FIGURE 8.

Model for the role of the β/δ subunit loop retention motifs in AChR trafficking. The biogenesis and trafficking of the AChR are regulated by a series of compartment-specific signals. First, in the ER, pentameric receptor is exported to the Golgi whereas most unassembled or partially assembled subunits are retained by transmembrane (black bar) and dibasic motifs and targeted for degradation. Second, in the Golgi complex, β and δ subunit cytoplasmic loop motifs (red bars) mediate retention by interacting with an as-yet unidentified protein (green). We propose that these motifs regulate the Golgi to surface trafficking of assembled AChR (i) and might also act in the Golgi and at the plasma membrane to retain and retrieve assembly intermediates that have escaped the ER (ii). This additional quality control mechanism could help ensure that only correctly assembled and fully functional AChR is expressed on the cell surface.