Research in the April Nature Methods shows how to make a good thing even better, modifying a popular system for protein labelling and modification to reduce the risk of unwanted cross-reactions.
With incredible specificity and powerful affinity for each other, the protein streptavidin and its small-molecule target biotin are truly the 'Dynamic Duo' of biological research, and a perennial favourite for use in the design of biochemical experimental techniques. For example, one can easily subject biotin-linked proteins to highly specific labelling with streptavidin-linked fluorophores. Nonetheless, there is an important limitation to the system - streptavidin naturally forms tetramers (assemblies of four protein molecules) that bind up to four molecules of biotin, creating the potential for unexpected cross-linking of biotinylated targets. Efforts to engineer monomeric streptavidin variants have generally resulted in diminished biotin affinity.
Alice Ting's lab now describes an alternative approach: engineering 'dead' streptavidin variants that can bind to each other but not to biotin. By combining the two types of streptavidin monomers in the proper proportions and isolating tetramers that consist of three dead subunits and one active subunit, they obtain streptavidin complexes that are functionally monomeric and bind only one molecule of biotin. Experiments demonstrate that the hybrid tetramers retain normal affinity for biotin but induce far less 'clumping' of biotinylated targets relative to wild-type streptavidin tetramers.
This approach also offers the possibility of building divalent and trivalent tetramers, and in an associated News & Views piece, Kai Johnsson comments that "the existing plentitude of applications of the streptavidin-biotin interaction provides an enormous playground for streptavidins with reduced but defined valencies."
Alice Y. Ting (Massachusetts Institute of Technology, Cambridge, MA, USA)
Kai Johnsson (École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland)
Abstract available online.
(C)Nature Methods press release.
Message posted by: Trevor M. D'Souza