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Protein Science (2005), 14:417-423. Published by Cold Spring Harbor Laboratory Press. Copyright © 2005 The Protein Society
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An avidin-like domain that does not bind biotin is adopted for oligomerization by the extracellular mosaic protein fibropellin

Itai Yanai1,4,5, Yong Yu2,3,4, Xiahui Zhu3,6, Charles R. Cantor2,3 and Zhiping Weng1,3

1 Bioinformatics Program, 2 Center for Advanced Biotechnology, and 3 Biomedical Engineering Department, Boston University, Boston, Massachusetts 02215, USA

(RECEIVED May 27, 2004; FINAL REVISION September 29, 2004; ACCEPTED October 9, 2004)

The protein avidin found in egg white seems optimized for binding the small vitamin biotin as a stable homotetramer. Indeed, along with its streptavidin ortholog in the bacterium Streptomyces avidinii, this protein shows the strongest known noncovalent bond of a protein with a small ligand. A third known member of the avidin family, as similar to avidin as is streptavidin, is found at the C-terminal ends of the multidomain fibropellin proteins found in sea urchin. The fibropellins form a layer known as the apical lamina that surrounds the sea urchin embryo throughout development. Based upon the structure of avidin, we deduced a structural model for the avidin-like domain of the fibropellins and found that computational modeling predicts a lack of biotin binding and the preservation of tetramerization. To test this prediction we expressed and purified the fibropellin avidin-like domain and found it indeed to be a homotetramer incapable of binding biotin. Several lines of evidence suggest that the avidin-like domain causes the entire fibropellin protein to tetramerize. We suggest that the presence of the avidin-like domain serves a structural (tetrameric form) rather than functional (biotin-binding) role and may therefore be a molecular instance of exaptation—the modification of an existing function toward a new function. Finally, based upon the oligomerization of the avidin-like domain, we propose a model for the overall structure of the apical lamina.

Keywords: avidin-like domain; biotin; fibropellin; streptavidin; avidin

Article and publication are at http://www.proteinscience.org/cgi/doi/10.1110/ps.04898705.

Reprint requests to: Zhiping Weng, Biomedical Engineering Department, 44 Cummington Street, Boston University, Boston, MA 02215, USA; e-mail: zhiping{at}bu.edu; fax: (617) 353-6766.


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