Structure-function studies have identified key functional motifs in the acetylcholine receptor, including residues that contribute to the ion channel and to the ligand-binding sites. Little is known, however, about determinants of channel gating kinetics. To identify structural correlates of gating, we examined the structual basis of the fetal-to-adult decrease in channel open time conferred by the presence of the~ subunit in place of the 7 subunit. By constructing chimeras composed of segments of the~ and 7 subunits, we show that the main determinant of this kinetic change is a 30 residue segment of a predicted amphipathic helix located between transmembrane domains M3 and M4. Further subdividing the amphipathic helix revealed that either multiple residues or its overall conformation confers this regulation of channel kinetics. We also show that L440 and