title: An avidin-like domain that does not bind biotin is adopted for oligomerization by the extracellular mosaic protein fibropellin.
abstract: 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.
authors.affiliation: Biomedical Engineering Department, 44 Cummington Street, Boston University, Boston, MA 02215, USA.
authors.forename: Itai
authors.initials: I
authors.lastname: Yanai
authors.forename: Yong
authors.initials: Y
authors.lastname: Yu
authors.forename: Xiahui
authors.initials: X
authors.lastname: Zhu
authors.forename: Charles R
authors.initials: CR
authors.lastname: Cantor
authors.forename: Zhiping
authors.initials: Z
authors.lastname: Weng
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001360:Avidin / Q000737:chemistry*; D001665:Binding Sites; D001710:Biotin / Q000737:chemistry*; D002942:Circular Dichroism; D018076:DNA, Complementary / Q000378:metabolism; D019281:Dimerization; D004591:Electrophoresis, Polyacrylamide Gel; D004815:Epidermal Growth Factor / Q000737:chemistry*; D004926:Escherichia coli / Q000378:metabolism; D016326:Extracellular Matrix Proteins / Q000737:chemistry*; D005822:Genetic Vectors; D008024:Ligands; D046911:Macromolecular Substances / Q000737:chemistry; D013058:Mass Spectrometry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry; D012617:Sea Urchins; D017386:Sequence Homology, Amino Acid; D012984:Software; D019809:Streptavidin / Q000737:chemistry; D013302:Streptomyces / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D018076:DNA, Complementary; D016326:Extracellular Matrix Proteins; D008024:Ligands; D046911:Macromolecular Substances; D011994:Recombinant Proteins; C062354:fibropellin protein, sea urchin; D001360:Avidin; D004815:Epidermal Growth Factor; D001710:Biotin; D019809:Streptavidin