Physicochemical and immunological studies of the N-terminal domain of the Torpedo acetylcholine receptor alpha-subunit expressed in Escherichia coli

The nicotinic acetylcholine receptor (AChR) from the electric organ of Torpedo species is an oligomeric protein composed of alpha2 beta gamma delta subunits. Although much is known about its tertiary and quaternary structure, the conformation of the large extracellular domains of each of the subunits has not been analysed in detail. In order to obtain information about the spatial structure of the extracellular domain, we have expressed the N-terminal fragment 1-209 of the Torpedo californica AChR alpha-subunit in Escherichia coli. Two vectors coding for a (His)6 tag, either preceding or following the 1-209 sequence, were used and the recombinant proteins obtained (designated alpha1-209pET and alpha1-209pQE, respectively) were purified by affinity chromatography on a Ni2+-agarose column. The chemical structure of both proteins was verified by Edman degradation and mass spectrometry. The proteins were soluble in aqueous buffers but to make possible a comparison with the whole AChR or its isolated subunits, the recombinant proteins were analyzed both in aqueous solution and with the addition of detergents. The two proteins bound [125I]alpha-bungarotoxin with equal potency (KD approximately 130 nm, Bmax approximately 10 nmol.mg-1). Both were shown to interact with several monoclonal antibodies raised against purified Torpedo AChR. The circular dichroism (CD) spectra of the two proteins in aqueous solution revealed predominantly beta-structure (50-56%), the fraction of alpha-helices amounting to 32-35%. Nonionic (beta-octylglucoside) and zwitterionic (CHAPS) detergents did not appreciably change the CD spectra, while the addition of SDS or trifluoroethanol decreased the percentage of beta-structure or increased the alpha-helicity, respectively. The predominance of beta-structure is in accord with recent data on the N-terminal domain of the mouse muscle AChR alpha-subunit expressed in the mammalian cells [West et al. (1997) J. Biol. Chem. 272, 25 468]. Thus, expression in E. coli provides milligram amounts of the protein that retains several structural characteristics of the N-terminal domain of the Torpedo AChR alpha-subunit and appears to share with the latter a similar secondary structure. The expression of recombinant polypeptides representing functional domains of the AChR provides an essential first step towards a more detailed structural analysis.

Reverse-phase chromatography isolation and MALDI mass spectrometry of the acetylcholine receptor subunits

A procedure for purifying the Torpedo californica nicotinic acetylcholine receptor subunits is proposed which involves preparative SDS-PAGE followed by reverse-phase HPLC on a C4 column in an acetonitrile-isopropanol system. By this method, the alpha-subunit can be completely separated from the 43-kDa protein which migrates very close to it on SDS-PAGE, and the delta-subunit can be isolated free from the beta-subunit of Na+, K(+)-ATPase comigrating with it on SDS-PAGE. The purity of all acetylcholine receptor subunits thus obtained was verified by Edman degradation and MALDI mass-spectrometric analysis which could be performed quite easily on the HPLC-purified samples. In general, we observed a good correlation between the experimentally determined molecular masses and those calculated from the amino acid sequences and when known, posttranslational modifications (glycosylation and phosphorylation) of individual receptor subunits. Transfer of the isolated receptor subunits into 1% octyl-beta-D-glucopyranoside generates samples suitable for functional studies and enzymatic proteolysis or deglycosylation.

The handedness of the subunit arrangement of the nicotinic acetylcholine receptor from Torpedo californica

Cross-linking an alpha-neurotoxin with a known three-dimensional structure and with photoactivatable groups in known positions to native membrane-bound acetylcholine receptor reveals its quaternary structure, including the handedness of its circular subunit arrangement. Photolabelling with alpha-neurotoxin carrying the photoactivatable group at position Lys46 is inhibited by the competitive antagonist (+)-tubocurarine in a biphasic manner, indicating that it reacts with both alpha-subunits that were shown to have different affinities for this antagonist [Neubig, R. R. & Cohen, J. B. (1979) Biochemistry 18, 5464-5475]. Lys46 is located on loop III of the neurotoxin. The other information necessary for the elucidation of the handedness was provided by the recent finding that the central loop of the toxin (loop II) is oriented towards the central pore of the receptor, securing the overall orientation of the bound toxin [Machold, J., Utkin, Y. N., Kirsch, D., Kaufmann, R., Tsetlin, V. & Hucho, F. (1995b) Proc. Natl Acad. Sci. USA 92, 7282-7286]. Looking at the receptor from the synaptic side of the postsynaptic membrane, it was concluded that the clockwise subunit arrangement is alpha H-gamma-alpha L-delta-beta (alpha H and alpha L are the alpha-subunits binding (+)-tubocurarine with high and low affinity, respectively). Its mirror image alpha alpha L-gamma-alpha H-beta-delta could thus be excluded.

Photolabeling reveals the proximity of the alpha-neurotoxin binding site to the M2 helix of the ion channel in the nicotinic acetylcholine receptor

A photoactivatable derivative of neurotoxin II from Naja naja oxiana containing a 125I-labeled p-azidosalicylamidoethyl-1,3'-dithiopropyl label at Lys-25 forms a photo-induced cross-link with the delta subunit of the membrane-bound Torpedo californica nicotinic acetylcholine receptor (AChR). The cross-linked radioactive receptor peptide was isolated by reverse-phase HPLC after tryptic digestion of the labeled delta subunit. The sequence of this peptide, delta-(260-277), and the position of the label at Ala-268 were established by matrix-assisted laser-desorption-ionization mass spectrometry based on the molecular mass and on post-source decay fragment analysis. With the known dimensions of the AChR molecule, of the photolabel, and of alpha-neurotoxin, finding the cross-link at delta Ala-268 (located in the upper part of the channel-forming transmembrane helix M2) means that the center of the alpha-neurotoxin binding site is situated at least approximately 40 A from the extracellular surface of the AChR, proximal to the channel axis.

Mapping the functional topography of a receptor

Photoactivatable derivatives of the alpha-neurotoxin II from Naja naja oxiana are useful tools for investigating the three dimensional architecture of the extra-membrane part of the nicotinic acetylcholine receptor from the electric tissue of Torpedo californica. Three derivatives, carrying an azidobenzoyl group in position Lys-15, Lys-26, and Lys-46, respectively, are shown to react differently within the receptor's quaternary structure. Especially the Lys-26 and Lys-46 derivatives can be used for differentiating between the two nonequivalent alpha-subunits. The Lys-26 derivative is applied for probing the receptor subunits next to the alpha-subunit: the gamma-subunit is shown to be located next to the alpha-subunit binding d-tubocurarine with high affinity. The delta-subunit is the neighbor of the low affinity alpha-subunit. We radioiodinated the toxin derivatives and localized the 125I at the His-31 residue of the toxin. Very little label was found in position Tyr-24, the only tyrosine residue of the toxin, or in position His-4, the only other histidine residue. This result is important for the cleavage experiments necessary in attempts to identify the receptor sequence which reacted with the photolabel.