EC Accepts, published online ahead of print on 1 May 2009

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Eukaryotic Cell doi:10.1128/EC.00165-08
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Characterization of a REG/PA28 proteasome activator homolog in Dictyostelium discoideum indicates that the ubiquitin- and ATP-independent REGgamma proteasome is an ancient nuclear protease

Patrick Masson, Daniel Lundin, Fredrik Söderbom, and Patrick Young^*

Swiss Institute of Bioinformatics Swiss-Prot Group CMU - 1, rue Michel Servet CH-1211 Geneva 4 Switzerland; Department of Molecular Biology and Functional Genomics, Stockholm University, S-10691 Stockholm, Sweden; Department of Molecular Biology, Swedish University of Agricultural Sciences (SLU), Uppsala Biomedical Center (BMC), Box 590, S-75124; Department of Genetics Microbiology and Toxicology, Stockholm University, S-10691 Stockholm, Sweden

^* To whom correspondence should be addressed. Email: patrick.young{at}gmt.su.se.

The nuclear proteasome activator REG/PA28 is an ATP-ubiquitin independent activator of the 20 S proteasome and has been proposed to degrade and thereby regulate both a key human oncogene, SRC-3/AIB1 coactivator and the cyclin-dependent kinase inhibitor, p21(Waf/Cip1). We report the identification and characterization of a PA28/REG proteasome activator homolog in Dictyostelium. Association of recombinant Dictyostelium REG activator with purified Dictyostelium 20 S proteasome led to the preferential stimulation of the trypsin-like proteasome peptidase activity. Immuno-localization studies demonstrate the proteasome activator is localized to the nucleus and is present in growing as well as starving Dictyostelium cells. Our results indicate that the Dictyostelium PA28/REG activator can stimulate both the trypsin-like and chymotrypsin-like activities of the 20 S proteasome and supports the idea that the REG-20 S proteasome represents an early unique nuclear degradation pathway for eukaryotic cells.