IDENTIFICATION OF A DETERMINANT OF ACETYLCHOLINE-RECEPTOR GATING KINETICS IN THE EXTRACELLULAR PORTION OF THE GAMMA-SUBUNIT

A large body of structure-function studies has identified many of the functional motifs underlying ion permeation through acetylcholine rece ptor (AChR) channels. The structural basis of channel gating kinetics is, however, incompletely understood. We have previously identified a novel shorter form of the AChR gamma subunit, which lacks the 52 amino acids within the extracellular amino-terminal half, encoded by exon 5 . To define the contribution of the missing domain to AChR channel fun ction, we have transiently coexpressed the mouse short gamma subunit ( gamma(s)) with alpha, beta and delta subunits in human cells and recor ded single-channel currents from the resulting AChRs. Our findings sho w that replacement of the gamma by the gamma(s) subunit confers a long duration characteristic to AChR channel openings without altering uni tary conductance sizes or receptor affinity for the transmitter. We al so show that alpha beta gamma(s) delta AChR channels exhibit a peculia r voltage sensitivity characterized by a short opening duration when t he membrane potential is hyperpolarized. Together, these findings indi cate that the domain in the extracellular amino-terminal half of the g amma subunit that encompasses a conserved disulphide loop and a critic al tyrosine residue implicated in receptor oligomerization and inserti on at the cell surface is a functional motif that also modulates AChR channel gating kinetics. The results also provide a molecular explanat ion of the functional diversity exhibited by skeletal muscle AChRs dur ing development.