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First published online May 20, 2005; 10.1104/pp.104.052456 Plant Physiology 138:654-662 (2005) © 2005 American Society of Plant Biologists Two Microtubule-Associated Proteins of the Arabidopsis MAP65 Family Function Differently on Microtubules1State Key Laboratory of Plant Physiology and Biochemistry, Department of Plant Sciences, College of Biological Sciences, China Agricultural University, Beijing 100094, China (T.M., L.J., H.L., M.Y.); and Section of Plant Biology, University of California, Davis, California 95616 (B.L.)
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.
1 This work was supported by the National Key Basic Research Project of China and the National Natural Science Foundation of China (grant no. 30128008 to B.L. and grant no. 30170057 to M.Y.). Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.052456. * Corresponding author; e-mail mingyuan{at}cau.edu.cn; fax 86–10–62733491. Received August 26, 2004; returned for revision December 28, 2004; accepted March 10, 2005. Related articles in Plant Physiol.:
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