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Plant and Cell Physiology Advance Access published online on February 2, 2005
Plant and Cell Physiology, doi:10.1093/pcp/pci043
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Plant and Cell Physiology 2005 © The Japanese Society of Plant Physiologists (JSPP); all rights reserved.
Revised December 25, 2004
Accepted January 7, 2005

Regular Paper

The Ubiquitin-Proteasome Pathway Is Involved in Rapid Degradation of Phosphoenolpyruvate Carboxylase Kinase for C4 Photosynthesis

Masakazu Agetsuma 1, Tsuyoshi Furumoto 1, Shuichi Yanagisawa 2, and Katsura Izui 1*

1 Graduate School of Biostudies, Kyoto University, Kyoto 606-8502, Japan
2 Graduate School of Agricultural and life Sciences, The University of Tokyo, Tokyo 113-8657, Japan

* To whom correspondence should be addressed.
Katsura Izui, E-mail: izui{at}kais.kyoto-u.ac.jp


  Abstract

In C4 photosynthesis, phosphoenolpyruvate carboxylase (PEPC) is the enzyme responsible for catalyzing the primary fixation of atmospheric CO2. Activity of PEPC is diurnally regulated by reversible phosphorylation. PEPC kinase (PEPCk), a protein kinase involved in this phosphorylation, is highly specific to PEPC and consists of only the core domain of protein kinase. Owing to its extremely low abundance in cells, analysis of its regulatory mechanism at the protein level has been difficult. Here we employed the transient expression system using maize mesophyll protoplasts. The PEPCk protein with a FLAG tag could be expressed correctly and detected with high sensitivity. Rapid degradation of PEPCk protein was confirmed and shown to be blocked by MG132, a 26S proteasome inhibitor. Furthermore, MG132 enhanced accumulation of PEPCk with increased molecular sizes at about 8 kDa intervals. Using anti-ubiquitin antibody, this increase was shown to be due to ubiquitination. This is the first report to show the involvement of the ubiquitin-proteasome pathway in PEPCk turnover. The occurrence of PEPCks with higher molecular sizes, which were noted previously with cell extracts from various plants, was also suggested to be due to ubiquitination of native PEPCk.

Keywords: C4 photosynthesis; phosphoenolpyruvate carboxylase kinase; 26S proteasome; protein degradation; ubiquitination.
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