The accumulation of misfolded proteins and protein aggregates can perturb protein homeostasis and cause extensive cellular damage. The cell has various quality control (QC) systems to recognize and dispose of misfolded and aggregated proteins. Failure of these QC systems can result in number of diseases, including the age-related diseases Alzheimer's disease, Parkinson's disease, and cataract. Cellular factors, including molecular chaperones and E3 ubiquitin ligases, recognize misfolded proteins and either attempt to refold the proteins or target them for degradation via the proteasome. Additionally, a poorly defined mechanism for selective quality control of proteins and aggregates via autophagy is known to exist. It is important to define the mechanisms for protein QC so that we may gain insight into the cellular basis of protein misfolding diseases. A number of diseases are caused by mutations to membrane proteins that disrupt their transmembrane domains, causing them to misfold, and be degraded by quality control machinery. Gonadotropin releasing hormone receptor (GnRHR), a G protein-coupled receptor (GPCR), contains no cytoplasmic or extracellular domains and is comprised of only a seven-transmembrane span domain. Therefore, study of GnRHR biogenesis provides an opportunity for understanding basic principles for QC of misfolded TM proteins. In addition, there are twenty-three known disease (hypogonadotropic hypogonadism) causing GnRHR mutations and many cause misfolding. Preliminary data in a cell culture model indicates that quality control pathways for various GnRHR mutants are very different. S168R GnRHR undergoes QC via the RMA1 E3 ubiquitin ligase complex and is degraded by the proteasome. In stark contrast, E90K GnRHR is degraded by autophagy. This represents the first example of a plasma membrane protein being selectively partitioned between autophagic and proteasomal quality control.
In Aim 1, I will determine how misfolded S168R GnRHR is recognized and targeted for proteasomal degradation.
In Aim 2, I will determine why and how E90K GnRHR is selected for autophagy instead of proteasomal quality control. The results obtained here will greatly further our knowledge of how the cell recognizes defects in transmembrane spans and the mechanisms by which those proteins are degraded.
A large number of diseases, such as Alzheimer's disease, Parkinson's disease, cataract, and mad cow disease are caused by the accumulation of toxic proteins. This project will define the mechanisms used by a cell to clear potentially toxic proteins.
| Houck, Scott A; Ren, Hong Yu; Madden, Victoria J et al. (2014) Quality control autophagy degrades soluble ERAD-resistant conformers of the misfolded membrane protein GnRHR. Mol Cell 54:166-179 |
