An avidin-like domain that does not bind biotin is adopted for oligomerization by the extracellular mosaic protein fibropellin.
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.
Yanai
Itai
I
Biomedical Engineering Department, 44 Cummington Street, Boston University, Boston, MA 02215, USA.
Yu
Yong
Y
Zhu
Xiahui
X
Cantor
Charles R
CR
Weng
Zhiping
Z
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
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