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J. Biol. Chem., Vol. 280, Issue 42, 35554-35561, October 21, 2005

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Arabidopsis BRS1 Is a Secreted and Active Serine Carboxypeptidase^*

From the Department of Botany and Microbiology, University of Oklahoma, Norman, Oklahoma 73019

The Arabidopsis BRS1 gene encodes a serine carboxypeptidase II-like protein. Its biological role in the brassinosteroid signaling pathway was first established by its capability to specifically suppress a weak brassinosteroid insensitive 1 (bri1) allele, bri1-5, when overexpressed. To gain additional insights into the molecular mechanisms of BRS1 function, the subcellular localization and the biochemical characteristics of BRS1 were determined by using transgenic plants harboring a 35S-BRS1-GFP construct and fusion proteins purified from 35S-BRS1-FLAG transgenic plants. The BRS1-GFP protein was mainly secreted and accumulated in the extracellular space. Immunological data suggest that BRS1 is proteolytically processed by an unknown endoproteinase in planta. Affinity-purified BRS1-FLAG from transgenic plants show strong hydrolytic activity with a broad P1 substrate preference including basic and hydrophobic groups on either side of the scissile bond. The hydrolytic activity of BRS1 can be strongly inhibited by a serine protease inhibitor, phenylmethylsulfonyl fluoride. The pH and temperature optima for the hydrolytic activity of BRS1 are pH 5.5 and 50 °C, respectively. These data demonstrate that BRS1 is a secreted and active serine carboxypeptidase, consistent with the hypothesis suggested by our previous genetic evidence that BRS1 may process a protein involved in an early event in the BRI1 signaling pathway.

Received for publication, March 25, 2005 , and in revised form, August 24, 2005.

^* This project was funded by National Science Foundation Grant IBN 0312279 and start-up funds from the University of Oklahoma (to J. 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.

¹ To whom correspondence should be addressed: Dept. of Botany and Microbiology, 770 Van Vleet Oval, University of Oklahoma, Norman, OK 73019. Tel.: 405-325-1653; Fax: 405-325-7619; E-mail: lij{at}ou.edu.

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