Antibodies to Complementary Peptides as Probes for Receptors

Abstract

Peptide hormones initiate their physiological responses by binding to receptor proteins embedded in the plasma membranes of their target cells. Mechanisms accounting for specific protein-protein interactions, such as peptide hormone binding by cell receptors or epitope binding by antibody have not been defined. A fundamental tenet of the immunological network hypothesis is the generation of anti-idiotypic antibodies to epitopes located in the hypervariable regions of antibody evoked in the same animal species. Anti-idiotypic antibodies to antibodies to peptide hormones with specificity for epitopes involving antibody binding sites may mimic the actions of the peptide hormone by binding to receptors and evoke cell responses associated with the hormone. A provocative relationship was identified in the genetic code, which recognized that complementary codons for strongly hydrophobic amino acids code for strongly hydrophilic amino acids. This led to the proposal and then to demonstration that peptide pairs based on the nucleotide sequences of complementary codons bind one another. It was then proposed that immunization with complementary peptides to peptide hormones may produce antibodies which, analogous to anti-idiotypic antibodies, may mimic the hormone. Some antibodies to complementary peptides for peptide hormones have been shown to mimic the peptide hormones by binding to their receptors and evoking cell responses characteristic of those of the hormones. Exploiting these relationships, some antibodies to complementary peptides for peptide hormones have been used to identify, purify, and characterize receptor proteins for peptide hormones. Polypeptide hormones initiate their characteristic physiologic effects by binding to specific receptor proteins located on the plasma membranes of their target cells. Cell receptor proteins for peptide hormones vary in their structural characteristics, in their mechanisms for signal transduction, and in their affinity and specificity for their peptide ligands. Receptor proteins have been classified based on their transmembrane-spanning segments, identified by defined consecutive groupings of hydrophobic and hydrophilic amino acids and the signal transduction mechanisms by which they evoke the intracellular events leading to physiological responses characteristically identified with the hormone. As for other proteins, the functions of receptor proteins, including the specificity and affinity of their ligand binding, are dictated by their primary amino acid sequence structures. Receptor proteins for peptide hormones in the plasma membrane are relatively few in number and undergo a series of intracellular trafficking steps following hormone binding. Protein-protein interactions are clearly of major biological importance and have been the subject of intensive investigation. These include interactions of antibody binding sites with their epitopes, proteolytic enzymes with their substrates, and receptor proteins with their peptide hormone ligands. The physical-chemical mechanisms which assign affinity and specificity for antibody binding of their epitopes and for binding of peptide hormones by their cell membrane receptors are not yet clearly understood. Structural complementarity has been viewed as a major, although not exclusive, component in producing protein-protein interactions. Examples include the configurations of antibody binding sites with their peptide epitopes and binding pockets of proteolytic enzymes with specific amino acid sequence sites for hydrolysis. This discussion summarizes the background, rationale, and application of antibodies to complementary peptides as probes for cell receptor proteins in studies in which these antibodies have been used to identify, characterize, and purify cell receptor binding proteins for peptide hormones.