Plant Physiology Preview
Published on May 20, 2005; 10.1104/pp.104.052456
Received August 26, 2004
Returned for revision December 28, 2004
Accepted March 10, 2005
Two Microtubule-Associated Proteins of the Arabidopsis MAP65 Family Function Differently on Microtubules
Tonglin Mao , Lifeng Jin , Hua Li , Bo Liu , and Ming Yuan *
State Key Laboratory of Plant Physiology and Biochemistry, Department of Plant Sciences, College of Biological Sciences, China Agricultural University, Beijing 100094, China
Section of Plant Biology, University of California, Davis, California 95616
* Corresponding author; email: mingyuan{at}cau.edu.cn.
The organization and dynamics of microtubules are regulated by microtubule-associated proteins, or MAPs. In Arabidopsis (Arabidopsis thaliana), nine genes encode proteins of the evolutionarily conserved MAP65 family. We proposed that different MAP65s might have distinct roles in the interaction with microtubules. In this study, two AtMAP65 proteins, AtMAP65-1 and AtMAP65-6, were chosen to test this hypothesis in vitro. Although both fusion proteins were able to cosediment with microtubules in vitro, different properties on tubulin polymerization and microtubule bundling were observed. AtMAP65-1 was able to promote tubulin polymerization, enhance microtubule nucleation, and decrease the critical concentration for tubulin polymerization. It also induced the formation of large microtubule bundles by forming cross-bridges between microtubules evenly along the whole length of microtubules. In the presence of AtMAP65-1, microtubule bundles were more resistant to cold and dilution treatments. AtMAP65-6, however, demonstrated no activity in promoting tubulin polymerization and stabilizing preformed microtubules. AtMAP65-6 induced microtubules to form a mesh-like network with individual microtubules. Cross-bridge-like interactions were only found at regional sites between microtubules. The microtubule network induced by AtMAP65-6 was more resistant to high concentration of NaCl than the bundles induced by AtMAP65-1. Purified monospecific anti-AtMAP65-6 antibodies revealed that AtMAP65-6 was associated with mitochondria in Arabidopsis cells. It was concluded that these two MAP65 proteins were targeted to distinct sites, thus performing distinct functions in Arabidopsis cells.
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