GABAAreceptors are pentameric ligand-gated ion channelsthat mediate inhibitory fast synaptic transmission in the centralnervous system. Consistent with recent pentameric ligand-gated ion channels structures, sequence analysis predicts ana-helix near the N-terminus of each GABAAreceptor subunit.Preceding each a-helix are 8–36 additional residues, which weterm the N-terminal extension. In homomeric GABACrecep-tors and nicotinic acetylcholine receptors, the N-terminal a-helix is functionally essential. Here, we determined the role ofthe N-terminal extension and putative a-helix in heteromerica1b2c2 GABAAreceptors. This role was most prominent in thea1 subunit, with deletion of the N-terminal extension or furtherdeletion of the putative a-helix both dramatically reduced thenumber of functional receptors at the cell surface. Conversely,deletion of the b2orc2 N-terminal extension had little effect onthe number of functional cell surface receptors. Additionaldeletion of the putative a-helix in the b2orc2 subunits did,however, decrease both functional cell surface receptors andincorporation of the c2 subunit into mature receptors. In the b 2subunit only, a-helix deletions affected GABA sensitivity anddesensitization. Our ﬁndings demonstrate that N-terminalextensions and a-helices make key subunit-speciﬁc contribu-tions to assembly, consistent with both regions being involvedin inter-subunit interactions.