TOPOGRAPHIC DETERMINANTS ON CYTOCHROME-C .1. COMPLETE ANTIGENIC STRUCTURES OF RABBIT, MOUSE, AND GUANACO CYTOCHROMES-C IN RABBITS AND MICE

  • 1 January 1977
    • journal article
    • research article
    • Vol. 118  (4) , 1170-1180
Abstract
Rabbit, mouse, and guanaco cytochromes c differ from each other by only 2 amino acid residues. The identification is described of all of the antigenic determinants of mouse and guanaco cytochromes c that elicit an antibody response in rabbits, and those of the rabbit and guanaco proteins that elicit antibodies in the mouse. All except one of these sites center around single amino acid residue differences between the antigen and the host cytochrome c. The corresponding antibody populations bind only to the areas of the protein in which the substitutions occur. Such antigenic determinants manifested in rabbits by guanaco and mouse cytochromes c are centered around residues 62 and 89, and residues 44 and 89, respectively. Mouse antibody recognizes sites containing residues 44 and 62 in guanaco cytochrome c, and residues 44 and 89 in rabbit cytochrome c. In no case has a change in sequence failed to produce an antibody response. Each of these determinants appears to elicit and bind to its antibody, independently of other determinants present on the protein. Two different autoantigenic responses were detected. The antibodies produced against the determinant formed by glutamyl residue 62 of the guanaco protein in rabbits and mice, the cytochromes c of which carry an aspartyl residue in that position, also bind to the aspartyl-containing region, but with lower affinity. Mouse and rabbit cytochrome c also elicit antibodies to the area of residue 62 in rabbits and mice, respectively; these antibodies still bind more strongly to the glutamyl- than to the aspartyl-containing determinant. This last response occurs only when there are residue substitutions elsewhere in the molecule, because mice and rabbits fail to respond to their own cytochrome c. Antibodies produced in mice against the change from alanyl to valyl residue 44 by rabbit and guanaco cytochromes c also bind to the alanyl-containing determinant, but they bind less tightly than to the valyl region. Antibodies raised in rabbits against the change from valyl to alanyl residue 44 only bind to this region when it carries an alanine. Antigenic determinants that arise as a result of amino acid residue substitutions between the immunizing and the corresponding host protein, without a change in the spatial arrangement of the polypeptide backbone, should be termed topographic determinants.