Question: Where is the proteasome located?

Context:
Cellular regulation by UPS- mediated protein degradation is a highly specific and selective process that depends on time (e.g. cell cycle) and location (nucleus, mitochondria or endoplasmic reticulum). 
In eukaryotic cells, regulated protein degradation of intracellular proteins is mediated largely by the ubiquitin proteasome system (UPS). 
We followed two key parameters of this process: the distribution of proteasomes in nuclear and cytosolic compartments, and the formation of cytoplasmic aggregate-like structures called proteasome storage granules (PSGs). 
Subsequently, cellular distribution of the PAI-2·proteasome complexes was established by immunogold staining and electron microscopy analyses. As judged by confocal microscopy, both proteins appeared in a diffuse cytosolic pattern, but they also could be found in a dense perinuclear and nuclear location. 
 Frequently ubiquitinated proteins are targeted to the proteasome for degradation in the cytosol. 
 More recently, proteasome-mediated degradation of cell cycle regulatory proteins, production and loading of antigenic peptides onto HLA molecules, and transient homing of diverse virion proteins required for entry and/or egress have been shown to be coordinated at the centrosome. 
We conclude that N-terminal sequences of Wld(S) protein influence the intranuclear location of both ubiquitin proteasome and NAD(+) synthesis machinery and that an evolutionary recent sequence mediates binding of mammalian Ube4b to VCP.
Two new forms of proteasomes, designated as the endoplasmic reticulum (ER) membrane-associated proteasome (ERa proteasome) and ER membrane-bound proteasome (ERb proteasome), were purified to homogeneity from 0.0125 and 2.5% sodium cholate extracts, respectively, of a rat liver microsomal fraction.
The proteasome is a large multicatalytic, proteinase complex located in the cytosol and the nucleus of eukaryotic cells.
Subcellular distribution of proteasomes implicates a major location of protein degradation in the nuclear envelope-ER network in yeast.
JAB1, which is thought to bind p27KIP1 and transport it from the nucleus to the cytoplasm for proteasome/ubiquitin-mediated degradation, was found to be localized both in the cytoplasm and the nucleus in undifferentiated and differentiating tumors whereas located predominantly in the nucleus of differentiated tumor cells.
20S proteasomes are present in all areas where ATPase subunits are detected, consistent with the presence of intact 26S proteasomes.
Inhibition of proteasomes located in axons resulted in an accumulation of ubiquitinated proteins in these axons.
All these observations indicate that the mouse sperm proteasome participates in the binding to the zona pellucida and the acrosome reaction and that there is a pool of proteasomes located on the sperm head..
Thus, newly synthesized apoB is localized throughout the entire ER and degraded homogeneously, most likely by neighboring proteasomes located on the cytosolic side of the ER membrane.
While misfolded and short-lived proteins are degraded in proteasomes located in the nucleus and cytoplasm, the degradation of organelles and long-lived proteins in the lysosome occurs by the process of autophagy.
The GFP-tagged proteasomes were located within both the cytoplasm and the nucleus.
Proteasomes are large multicatalytic proteinase complexes located in the cytosol and the nucleus of eukaryotic cells.
Immunohistochemically, proteasomes (26S) were located in the cytoplasm in normal human muscle, but the staining intensity was weak.
In DMRV, proteasomes often were located within or on the rim of rimmed vacuoles, and in the cytoplasm of atrophic fibers.

Answer: The proteasome can be found in perinuclear and nuclear location, as well as in cytosolic compartments, such as mitochondria and endoplasmic reticulum. Proteasome-mediated degradation of cell cycle regulatory proteins, production and loading of antigenic peptides onto HLA molecules, and transient homing of diverse virion proteins required for entry and/or egress have been also shown to be coordinated at the centrosome.