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MBE Advance Access originally published online on June 8, 2005
Molecular Biology and Evolution 2005 22(10):2000-2009; doi:10.1093/molbev/msi190
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© The Author 2005. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oupjournals.org

Research Article

Structural and Functional Implications of an Unusual Foraminiferal ß-Tubulin

Andrea Habura*, Laura Wegener*,{dagger}, Jeffrey L. Travis{dagger} and Samuel S. Bowser*,{ddagger}

* Wadsworth Center, New York State Department of Health, Albany, New York; {dagger} Department of Biological Sciences, State University of New York at Albany; and {ddagger} Department of Biomedical Sciences, State University of New York at Albany

E-mail: habura{at}wadsworth.org.

We have obtained sequence data for ß-tubulin genes from eight species of Foraminifera (forams) and {alpha}-tubulin sequences from four species, sampling major taxonomic groups from a wide range of environments. Analysis of the ß-tubulin sequences demonstrates that foram ß-tubulins possess the highest degree of divergence of any tubulin gene sequenced to date and represent a novel form of the protein. In contrast, foram {alpha}-tubulin genes resemble the conventional {alpha}-tubulins seen in other organisms. Partition homogeneity analysis shows that the foraminiferal ß-tubulin gene has followed an evolutionary path that is distinct from that of all other organisms. Our findings indicate that positive selective pressure occurred on the ß-tubulin subunit in ancestral forams prior to their diversification. The specific substitutions observed have implications for microtubule (MT) assembly dynamics. The regions most strongly affected are implicated in lateral contacts between protofilaments and in taxol binding. We predict that these changes strengthen lateral contacts between adjacent dimers in a manner similar to that induced by taxol binding, thus allowing the formation of the tubulin "helical filaments" observed in forams by electron microscopy. Our results also indicate that substantial changes to these portions of the ß-tubulin molecule can be made without sacrificing essential MT functions.

Key Words: M-loop • microtubule • tubulin polymorph • helical filament • Foraminifera


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