Streptavidin. Streptavidin is a 60 kDa non-glycosylated protein composed of four identical subunits, each of which has a binding site for biotin, and is purified from the microorganism Streptomyces avidinii. This protein resembles egg white avidin in its biotin binding properties. In most applications, streptavidin and avidin conjugates are interchangeable.