Review
Illuminating the ubiquitin/proteasome system

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Abstract

The ubiquitin/proteasome system (UPS) is responsible for the regulated processive degradation of proteins residing in the cytosol, nucleus, and endoplasmic reticulum. The two central players are ubiquitin, a small protein that is conjugated to substrates, and the proteasome, a large multi-subunit proteolytic complex that executes degradation of ubiquitylated proteins. Ubiquitylation and proteasomal degradation are highly dynamic processes. During the last decade, many researchers have started taking advantage of fluorescent proteins, which allow studying the dynamic nature of this system in the context of its natural environment: the living cell. In this review, we will summarize studies that have implemented this approach to examine the UPS and discuss novel insights in the dynamic organization of the UPS.

Section snippets

General introduction

Proteolysis is crucial for maintaining the quality of the cellular proteome. Among the various proteolytic systems in eukaryotic cells, the ubiquitin/proteasome system (UPS) is responsible for the controlled processive degradation of proteins residing in the nucleus, cytosol, and endoplasmic reticulum. This requires the coordinated actions of a large number of proteins that are involved successively in the recognition of substrates, the tagging of substrates with poly-ubiquitin chains, and the

Fluorescent ubiquitin

Covalent linkage of the C terminus of ubiquitin to a lysine residue in substrates, a process known as ubiquitylation, has a central regulatory role in a variety of cellular mechanisms. Besides its canonical role in targeting proteins for proteasomal degradation, ubiquitylation can also change the fate of a protein in very different ways. Non-proteolytic functions of ubiquitin are important in, for example, chromatin remodeling, DNA repair, endocytosis, signal transduction, and transcription [6]

Fluorescent proteasome

The 26S proteasome is a large proteolytic complex consisting of a 20S core particle and one or two 19S regulatory particles (Fig. 2A). The 20S core particle consists of four rings, each containing seven subunits. For simplicity, we will use throughout this review the unified nomenclature for the proteasome subunits [3], [27]. Each of the two outer rings is composed of seven unique α subunits, while the two inner rings each consist of seven unique β subunits [28] (Fig. 2B). The proteolytic

Concluding remarks

Fluorescent labeling of UPS components has given a deeper understanding of the behavior of this proteolytic system in living cells. The recent development of new fluorescent techniques and the generation of new fluorescent proteins, such as proteins that undergo photoconversion after exposure to light with a specific wavelength, like PAGFP [42] and mOrange [43], will further increase the power of live cell imaging in studies on the UPS. The progress in automated microscopy and data analysis can

Acknowledgments

The research in the Dantuma laboratory is supported by the Swedish Research Council, the Swedish Cancer Society, the Nordic Center of Excellence Neurodegeneration, and the European Community Network of Excellence RUBICON (Project no LSHC-CT-2005-018683).

References (47)

  • W. Baumeister et al.

    The proteasome: paradigm of a self-compartmentalizing protease

    Cell

    (1998)
  • H.C. Yen et al.

    Rpn5 is a conserved proteasome subunit and required for proper proteasome localization and assembly

    J. Biol. Chem.

    (2003)
  • H.C. Yen et al.

    Schizosaccharomyces pombe Int6 and Ras homologs regulate cell division and mitotic fidelity via the proteasome

    Cell

    (2003)
  • M. Verdoes et al.

    A fluorescent broad-spectrum proteasome inhibitor for labeling proteasomes in vitro and in vivo

    Chem. Biol.

    (2006)
  • A. Hershko et al.

    The ubiquitin system

    Annu. Rev. Biochem.

    (1998)
  • C.M. Pickart

    Mechanisms underlying ubiquitination

    Annu. Rev. Biochem.

    (2001)
  • D. Finley

    Recognition and processing of ubiquitin–protein conjugates by the proteasome

    Annu. Rev. Biochem.

    (2009)
  • A.L. Schwartz et al.

    The ubiquitin–proteasome pathway and pathogenesis of human diseases

    Annu. Rev. Med.

    (1999)
  • L. Hicke

    Protein regulation by monoubiquitin

    Nat. Rev. Mol. Cell Biol.

    (2001)
  • I. Dikic et al.

    Ubiquitin-binding domains—from structures to functions

    Nat. Rev. Mol. Cell Biol.

    (2009)
  • D.L. Stenoien et al.

    Intranuclear ataxin1 inclusions contain both fast- and slow-exchanging components

    Nat. Cell Biol.

    (2002)
  • P.S. Bilodeau et al.

    Vps27-Hse1 and ESCRT-I complexes cooperate to increase efficiency of sorting ubiquitinated proteins at the endosome

    J. Cell Biol.

    (2003)
  • H. Lelouard et al.

    Dendritic cell aggresome-like induced structures are dedicated areas for ubiquitination and storage of newly synthesized defective proteins

    J. Cell Biol.

    (2004)
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