Proteins are desirable building blocks to create self-assembled, spatially defined structures and interfaces on length-scales that are inaccessible by traditional methods. Here, we describe a novel approach to create functionalized monolayers using the proteins BsIA and SpyCatcher/SpyTag. BsIA is a bacterial hydrophobin whose amphiphilic character underlies its ability to assemble into a monolayer at both air/water and oil/water interfaces. We demonstrate that Bsa1A having the SpyTag peptide fused at the N- or C-terminus does not affect the formation of such monolayers. We establish the creation of stable oil-in-water microcapsules using Bs1A, and also show the fabrication of capsules outwardly displaying the reactive SpyTag peptide by fusing it to the C-terminus of Bs1A. Such capsules can be covalently labeled by reacting the surface-displayed SpyTag with SpyCatcher fused to any desired protein. We demonstrate this principle by labeling microcapsules using green fluorescent protein (GFP). All components are genetically encodable, the reagents can be readily prepared in large quantities, and all reactions occur at ambient temperature in aqueous solution. Thus, this straightforward, modular, scalable strategy has myriad potential applications in the creation of novel, functional materials, and interfaces.
