Determinants responsible for assembly of the nicotinic acetylcholine receptor

SH Keller, P Taylor - The Journal of general physiology, 1999 - rupress.org
SH Keller, P Taylor
The Journal of general physiology, 1999rupress.org
The assembly of the four homologous, but distinct,,,, and subunits of the nicotinic
acetylcholine receptor (nAChR) into the pentamer 2 presents a unique opportunity to
delineate the individual amino acid side chains that contribute to the assembly process, and
to examine the pathway responsible for subunit assembly and expression at the cell surface.
It is well established that subunits assemble into the circular order of, where the subunit
resides between the two subunits and two binding sites are found at the and interfaces (for …
The assembly of the four homologous, but distinct,,,, and subunits of the nicotinic acetylcholine receptor (nAChR) into the pentamer 2 presents a unique opportunity to delineate the individual amino acid side chains that contribute to the assembly process, and to examine the pathway responsible for subunit assembly and expression at the cell surface. It is well established that subunits assemble into the circular order of, where the subunit resides between the two subunits and two binding sites are found at the and interfaces (for detailed reviews, see Karlin and Akabas, 1995; Hucho et al., 1996). Chirality of the order of the subunits has also been proposed on the basis of cross-linking of toxins with known structures to the receptor (Machold et al., 1995; Utkin et al., 1997). The subunits are glycoproteins composed of 450–520 amino acids that traverse the membrane four times (Karlin and Akabas, 1995); the extracellular domain is formed from the amino terminal 210 residues (Chavez and Hall, 1991; Fig. 1). Sequence elements in this domain specify both ligand recognition and the arrangement of subunits (Blount and Merlie, 1989; Sine and Claudio, 1991; Yu and Hall, 1991; Verrall and Hall, 1992; Kreienkamp et al., 1995). An extended cytoplasmic loop after the third transmembrane domain contains numerous lysine residues (Boulter et al., 1990) that may also encode signals for the stability and trafficking of the subunits. A structural feature common to ligand-gated ion channels is the presence of multiple membrane spans extending to large extracellular and cytoplasmic domains. This separates the multisubunit ion channels from the well-studied receptors of the immune system, such as T cell–receptor subunits that traverse the membrane once. Owing to amino acid sequence similarity, hetero-oligomeric subunit composition, and the conserved positioning of disulfide loops and glycosylation sites, assembly and expression pathways should be shared by the family of ligand-gated ion channels. Therefore, the overall characteristics of assembly and expression identified for acetylcholine receptor biogenesis should be applicable to the other less well studied ligand-gated ion channels.
What is the pathway by which the subunits of the nAChR assemble in the endoplasmic reticulum (ER) and become expressed at the cell surface? Four major features in processing of the subunits appear to direct expression of the ion channel. First, subunits are inserted in the endoplasmic reticulum membrane and undergo concurrent folding transitions and other posttranslational modifications throughout the assembly process. Second, unassembled subunits are susceptible to rapid degradation (Claudio et al., 1989; Blount and Merlie, 1990), and association with chaperones and assembly with neighboring subunits both enhance the stability of the emerging complex (Keller et al., 1996, 1998). Third, the subunits are ordered into the pentameric structure to compose the subunit arrangement of----(see Hucho et al., 1996); specific amino acid residues in the NH2-terminal domain dictate the
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