STRUCTURAL BASIS OF THE DIFFERENT GATING KINETICS OF FETAL AND ADULT ACETYLCHOLINE-RECEPTORS

Structure-function studies have identified key functional motifs in th e acetylcholine receptor, including residues that contribute to the io n channel and to the ligand-binding sites. Little is known, however, a bout determinants of channel gating kinetics. To identify structural c orrelates of gating, we examined the structual basis of the fetal-to-a dult decrease in channel open time conferred by the presence of the ep silon subunit in place of the gamma subunit. By constructing chimeras composed of segments of the epsilon and gamma subunits, we shaw that t he 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 eithe r multiple residues or its overall conformation confers this regulatio n of channel kinetics. We also show that L440 and M442, conserved resi dues within M4 of the gamma subunit, contribute to long duration openi ngs characteristic of the fetal receptor.