| Neuronal nicotinic acetylcholine receptors (nAChRs) play important roles in both the peripheral and central nervous systems. Considerable effort has been made to understand the molecular structure, physiology, and pharmacology of neuronal nAChRs. Less progress has been made, however, in resolving the cellular events and molecular mechanisms that guide the assembly and trafficking of neuronal nAChRs to the surface membrane. Here, we describe the identification of ubiquilin-1 as a nAChR subunit-interacting protein. We describe a role for ubiquilin-1, a ubiquitin-like protein with the capacity to interact with both the proteasome and ubiquitin ligases, in regulating surface expression of neuronal nAChRs. Ubiquilin-1 interacts with unassembled alpha3 and alpha4 subunits when coexpressed in heterologous cells and interacts with the endogenous nAChR subunits in neurons. Coimmunostaining shows that the interaction of ubiquilin-1 with the alpha3 subunit draws the receptor subunit and proteasome into a complex. Coexpression of ubiquilin-1 and neuronal nAChRs in heterologous cells dramatically reduces the expression of the receptors on the cell surface. In cultured superior cervical ganglion neurons, expression of ubiquilin-1 abolishes nicotine-induced upregulation of nAChRs, a process recently suggested to act through increased assembly and maturation of receptor subunits into functional pentameric receptors. Taken together, these data suggest that ubiquilin-1 limits the availability of unassembled nAChR subunits for further assembly and trafficking by drawing them to the proteasome. This provides a role for ubiquilin-1 in regulating surface expression of neuronal nAChRs, in particular nicotine-induced upregulation of surface receptors. |
