Unusual molecular properties of human IgG3 proteins due to an extended hinge region.

The molecular weight of IgG3 myeloma proteins is about 170,000 as judged by gel filtration analyses of the light and heavy chains in 5 M guanidine-1 M acetic acid. The heavy chain of IgG3 has a molecular weight of about 60,000, which is larger by 9,000 to 10,000 than the molecular weight of the heavy chain of IgGl. Under nondenaturing conditions, an apparent molecular weight of 200,000 is obtained for the intact IgG3 protein. A similar molecular weight discrepancy is found when the pepsin fragment, F(ab’)*, is analyzed by gel filtration under dissociating and nondissociating conditions. In contrast, the papain fragments, Fc and Fab, showed no anomalous gel filtration behavior when molecular weights were determined in the presence or in the absence of guanidine HCl. On the basis of these observations, and from immunological comparisons and amino acid compositions of the IgG3 proteins and their enzymic fragments, it is proposed that the IgG3 heavy chain has a hinge region of increased length. It is suggested that this extended hinge region results in an unusual molecular size and shape for the protein, which consequently behaves abnormally during gel filtration. This abnormal hinge region explains the appearance of Fab’-like and Fch fragments, obtained by pepsin and papain treatment, respectively. Both fragments contain partly intact hinge regions and on gel filtration under nondissociating conditions behave as species with an apparent molecular weight of about 100,000, while the correct values are 60,000 to 70,000.