EVOLUTION OF MAMMALIAN APOLIPOPROTEIN-A-I AND CONSERVATION OF ANTIGENICITY - CORRELATION WITH PRIMARY AND SECONDARY STRUCTURE

We have evaluated the immunoreactivity of 20 monoclonal antibodies (mA bs) directed against human apolipoprotein (apo)A-I with a panel of hig h density lipoproteins (HDL) from 13 mammalian species. The pattern of cross-reactivity showed that 20 mAbs had different specificity. While not all mAbs recognized apoA-l from all of the different species, the antigenicity of some sequences was well conserved. Thus, mAb A05 cros s-reacted with all species except guinea pig and rat. In contrast, the mAb 4H1, which recognized residues 28, required a specific proline in position 3, as no immunoreactivity was found in the species missing t his amino acid. Furthermore, the presence of a threonine residue in pl ace of serine (in position 6) in the cynomolgus monkey was associated with a 20-fold loss of immunoreactivity in radioimmunometric assay wit h 4H1. As most of the epitopes were found in CNBr fragments 2 and 3, w e sequenced these regions in four species (horse, goat, sheep, and cat ) and analyzed the alignment of most known sequences to evaluate their consensus. Except for the rat and the chicken, considerable identity was observed. This permitted us to deduce the involvement of the resid ues in some antigenic epitopes. In the middle of apoA-I, a conservativ e mutation Asp(108) --> Glu was found sufficient to eliminate all reac tivity of this epitope for All (residues 99-105... 126-132) in five sp ecies (rabbit, cow, goat, sheep, and rat). The residues essential to t he expression of two other epitopes overlapping with All were also cha racterized. Edmundson-wheel representation of 18-residue repeated sequ ences of the different apoA-I species (for the eight amphipatic helice s of residues 46-63, 68-85, 101-115, 123-140, 143-160, 167-184, 189-20 6, and 222-239) showed that secondary structure of apoA-I was more con served than the antigenic epitopes. The N-terminal region, residues 1 to about 98, is rich in both strictly preserved sequences and epitope expression in most of the species surveyed. This evolutionary conserva tion of the N-terminal domain suggests an important yet unknown functi on.