Streptavidin for high affinity to biotinylated molecules
Streptavidin is one of the most useful proteins in the modern biotechnology toolkit. This is because the binding of streptavidin and its ligand, biotin, is the highest-affinity noncovalent binding interaction known to science. For all practical purposes biotin (and by extension other molecules coupled to biotin) are essentially irreversibly bound by streptavidin. Furthermore, streptavidin is a tetramer of biotin-binding subunits, and as such is capable of multivalent binding. These properties, along with the rugged stability of streptavidin, make it suitable for mediating all sorts of binding interactions in diagnostic and research applications.