The distribution of charged amino acid residues and the Ca(2+) permeability of nicotinic acetylcholine receptors: a predictive model

Nicotinic acetylcholine receptors (nAChRs) are cation-selective ligand-gated ion channels exhibiting variable Ca(2+) permeability depending on their subunit composition. The Ca(2+) permeability is a crucial functional parameter to understand the physiological role of nAChRs, in particular considering their ability to modulate Ca(2+)-dependent processes such as neurotransmitter release. The rings of extracellular and intracellular charged amino acid residues adjacent to the pore-lining TM2 transmembrane segment have been shown to play a key role in the cation selectivity of these receptor channels, but to date a quantitative relationship between these structural determinants and the Ca(2+) permeability of nAChRs is lacking. In the last years the Ca(2+) permeability of several nAChR subtypes has been experimentally evaluated, in terms of fractional Ca(2+) current (Pf, i.e., the percentage of the total current carried by Ca(2+) ions). In the present study, the available Pf-values of nAChRs are used to build a simplified modular model describing the contribution of the charged residues in defined regions flanking TM2 to the selectivity filter controlling Ca(2+) influx. This model allows to predict the currently unknown Pf-values of existing nAChRs, as well as the hypothetical Ca(2+) permeability of subunit combinations not able to assemble into functional receptors. In particular, basing on the amino acid sequences, a Pf > 50% would be associated with homomeric nAChRs composed by different α subunits, excluding α7, α9, and α10. Furthermore, according to the model, human α7β2 receptors should have Pf-values ranging from 3.6% (4:1 ratio) to 0.1% (1:4 ratio), much lower than the 11.4% of homomeric α7 nAChR. These results help to understand the evolution and the function of the large diversity of the nicotinic receptor family.