UC San Diego

The acquisition of cellular identity requires large-scale alterations in cellular state. A cell must not only reorganize its nuclear contents to facilitate the expression of necessary genes while silencing those required for stem-like character, but differentiation also often entails dramatic reorganization of the cytoplasmic contents. The number of proteins required for this process is prodigious and may well count among its number the non-canonical proteasome activator PSME3. PSME3 is known to regulate diverse cellular processes in both the nucleus and cytoplasm, but its importance for differentiation remains unclear. To this end, we have undertaken a series of studies using the C2C12 mouse myoblast cell line.

In Chapter 1, I provide a detailed introduction of proteasome biology with a particular focus on PSME3. I discuss major findings pertaining to PSME3’s function, enumerate its many degradation targets, discuss its known biochemical properties, and note the existing evidence for its role in differentiation.

In Chapter 2, I investigate the association of PSME3 with the chromatin and find its widespread association with highly active promoter regions. I further identify the transcriptional protein RPRD1A as an interaction partner that may facilitate this association. Finally, I demonstrate that, though it associates with highly active genes, loss of PSME3 has no effect on gene expression across differentiation.

In Chapter 3, I show that PSME3 is important in differentiating myoblasts for its ability to regulate cell migration and the formation of myotubes. I demonstrate that loss of PSME3 is accompanied by an altered abundance of cell-adhesion-related proteins, which may be affected by an interaction of PSME3 with the chaperone NUDC. I attempt to elucidate a potential mechanism for these functions and show PSME3 mediates differentiation in a cell-intrinsic, proteasome-independent mechanism.

In Chapter 4, I analyze the sum of my work, place it into the broader context of PSME3 function, and speculate on what future efforts would prove fruitful to better understand this elusive protein.