orientation. This mutation rotates the hydrophobic face of repeat 6 approximately 80 degrees. Lipid-free RO6 apoA-I showed a marked stabilization when denatured by guanidine hydrochloride, but showed significant destabilization to guanidine hydrochloride denaturation in the lipid-bound state compared with wild-type apoA-I. Recombinant high density lipoprotein discs (rHDL) formed from RO6 apoA-I, sn-1-palmitoyl-sn-2-oleoyl phosphati-dylcholine, and cholesterol were approximately 12 A smaller than wild-type apoA-I rHDL. The reduced size suggests that one of the repeats did not effectively participate in phospholipid binding and organization. The sn-1-palmitoyl-sn-2-oleoyl phosphatidylcholine RO6 rHDL were a less effective substrate for LCAT. Mapping the entire lipid-free and lipid-bound RO6 apoA-I with a series of monoclonal antibodies revealed that both the lipid-free and lipid-bound RO6 apoA-I displayed altered or absent epitopes in domains within and adjacent to repeat 6. Together, these results suggest that the proper alignment and orientation of the hydrophobic face of repeat 6 is an important determinant for maintaining and stabilizing helix-bilayer and helix-helix interactions.