Folding Mechanism of the CH2 Antibody Domain

The immunoglobulin CH2 domain is a simple model system suitable for the study of the folding of all-β-proteins. Its structure consists of two β-sheets forming a greek-key β-barrel, which is stabilized by an internal disulfide bridge located in the hydrophobic core. Crystal structures of various antibodies suggest that the CH2 domains of the two heavy chains interact with their sugar moieties and form a homodimer. Here, we show that the isolated, unglycosylated CH2 domain is a monomeric protein. Equilibrium unfolding was a two-state process, and the conformational stability is remarkably low compared to other antibody domains. Folding kinetics of CH2 were found to consist of several phases. The reactions could be mapped to three parallel pathways, two of which are generated by prolyl isomerizations in the unfolded state. The slowest folding reaction, which was observed only after long-term denaturation, could be catalyzed by a prolyl isomerase. The majority of the unfolded molecules, however, folded more rapidly, on a time-scale of minutes. Presumably, these molecules also have to undergo prolyl isomerization before reaching the native state. In addition, we detected a small number of fast-folding molecules in which all proline residues appear to be in the correct conformation. On both prolyl isomerization limited pathways, the formation of partly structured intermediates could be observed.