C. Enenkel et al., SUBCELLULAR-DISTRIBUTION OF PROTEASOMES IMPLICATES A MAJOR LOCATION OF PROTEIN-DEGRADATION IN THE NUCLEAR-ENVELOPE ER NETWORK IN YEAST, EMBO journal (Print), 17(21), 1998, pp. 6144-6154
26S proteasomes are the key enzyme complexes responsible for selective
turnover of short-lived and misfolded proteins. Based on the assumpti
on that they are dispersed over the nucleoplasm and cytoplasm in all e
ukaryotic cells, we wanted to determine the subcellular distribution o
f 26S proteasomes in living yeast cells. For this purpose, we generate
d yeast strains that express functional green fluorescent protein (GFP
) fusions of proteasomal subunits, An a subunit of the proteolytically
active 20S core complex of the 26S proteasome, Pre6/YOL038w, as well
as an ATPase-type subunit of the regulatory 19S cap complex, Cim5/YOL1
45w, were tagged with GFP. Both chimeras were shown to be incorporated
completely into active 26S proteasomes. Microscopic analysis revealed
that GFP-labelled 20S as well as 19S subunits are accumulated mainly
in the nuclear envelope (NE)-endoplasmic reticulum (ER) network in yea
st, These findings mere supported by the co-localization and co-enrich
ment of 26S proteasomes with NE-ER marker proteins. A major Location o
f proteasomal peptide cleavage activity was visualized in the NE-ER ne
twork, indicating that proteasomal degradation takes place mainly in t
his subcellular compartment in yeast.