Factors Affecting the Expression of Apolipoprotein A-I Epitopes and Screening for Mutants

Abstract

Before applying monoclonal antibodies (Mabs) to the screening of apolipoprotein mutants, we should first consider the limitations of such an approach. To detect mutations randomly distributed along the primary sequence of an antigen, we need antibodies directed against epitopes that are also widely distributed. Early experimental evidence indicated that discrete antigenic epitopes exist, implying that specific regions of a protein are more antigenic than others. However as more experimental data accumulate, it appears that many overlapping epitopes exist on protein surfaces and that the whole surface may be antigenic¹. Nevertheless, accessibility, flexibility and shape will determine which sites on proteins are most likely to be antigenic². Novotny et al³ have proposed that the primary reason for the antigenicity of certain polypeptide-chain segments is their exceptional surface exposure which make them readily available for contact with antibody combining sites. Because exposure of peptide segments results frequently in high mobility, the reported correlation between antigenicity and segmental flexibility may in fact be secondary to the first correlation between accessibility and antigenicity. These principles imply that, in the case of an apolipoprotein the amino acids that constitute the hydrophobic face of the amphipathic α-helices and β-sheets and are in contact with the lipid phase of the lipoproteins, should be the least susceptible to represent the contact residues of an epitope. The opposite might be true for the amino acids constituting the hydrophilic face which should be both highly accessible and antigenic.