Site-specific labeling of surface proteins on living cells using genetically encoded peptides that bind fluorescent nanoparticle probes.

We report a highly specific, robust, and generic method for noncovalent labeling of cellular proteins with highly fluorescent core-shell silica nanoparticles termed C dots. Our approach uses short genetically engineered peptides with affinity for silica (GEPS) that are site-specifically introduced at the termini or in loops of cellular proteins. Because GEPS are absent from native cell surface proteins, GEPS-tagged recombinant proteins can be selectively and rapidly labeled with fluorescent C dots. To demonstrate the versatility of our method, we targeted 30 nm C dots to two structurally distinct integral outer membrane proteins in Escherichia coli, FhuA and OmpX. Efficient labeling was achieved in 15 min or less and was observed to be highly sensitive and specific. This strategy provides a powerful technique, comparable to other chemical and biological labeling strategies, for efficient and quantitative investigation of protein function in live biological cells.