Unfolding and refolding of a type kappa immunoglobulin light chain and its variable and constant fragments.

By limited proteolysis of a type kappa immunoglobulin light chain (Oku) with clostripain, both the VL and CL fragments were obtained with a high yield. The unfolding and refolding by guanidine hydrochloride of light chain Oku and its VL and CL fragments were studied at pH 7.5 and 25 degrees C with circular dichroism and tryptophyl fluorescence. The concentration of guanidine hydrochloride at the midpoint of the unfolding curve was 1.2 M for the VL fragment and 0.9 M for the CL fragment. As in the case of the CL fragment of light chain Nag (type lambda) [Goto, Y., & Hamaguchi, K. (1982) J. Mol. Biol. 156, 891-910], the unfolding and refolding kinetics of the CL fragment could be explained in principle on the basis of the three-species mechanism U1 in equilibrium U2 in equilibrium N, where N is native protein and U1 and U2 are the slow-folding and fast-folding species, respectively, of unfolded protein. The unfolding and refolding kinetics of the VL(Oku) fragment were described by a single exponential term. Double-jump experiments, however, showed that two forms of the unfolding molecule exist. The equilibrium and kinetics of unfolding of light chain Oku were explained by the sum of those of the VL and CL fragments. On the other hand, the refolding kinetics of light chain Oku were greatly different from the sum of those of the VL and CL fragments.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of amino-terminal residues in the folding of the constant fragment of the immunoglobulin light chain.

Three constant fragments with different amino terminals, CL(105-214), CL(109-214), and CL(113-214), were obtained by limited proteolysis with trypsin or papain of a type lambda immunoglobulin light chain. The conformations of the three CL fragments were indistinguishable on the basis of circular dichroism and tryptophyl fluorescence spectra. The stability to heat and guanidine hydrochloride of CL(105-214) was almost the same as that of CL(109-214), but the stability of CL(113-214) was slightly lower than that of CL(105-214) or CL(109-214). The midpoint of the thermal unfolding transition at pH 7.5 was at 60.0 degrees C for CL(105-214), 60.4 degrees C for CL(109-214), and 57.5 degrees C for CL(113-214). The midpoint of the unfolding transition by guanidine hydrochloride at pH 7.5 and 25 degrees C was 1.2 M for CL(105-214) and CL(109-214) and at 1.0 M for CL(113-214). The kinetics of unfolding and refolding by guanidine hydrochloride of these CL fragments were analyzed on the basis of the three-species mechanism, U1 in equilibrium with U2 in equilibrium with N, where U1 and U2 are the slow-folding and fast-folding species, respectively, of unfolded protein and N is native protein. It was found that only the microscopic unfolding rate constant for CL(113-214) is 2-3 times greater than that for CL(105-214) or CL(109-214) and that the other microscopic rate constants for the three CL fragments are all the same. These findings indicated that the amino-terminal residues, Gly-109-Lys-112, or a part of them, stabilize the CL(113-214) fragment by decreasing only the unfolding rate, that the transition state of the folding of the CL fragment is independent of the presence or absence of this peptide, and that, at the last step of folding, the peptide is incorporated into the globular domain, thus stabilizing it.