Pore size and negative charge as structural determinants of permeability in the Torpedo nicotinic acetylcholine receptor channel

Abstract

To gain an insight into the molecular basis of ion permeation mechanism through the nicotinic acetylcholine receptor (AChR) channel, we have determined permeability ratios of organic cations relative to Na+ of specifically mutated Torpedo californica AChR channels expressed in Xenopus oocytes. The mutations involved mainly the side chains of the amino acid residues in the intermediate ring, where mutations have been found to exert strong effects on single-channel conductance and ion selectivity among alkali metal cations. The results obtained reveal that both the size and the net charge of the side chains of the intermediate ring are involved in determining the permeability, and provide experimental evidence that the pore size at the intermediate ring is a critical determinant of permeability. Our findings further suggest that changes in net charge exert effects on permeability by affecting the pore size of the channel.

A prokaryotic perspective on pentameric ligand-gated ion channel structure

“…This asymmetric behavior has also been shown for the heteromeric muscle nAChR, where mutations at the -1 position have a stronger effect if they occur in the γ subunit than in any of the other subunits [93,96,102] . The asymmetry in most channels from the pLGIC family offers fine tuning possibilities regarding ion selectivity and conductance.…”

“…Depending on the subtype of receptor it also allows Ca 2+ ions to pass the ion channel to different extents [92] . The narrowest part of the pore is believed to be around 7.4 Å for the Torpedo nAChR [93] or 8.4 Å for the nAChR homologue from the mouse [91] , whereas in the anion-selective branch of the family the diameter is suggested to have a smaller size of about 5−6 Å [94,95] . The narrowest part of the pore of the nAChR, which most likely represents the selectivity filter, has been assigned a longitudinal dimension of 3−6 Å, which would correspond to two or three turns of an α-helix pointing amino acids into the pore [69] .…”

Novel Seizure Phenotype and Sleep Disruptions in Knock-In Mice with Hypersensitive α4*Nicotinic Receptors

“…Previous studies show that mutations in the M2 region can alter the ion selectivity of nicotinic receptors, although mutations at the 2Ј position appear to have the greatest effect (Cohen et al, 1992;Wang and Imoto, 1992). To determine whether the L9ЈA mutation altered ion selectivity of ␣4␤2 receptors for three physiologically relevant cations (Na ϩ , K ϩ , and Ca 2ϩ ), we substituted K ϩ or Ca 2ϩ for extracellular Na ϩ and measured the shift in reversal potential (E r ) of the ACh response (see Materials and Methods).…”

The structural basis of function in Cys-loop receptors

“…Five symmetrically placed M2 helices from each of the five subunits create a hydrophobic region that is 3 Å at its narrowest and less than 3Á5 Å over a distance of approximately 8 Å in the closed state, and has been referred to as a hydrophobic girdle (Miyazawa et al 2003). Ion permeabilities suggest that the diameter of the open channel is between 7Á4 Å and 8Á4 Å for cation channels and between 5Á2 Å and 6Á2 Å for anion channels (Brown et al 1998 ;Cohen et al 1992 ;Fatima-Shad & Barry, 1993 ;Rundstrom et al 1994 ;Wang & Imoto, 1992). Originally, the channel gate was predicted to be close to the cytoplasmic end of M2 (Wilson & Karlin, 1998 ;Wilson et al 2000).…”