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Molecular and Cellular Biology, December 2001, p. 8007-8021, Vol. 21, No. 23
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.23.8007-8021.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Construction and Analysis of Mouse Strains Lacking the Ubiquitin Ligase UBR1 (E3) of the N-End Rule Pathway

Yong Tae Kwon,¹ Zanxian Xia,¹ Ilia V. Davydov,^1, Stewart H. Lecker,² and Alexander Varshavsky^1,*

Division of Biology, California Institute of Technology, Pasadena, California 91125,¹ and Renal Unit, Beth Israel Deaconess Medical Center, Boston, Massachusetts 02115²

Received 6 June 2001/Accepted 6 September 2001

The N-end rule relates the in vivo half-life of a protein to the identity of its N-terminal residue. In the yeast Saccharomyces cerevisiae, the UBR1-encoded ubiquitin ligase (E3) of the N-end rule pathway mediates the targeting of substrate proteins in part through binding to their destabilizing N-terminal residues. The functions of the yeast N-end rule pathway include fidelity of chromosome segregation and the regulation of peptide import. Our previous work described the cloning of cDNA and a gene encoding the 200-kDa mouse UBR1 (E3). Here we show that mouse UBR1, in the presence of a cognate mouse ubiquitin-conjugating (E2) enzyme, can rescue the N-end rule pathway in ubr1 S. cerevisiae. We also constructed UBR1^/ mouse strains that lacked the UBR1 protein. UBR1^/ mice were viable and fertile but weighed significantly less than congenic +/+ mice. The decreased mass of UBR1^/ mice stemmed at least in part from smaller amounts of the skeletal muscle and adipose tissues. The skeletal muscle of UBR1^/ mice apparently lacked the N-end rule pathway and exhibited abnormal regulation of fatty acid synthase upon starvation. By contrast, and despite the absence of the UBR1 protein, UBR1^/ fibroblasts contained the N-end rule pathway. Thus, UBR1^/ mice are mosaics in regard to the activity of this pathway, owing to differential expression of proteins that can substitute for the ubiquitin ligase UBR1 (E3). We consider these UBR1-like proteins and discuss the functions of the mammalian N-end rule pathway.

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^* Corresponding author. Mailing address: Division of Biology, 147-75, Caltech, 1200 East California Blvd., Pasadena, CA 91125. Phone: (626) 395-3785. Fax: (626) 440-9821. E-mail: avarsh{at}caltech.edu.

Present address: IGEN International, Inc., Gaithersburg, MD 20877.

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Molecular and Cellular Biology, December 2001, p. 8007-8021, Vol. 21, No. 23
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.23.8007-8021.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

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