Use of fluorescence energy transfer to characterize the compactness of the constant fragment of an immunoglobulin light chain in the early stage of folding.

The CL fragment of a type-K immunoglobulin light chain in which the C-terminal cysteine residue was modified with N-(iodoacetyl)-N'-(5-sulfo- 1-naphthy1)ethylenediamine (CL-AEDANS fragment) was prepared. This fragment has only one tryptophan residue at position 148. The compactness of the fragment whose intrachain disulfide bond was reduced in order for the tryptophan residue to fluoresce (reduced CL-AEDANS fragment) was studied in the early stages of refolding from 4 M guanidine hydrochloride by fluorescence energy transfer from Trp 148 to the AEDANS group. The AEDANS group attached to the SH group of a cysteine scarcely fluoresced when excited at 295 nm. For the reduced CL-AEDANS fragment, the fluorescence emission band of the Trp residue overlapped with the absorption band of the AEDANS group, and the fluoresence energy transfer was observed between Trp 148 and the AEDANS group in the absence of guanidine hydrochloride. In 4 M guanidine hydrochloride, the distance between the donor and the acceptor was larger, and the efficiency of the energy transfer became lower. The distance between Trp 148 and the AEDANS group for the intact protein estimated by using the energy-transfer data was in good agreement with that obtained by X-ray crystallographic analysis. By the use of fluoresence energy transfer, tryptophyl fluoresence, and circular dichroism at 2 18 nm, the kinetics of unfolding and refolding of the reduced fragment were studied. These three methods gave the same unfolding kinetic pattern. However, the refolding kinetics measured by fluoresence energy transfer were different from those measured by tryptophyl fluorescence and circular dichroism, the latter two giving the same kinetic pattern. In addition to the two phases observed by using tryptophyl fluoresence or circular dichroism, a very much faster phase was detected by fluoresence energy transfer. The energy-transfer efficiency reached the same level as that of the intact protein at a very early stage of refolding. Double-jump experiments also gave the same result. These findings indicate that a structure as compact as that of the native protein is formed immediately after refolding, and then the compact molecule converts slowly to the native protein by rearrangement of groups, probably involving cis-trans isomerization of the prolyl residue.