Evolutionary substitutions and the antigenic structure of globular proteins

Abstract

A FEW years ago, Crumpton¹ and Reichlin² reviewed information about the antigenic structure of globular proteins of known amino acid sequence and three-dimensional structure. It seemed that only a small fraction of the amino acid residues in such proteins participated directly in antibody binding. This impression was reinforced by Atassi and co-workers, who proposed antigenic structures of two such proteins—sperm whale myoglobin³ and chicken lysozyme c (ref.4). These workers claim to have identified all those amino acid residues which bind to antibodies present in antisera directed against the protein. These claims stem from extensive studies with chemically modified forms of the protein as well as with antigenically active fragments obtained by specific cleavage of the protein. The antigenic residues are arranged in groups called determinants, each of which binds a specific class of antibodies. A typical determinant contains four to seven residues. For both lysozyme and myoglobin, the proportion of antigenic residues is about 15% of the total number of amino acid residues in the protein^3,4. Information has also accumulated concerning the power of antisera to detect evolutionary substitutions of amino acids in globular proteins. Quantitative immunological comparisons of related proteins of known amino acid sequence have consistently shown a strong correlation between degree of sequence difference and degree of antigenic difference. This has been demonstrated most thoroughly for monomeric globular proteins such as lysozymes^5,6, ribonucleases⁷, azurins⁸ and cytochromes c (refs 9, 10). Less complete information suggests that the correlation holds also for plastocyanins¹⁰, tryptophan synthetase α subunits¹¹, serum albumins¹², carbonic anhydrases and myoglobins¹³ (A. B. Champion, E. M. Prager, S. L. Welch, and A. C. Wilson, unpublished), and ferredoxins¹⁴. From the strength of the correlations observed for lysozymes, ribonucleases, azurins and cytochromes (r > 0.9), one may make the statistical inference that about 80% (that is, 0.9 squared) of those amino acid substitutions which accumulate during evolution are immunologically detectable⁷. Here we summarise additional evidence that the majority of evolutionary substitutions are immunologically detectable, and we discuss how to reconcile this evidence with the chemical evidence that only a small fraction of the amino acids are antigenic.