Localization of Gm markers to different molecular regions of the Fc fragment.

pFc' fragments (corresponding to homology region Cγ3) and smaller subfragments produced by trypsin and papain have been studied by haemagglutination inhibition techniques for the presence of a range of Fcγ allotypic markers. The markers which were identified in the pFc' fragment included Gm(a), (x), (b), (b), (b), (b), (c), (c) and non a'. In addition, an IgG 4 marker which possibly occupies the same molecular region as the Gm(a) and non a' antigens was detected in pFc' fragments from IgG 4 proteins. Limited tryptic cleavage of pFc' fragment resulting in the loss of approximately seven N-terminal residues had no effect on the antigenic expression of the genetic markers. More extensive degradation of pFc' fragments with papain yielded Fc'-like fragments which are presumed to lack a C-terminal tridecapeptide as well as eight or nine N-terminal residues. In these fragments antigenic expression of Gm(a), (x) and non a' was lost, but Gm(b), (b), (b) and (b) antigens were still detectable. The other allotypic markers of the Fc region (Gm(n), (b), (g) and non g') were not detected in any of the fragments investigated and it is probable that they are in homology region Cγ2.