Roles of nicotinic acetylcholine receptor β subunit cytoplasmic loops in acute desensitization and single-channel features.

To evaluate physiological roles of the large, second cytoplasmic loops (C2) situated between the M3 and M4 transmembrane domains of nicotinic acetylcholine receptor (nAChR) subunits. We have constructed chimeric β2 (β2χ) and β4 (β4χ) subunits in which the "nested" C2 domains (but not the "proximal" sequences of ∼14 residues immediately adjacent to the M3 or M4 domains) of these β subunits were replaced by the corresponding sequence from the serotonin 5-HT3A receptor subunit. We previously reported that heterologously expressed nAChR containing α4 and β2χ subunits displayed a faster whole-cell current decay in its agonist response compared to responses of all-wild-type α4β2-nAChR. This suggests an unexpected, functional role for the C2 domain of the β2 subunit in α4β2-nAChR acute desensitization. Here we report that there also is faster desensitization of α4β4χ-nAChR relative to α4β4-nAChR stably and heterologously expressed in the human SH-EP1 cell-line. In addition, cell-attached, single-channel recording shows that both acetylcholine-activated α4β2χ- and α4β4χ-nAChR have a significantly lower mean open probability, shorter mean open-time, and a longer mean closed-time than their fully wild-type counterparts while not having different conductance amplitudes. These findings reveal microscopic bases for the faster desensitization of α4(∗)-nAChR containing chimeric instead of wild-type β subunits. Our findings also remain consistent with novel and unexpected roles of β subunit-nested C2 domains in modulation of α4(∗)-nAChR function.