Avidin-like domain in an epidermal growth factor homolog from a sea urchin.

We have found that a protein from the purple sea urchin has a carboxyl-terminal domain with striking sequence similarity to chicken avidin and bacterial streptavidin. All our evidence supports the homology of these sequences. Tetramers of avidin and streptavidin bind biotin strongly; the biotin binding site involves two to four tryptophans and probably an adjacent lysine in each chain. The presence of four tryptophans at equivalent positions in the sea urchin protein domain suggests that it may also be able to bind biotin and inhibit cell growth, as do the two other proteins. Alternatively, this domain may have acquired a new role as part of a multidomain protein.