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J. Biol. Chem., Vol. 281, Issue 37, 27145-27157, September 15, 2006

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MUBs, a Family of Ubiquitin-fold Proteins That Are Plasma Membrane-anchored by Prenylation^*

Brian P. Downes¹², Scott A. Saracco¹, Sang Sook Lee, Dring N. Crowell, and Richard D. Vierstra³

From the Department of Genetics, University of Wisconsin, Madison, Wisconsin 53706-1574 and the Department of Biology, Indiana University-Purdue University, Indianapolis, Indiana 46202-5132

Ubiquitin (Ub)-fold proteins are rapidly emerging as an important class of eukaryotic modifiers, which often exert their influence by post-translational addition to other intracellular proteins. Despite assuming a common -grasp three-dimensional structure, their functions are highly diverse because of distinct surface features and targets and include tagging proteins for selective breakdown, nuclear import, autophagic recycling, vesicular trafficking, polarized morphogenesis, and the stress response. Here we describe a novel family of Membrane-anchored Ub-fold (MUB) proteins that are present in animals, filamentous fungi, and plants. Extending from the C terminus of the Ub-fold is typically a cysteine-containing CAAX (where A indicates aliphatic amino acid) sequence that can direct the attachment of either a 15-carbon farnesyl or a 20-carbon geranylgeranyl moiety in vitro. Modified forms of several MUBs were detected in transgenic Arabidopsis thaliana, suggesting that these MUBs are prenylated in vivo. Both cell fractionation and confocal microscopic analyses of Arabidopsis plants expressing GFP-MUB fusions showed that the modified forms are membrane-anchored with a significant enrichment on the plasma membrane. This plasma membrane location was blocked in vivo in prenyltransferase mutants and by mevinolin, which inhibits the synthesis of prenyl groups. In addition to the five MUBs with CAAX boxes, Arabidopsis has one MUB variant with a cysteine-rich C terminus distinct from the CAAX box that is also membrane-anchored, possibly through the attachment of a long chain acyl group. Although the physiological role(s) of MUBs remain unknown, the discovery of these prenylated forms further expands the diversity and potential functions of Ub-fold proteins in eukaryotic biology.

Received for publication, March 10, 2006 , and in revised form, June 28, 2006.

^* This work was supported by a National Science Foundation Arabidopsis 2010 Program Grant MCB-0115870 (to R. D. V.), United States Department of Agriculture-National Research Initiative Competitive Program Grant 03-02151 (to D. N. C.), NSRA Postdoctoral Fellowship 5-F32-GM065736-02 from the National Institutes of Health (to B. P. D.), and a Korean Science and Engineering Foundation postdoctoral fellowship (to S. S. L.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Table 1.

¹ Both authors contributed equally to this work.

² Present address: Dept. of Biological Sciences, 3507 Laclede Ave., Saint Louis University, St Louis, MO 63103-2010.

³ To whom correspondence should be addressed: Dept. of Genetics, 425-G Henry Mall, University of Wisconsin, Madison, WI 53706-1574. Fax: 608-262-2976; E-mail: vierstra{at}wisc.edu.

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