|Avidin  is a minor constituent of egg white in several groups of oviparous
vertebrates. Avidin, which was discovered in the 1920's, takes its name from
the avidity with which it binds biotin. These two molecules bind so strongly
that is extremely difficult to separate them. Streptavidin is a protein produced
by Streptomyces avidinii which also binds biotin and whose sequence is
evolutionary related to that of avidin.
Avidin and streptavidin both form homotetrameric complexes of noncovalently
associated chains. Each chain forms a very strong and specific non-covalent
complex with one molecule of biotin.
The three-dimensional structures of both streptavidin [2928324, 8515446] and avidin 
have been determined and revealed them to share a common fold: an eight
stranded anti-parallel beta-barrel with a repeated +1 topology enclosing an
internal ligand binding site.
Fibropellins I and III  are proteins that form the apical lamina of the sea
urchin embryo, a component of the extracellular matrix. These two proteins
have a modular structure composed of a CUB domain (see), followed
by a variable number of EGF repeats and a C-terminal avidin-like domain.