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Plant and Cell Physiology Advance Access originally published online on May 15, 2007
Plant and Cell Physiology 2007 48(6):866-874; doi:10.1093/pcp/pcm059
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© The Author 2007. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

The AtMAP65-1 Cross-Bridge Between Microtubules is Formed by One Dimer

Hua Li1, Tonglin Mao1, Ziding Zhang and Ming Yuan*

State Key Laboratory of Plant Physiology and Biochemistry; Department of Plant Sciences, College of Biological Sciences, China Agricultural University, Beijing 100094, PR China

*Corresponding author: E-mail, mingyuan{at}cau.edu.cn; Fax, +86-10–62733491


  Abstract

The microtubule-associated protein AtMAP65-1 from Arabidopsis thaliana dimerizes and forms 25 nm cross-bridges between microtubules, but the exact mechanism is unknown. Here, we used the predicted three-dimensional structure of AtMAP65-1 as a basis for analyzing the actual cross-bridging in detail. Fold-recognition predicts that AtMAP65-1 contains four coiled-coil domains and a flexible extended loop. The length of these coiled-coil domains is about 25 nm, suggesting that one molecule could span the gap, hence forming an antiparallel overlapping dimer instead of an end-to-end dimer. We then tested this model by using truncations of AtMAP65-1. EDC {[3-(dimethylamino) propyl] carbodiimide} cross-linking analysis indicated that the N-terminus of the rod domain of AtMAP65-1 (amino acids 1–339) binds to the C-terminus of the rod domain (amino acids 340–494) and also participates in connecting the two antiparallel proteins in the cross-bridge. Nevertheless, microtubules can still form bundles in the presence of AtMAP65-1 340–587 (amino acids 340–587) or AtMAP65-1 1–494 (amino acids 1–494). Comparing the cold stability of microtubule bundles induced by full-length AtMAP65-1 with that of AtMAP65-1 340–587 or AtMAP65-1 1–494, we conclude that AtMAP65-1 495–587 acts as a flexible extended loop, playing a crucial role in binding to and stabilizing microtubules in the AtMAP65-1 cross-bridge.

Keywords: Arabidopsis thaliana - AtMAP65-1 - Bundles - Cross-bridge - Microtubules

Abbreviations: EDC, [3-(dimethylamino) propyl] carbodiimide; EGTA, ethylene glycol-bis-(ß-aminoethyl ether)-N,N,N',N'-tetraacetic acid; GFP, green fluorescent protein; GST, glutathione S-transferase; MAP, microtubule-associated protein; NORS, non-regular secondary structure; PIPES, piperazine-N,N'-bis (2-ethanesulfonic acid; 1,4-piperazinediethanesulfonic acid).

1 These authors contributed equally to this work.

(Received April 9, 2007; Accepted May 10, 2007)
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