Tryptophan scanning mutagenesis reveals distortions in the helical structure of the δM4 transmembrane domain of the Torpedo californica nicotinic acetylcholine receptor.

The lipid-protein interface is an important domain of the nicotinic acetylcholine receptor (nAChR) that has recently garnered increased relevance. Several studies have made significant advances toward determining the structure and dynamics of the lipid-exposed domains of the nAChR. However, there is still a need to gain insight into the mechanism by which lipid-protein interactions regulate the function and conformational transitions of the nAChR. In this study, we extended the tryptophan scanning mutagenesis (TrpScanM) approach to dissect secondary structure and monitor the conformational changes experienced by the δM4 transmembrane domain (TMD) of the Torpedo californica nAChR, and to identify which positions on this domain are potentially linked to the regulation of ion channel kinetics. The difference in oscillation patterns between the closed- and open-channel states suggests a substantial conformational change along this domain as a consequence of channel activation. Furthermore, TrpScanM revealed distortions along the helical structure of this TMD that are not present on current models of the nAChR. Our results show that a Thr-Pro motif at positions 462-463 markedly bends the helical structure of the TMD, consistent with the recent crystallographic structure of the GluCl Cys-loop receptor which reveals a highly bent TMD4 in each subunit. This Thr-Pro motif acts as a molecular hinge that delineates two gating blocks in the δM4 TMD. These results suggest a model in which a hinge-bending motion that tilts the helical structure is combined with a spring-like motion during transition between the closed- and open-channel states of the δM4 TMD.

Fourier transform coupled tryptophan scanning mutagenesis identifies a bending point on the lipid-exposed δM3 transmembrane domain of the Torpedo californica nicotinic acetylcholine receptor.

The nicotinic acetylcholine receptor (nAChR) is a member of a family of ligand-gated ion channels that mediate diverse physiological functions, including fast synaptic transmission along the peripheral and central nervous systems. Several studies have made significant advances toward determining the structure and dynamics of the lipid-exposed domains of the nAChR. However, a high-resolution atomic structure of the nAChR still remains elusive. In this study, we extended the Fourier transform coupled tryptophan scanning mutagenesis (FT-TrpScanM) approach to gain insight into the secondary structure of the δM3 transmembrane domain of the Torpedo californica nAChR, to monitor conformational changes experienced by this domain during channel gating, and to identify which lipid-exposed positions are linked to the regulation of ion channel kinetics. The perturbations produced by periodic tryptophan substitutions along the δM3 transmembrane domain were characterized by two-electrode voltage clamp and (125)I-labeled α-bungarotoxin binding assays. The periodicity profiles and Fourier transform spectra of this domain revealed similar helical structures for the closed- and open-channel states. However, changes in the oscillation patterns observed between positions Val-299 and Val-304 during transition between the closed- and open-channel states can be explained by the structural effects caused by the presence of a bending point introduced by a Thr-Gly motif at positions 300-301. The changes in periodicity and localization of residues between the closed-and open-channel states could indicate a structural transition between helix types in this segment of the domain. Overall, the data further demonstrate a functional link between the lipid-exposed transmembrane domain and the nAChR gating machinery.

Potential role of caveolin-1-positive domains in the regulation of the acetylcholine receptor's activatable pool: implications in the pathogenesis of a novel congenital myasthenic syndrome.

Cholesterol modulates the plasmalemma's biophysical properties and influences the function and trafficking of membrane proteins. A fundamental phenomenon that remains obscure is how the plasmalemma's lipid composition regulates the activatable pool of membrane receptors. An outstanding model to study this phenomenon is the nicotinic acetylcholine receptor (nAChR), since the nAChR activatable pool has been estimated to be but a small fraction of the receptors present in the plasmalemma. Studies on the effect of cholesterol depletion in the function of the Torpedo californica nAChR, using the lipid-exposed nAChR mutation (alpha C418W) that produces a congenital myasthenic syndrome (CMS), demonstrated that cholesterol depletion causes a remarkable increase in the alpha C418W nAChR's macroscopic current whereas not in the wild-type (WT). A variety of approaches were used to define the mechanism responsible for the cholesterol depletion mediated-increase in the alpha C418W nAChR's macroscopic current. The present study suggests that a substantial fraction of the alpha C418W nAChRs is located in caveolin-1-positive domains, "trapped" in a non-activatable state, and that membrane cholesterol depletion results in the relocation of these receptors to the activatable pool. Co-fractionation and co-immunoprecipitation of the alpha C418W nAChR and the membrane raft protein caveolin-1 (cav1) support the notion that interactions at lipid-exposed domains regulate the partition of the receptor into membrane raft microdomains. These results have potential implications as a novel mechanism to fine-tune cholinergic transmission in the nervous system and in the pathogenesis associated to the alpha C418W nAChR.