Studies on the amino acid sequence of heavy chains from rabbit immunoglobulin G

Abstract

It is now generally agreed that the four-chain subunit structure of Immunoglobulins which was first proposed by Porter (1962), accurately represents the gross structure of immunoglobulin G (IgG) and specific antibodies (Fleischman, Porter & Press 1963; Edelman & Gally 1964; Marler, Nelson & Tanford 1964; Nelson et al . 1965). However, an understanding of the structural basis of antibody specificity requires greater insight into the amino acid sequence of the polypeptide chain components of specific antibodies. Isolated light chains from specific antibodies and inert IgG, show a considerable degree of electrophoretic heterogeneity (Edelman & Gally 1964; Cohen & Porter 1964; Poulik 1964). Tryptic peptide maps of light chains (Nelson et al . 1965) have suggested that this heterogeneity may be accounted for by differences in amino acid sequence. This view has received considerable support from the observation that Bence-Jones proteins, which may be regarded as light chains, vary significantly in amino acid sequence (Hilschman & Craig 1965; Milstein 1966; Titani, Whitley & Putman 1966). A similar but less well-defined sequence heterogeneity has been suggested to exist in the heavy chains of specific antibodies (Feinstein 1964). However, the Fc fragment of the heavy chains has been thought to possess a regular amino acid sequence which may be similar, if not identical, among all specific antibodies (Porter 1959; Nelson et al . 1965). This paper summarizes the results of studies on the amino acid sequence of heavy chains and that portion of heavy chain, Fc fragment, which is obtained on treatment of rabbit IgG with papain (Porter 1959). These studies were designed to determine how much of the amino acid sequence of heavy chain could be accounted for by a unique, regular amino acid sequence which was common to most, if not all, IgG antibodies. In addition, attempts were made to locate regions of heavy chains which varied in amino acid sequence. Although structural variants appear to occur among the heavy chains found in non-specific IgG, it would be desirable to know what portion of the heavy chain sequence is invariant among all antibodies. If antibody specificity results from sequence heterogeneity in light and heavy chains, then knowledge of the variant and invariant portions of these chains may provide insight into the nature of specific binding sites in anti-­bodies.