Heterologous expression of the extracellular domains (ECDs) of the nicotinic acetylcholine receptor (AChR) subunits may give large amounts of proteins for studying the functional and spatial characteristics of their ligand-binding sites. The ECD of the α7 subunit of the homo-oligomeric α7 neuronal AChR appears to be a more suitable object than the ECDs of other heteromeric neuronal or muscle-type AChRs. The rat α7 ECDs (amino-acid residues ≈1-210) were recently expressed in Escherichia coli as fusion proteins with maltose-binding protein [Fischer, M., Corringer, P., Schott, K., Bacher, A. & Changeux, J. (2001) Proc. Natl Acad. Sci. USA 98, 3567-3570] and glutathione S-transferase (GST) [Utkin, Y., Kukhtina, V., Kryukova, E., Chiodini, F., Bertrand, D., Methfessel, C. & Tsetlin, V. (2001) J. Biol. Chem. 276, 15810-15815]. However, these proteins exist in solution mostly as high-molecular mass aggregates rather than monomers or oligomers. In the present work it is found that refolding of GST-α7-(1-208) protein in the presence of 0.1% SDS considerably decreases the formation of high-molecular mass aggregates. The C116S mutation in the α7 moiety was found to further decrease the aggregation and to increase the stability of protein solutions. This mutation slightly increased the affinity of the protein for α-bungarotoxin (from Kd ≈ 300 to 150 nM). Gel-permeation HPLC was used to isolate the monomeric form of the GST-α7-(1-208) protein and its mutant almost devoid of SDS. CD spectra revealed that the C116S mutation considerably increased the content of β structure and made it more stable under different conditions. The monomeric C116S mutant appears promising both for further structural studies and as a starting material for preparing the α7 ECD in an oligomeric form.