Nicotinic acetylcholine receptor and the structural basis of neuromuscular transmission: insights fromTorpedopostsynaptic membranes

Abstract

The nicotinic acetylcholine (ACh) receptor, at the neuromuscular junction, is a neurotransmitter-gated ion channel that has been fine-tuned through evolution to transduce a chemical signal into an electrical signal with maximum efficiency and speed. It is composed from three similar and two identical polypeptide chains, arranged in a ring around a narrow membrane pore. Central to the design of this assembly is a hydrophobic gate in the pore, more than 50 Å away from sites in the extracellular domain where ACh binds. Although the molecular properties of the receptor have been explored intensively over the last few decades, only recently have structures emerged revealing its complex architecture and illuminating how ACh entering the binding sites opens the distant gate. Postsynaptic membranes isolated from the (muscle-derived) electric organ of the Torpedo ray have underpinned most of the structural studies: the membranes form tubular vesicles having receptors arranged on a regular surface lattice, which can be imaged directly in frozen physiological solutions. Advances in electron crystallographic techniques have also been important, enabling analysis of the closed- and open-channel forms of the receptor in unreacted tubes or tubes reacted briefly with ACh. The structural differences between these two forms show that all five subunits participate in a concerted conformational change communicating the effect of ACh binding to the gate, but that three of them (αγ, β and δ) play a dominant role. Flexing of oppositely facing pore-lining α-helices is the principal motion determining the closed/open state of the gate. These results together with the findings of biochemical, biophysical and other structural studies allow an integrated description of the receptor and of its mode of action at the synapse.

Cryo-EM Structures of the Magnesium Channel CorA Reveal Symmetry Break upon Gating

“…Our findings thus potentially integrate the functional mechanisms of gating in other ligand-gated channels such as Cys-loop receptors (Du et al, 2015; Miyazawa et al, 2003; Unwin and Fujiyoshi, 2012; Wilson and Karlin, 2001) and P2X receptor family (Baconguis et al, 2013; Hattori and Gouaux, 2012; Heymann et al, 2013), where interfacial ligand binding energies have been surmised to play an important role in driving gating transitions. The quaternary structural changes in CorA appear to be an interesting variant of what is observed in the vast majority of ligand-gated channels, where ligand binding energies are typically used to transition from an unliganded (apo) closed state, to the ligand-bound activated (conducting) open state (Auerbach, 2013; Hille, 2001; Unwin, 2013). In CorA, the changes are in the opposite direction, and it is likely that the reduction in inter-subunit stabilization energies due to the release of Mg 2+ leads to an increase in overall dynamics and conformational flexibility, thus leading to opening of the channel.…”

Mechanisms Regulating Neuromuscular Junction Development and Function and Causes of Muscle Wasting

“…Rapsyn is a cytoplasmic scaffolding protein expressed constitutively in myotubes; it is present at the NMJ from the earliest stages of development, and in adult muscle, expression is largely restricted to the synaptic region. Rapsyn binds tightly to AChRs to form a high-density network of the two proteins (434). Rapsyn also binds to dystroglycan (26) and is thus thought to link AChRs to the postsynaptic actin cytoskeleton (FIGURE 3B).…”

Functional differences between neurotransmitter binding sites of muscle acetylcholine receptors

“…The muscle AChR has central pore surrounded by five subunits of composition α 2 βδe in adult-type and α 2 βδγ in fetal-type (Fig. 1A) (4). The fetal, γ, subunit is essential for proper synapse maturation, and the adult, e, subunit is necessary for proper function of mature synapses (5)(6)(7).…”