The structure and function of immunoglobulin domains: studies with beta-2-microglobulin on the role of the intrachain disulfide bond.

Beta-Microglobulin, a low-molecular-weight protein structurally related to the homology regions of immunoglobulins, has been used to study the role of the intrachain disulfide bond in the unfolding of immunoglobulin domains. The intact protein could be reversibly unfolded in guanidine hydrochloride, as judged by circular dichroism and optical rotation. Similarly, reoxidation of the reduced protein, during transfer from high concentrations of guanidine to neutral aqueous buffer, yielded a product with spectral characteristics typical of the native protein. However, if the free SH groups were prevented from reoxidizing either by chemical modification or by holding them in the reduced state, the molecule appeared to be in the randomly coiled state even under conditions where the intact protein is in the native conformation, judged on the basis of chiroptical measurements. The complement-fixing activity exhibited by native beta-2-microglobulin was retained by the reduced and alkylated derivative, suggesting that the site may be formed from a linear array of amino acids. We suggest a model for the folding of beta-2-microglobulin (and immunoglobulin domains) in which one of the early folding events results in disulfide bond formation, the latter being an obligatory step for continued folding to the native state.