ReviewUbiquitin-like proteins: new wines in new bottles
Introduction
Since the identification of ubiquitin as the heat-stable factor in an ATP-dependent proteolysis assay two decades ago, tremendous strides have been made in our understanding of the biology and biochemistry of ubiquitination (Ciechanover, 1998, Hershko and Ciechanover, 1998). A ubiquitin-like protein, UCRP/ISG15, which contains two ubiquitin-like domains in tandem, was reported in 1987 (Haas et al., 1987, Loeb and Haas, 1992). Additional ubiquitin-like sequences have also been reported over the years (Biggins et al., 1996, Michiels et al., 1993, Schauber et al., 1998); however, only ubiquitin and UCRP/ISG15 were shown to possess the ability to covalently modify other cellular proteins. The field changed dramatically in 1996 with the discovery of a novel ubiquitin-like protein modifier, Sentrin-1 (also called SUMO-1, PIC-1, GMP-1, UBL1, SMT3C), by several laboratories (Boddy et al., 1996, Kamitani et al., 1997a, Lapenta et al., 1997, Mahajan et al., 1997, Mannen et al., 1996, Matunis et al., 1996, Okura et al., 1996, Shen et al., 1996b, Tsytsykova et al., 1998). In 1997, another novel ubiquitin-like protein, NEDD8, was shown to define yet another protein modification pathway (Kamitani et al., 1997a). Finally, Apg12 was found to represent the fourth ubiquitin-like protein modification pathway required for autophagy (Mizushima et al., 1998a, Mizushima et al., 1998b). The discoveries of these novel proteins have sparked tremendous interest in ubiquitin-like protein modification pathways. This review will focus exclusively on the biology and biochemistry of Sentrin and NEDD8 modification pathways. Similarity in the enzymatic mechanism between ubiquitin and ubiquitin-like modification will be contrasted with the richness in functional diversity of these pathways.
Section snippets
Ubiquitination
Ubiquitin is a 76 amino acid polypeptide, which is highly conserved in evolution with only three amino acid differences between the human and yeast homologues. The C-terminus of ubiquitin contains a conserved Gly residue, which is activated in an ATP-dependent manner to form a thiol ester linkage with the cysteine residue of the ubiquitin-activating enzyme (E1) (Haas and Siepmann, 1997) (Fig. 1). Activated ubiquitin is then transferred to a carrier protein (E2) to form another thiol ester
Sentrin family of ubiquitin-like proteins
The Sentrin family of ubiquitin-like proteins consists of Sentrin-1, Sentrin-2, and Sentrin-3. Sentrin-1 is a 101 amino acid protein containing a ubiquitin-homology domain (residues 22–97), which is 18% identical and 48% homologous to human ubiquitin (see Fig. 2). Sentrin-2 is a 95 amino acid polypeptide, which is 46% identical and 66% similar to Sentrin-1 in the ubiquitin-homology domain. Sentrin-3 is a 103 amino acid polypeptide, which is 97% identical to Sentrin-2 in the ubiquitin-homology
NEDD8 and Rub1
NEDD8 (neural precursor cell-expressed developmentally downregulated) was originally reported as a novel mRNA highly enriched in fetal mouse brain (Kumar et al., 1992). Northern blot analysis showed that the NEDD8 message was developmentally downregulated (Kamitani et al., 1997a). In adult tissues NEDD8 expression was mostly restricted to the heart and skeletal muscle. Antiserum specific for NEDD8 detected a 6 kDa NEDD8 monomer, and a series of higher molecular weight NEDD8-conjugated proteins
Conclusions
In this review, we concisely summarized our current understanding of two novel ubiquitin-like proteins. It is clear that the sentrinization and NEDD8 modification define two enzymatic pathways distinct from ubiquitination. Nonetheless, the enzymatic principles, such as activation, conjugation, and ligation, appear to be conserved among these pathways. NEDD8 is structurally more related to the ubiquitin and the NEDD8 modification pathway appears to play an important role in ubiquitin-mediated
Acknowledgements
This work was supported in part by the Institute of Molecular Medicine for the Prevention of Human Diseases, Grants from the National Institutes of Health, the DREAM Project, American Heart Association, and Arthritis Foundation.
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