| In order to adapt to changing conditions and life stages, cells must remodel the composition of the proteome by specifically removing obsolete or damaged proteins. Failure to remove proteins at the appropriate time can cause a variety of cellular dysfunctions that can lead to cell death and conditions such as cancer, Huntington's, Parkinson's and Alzheimer's disease. Protein quality control is especially important during aging and stress, as the accumulating load of damaged, misfolded, and aggregated proteins can overwhelm the cell's ability to maintain the proteome. The ubiquitin-proteasome system is responsible for the specific degradation of many proteins within eukaryotic cells. The 26S proteasome degrades proteins that have been "tagged" with a chain of ubiquitin molecules. Ubiquitin is a highly-conserved 8.5 kDa protein that is conjugated post-translationally to a target protein after passing through a complex series of enzymes. Using the nematode C. elegans as a model, we sought to 1) Characterize the roles of specific ubiquitin-conjugation enzymes (UBCs) in the response to aggregation of a polyglutamine-containing protein in body wall muscle cells and 2) Examine the subcellular localization of proteasomes in these cells during normal aging and stress. Q82::GFP, a fusion protein expressed from a transgene encoding a polyglutamine tract fused to Green Fluorescent Protein (GFP), was found to aggregate rapidly (~58 minutes) in a manner that is not directly dependent on ubiquitin but dependent on the concentration of Q82::GFP protein, which was altered in response to RNA interference of several E2 ubiquitin-conjugating enzymes. In a separate transgenic worm strain, a fluorescently-labeled proteasome subunit, RPT-1, was observed in live animals to localize to the nucleus and cytoplasm during the adult stage. Within the nucleus, RPT-1::GFP localizes diffusely and to foci, and was excluded from heterochromatin. Within the muscle contractile apparatus, RPT-1::GFP localizes to I-bands, regions of thin actin filaments, and is excluded from dense bodies. After a prolonged heat stress, RPT-1::GFP in adult worms relocalizes to dense bodies. After starvation, these foci are associated with high proteolytic activity. These studies demonstrate some of the spatial dynamics of the ubiquitin-proteasome system in response to development, aging, and stress. |
