Immunochemical characteristics of subfragments from the CH3 homology region of human IgG.

A subfragment (peak IV) of Fc from human IgG belonging to the CH3 region and found to be slightly different from both pFc and tFc' fragments was released after tryptic digestion of acid-treated Fc. Gel filtration at dissociating conditions of reduced and alkylated fragments indicated a molecular weight of 10,200. The Fc, pFc', tFc', and peak IV fragments all interacted with a rheumatoid factor having anti-Gm(1) specificity and showed a reaction of identity in agar diffusion against both anti-Fc and anti-pFc' sera. In immunoelectrophoresis, peak IV and tFc' fragments behaved similarly but with a more anodical movement than pFc' fragments. The probable location of the new fragment in terms of the sequenced gamma1-chain is proposed to be the residues 335 to 439.