SNAP-tag is a self-labeling protein tag commercially available in various expression vectors. SNAP-tag is a 182 residues polypeptide that can be fused to any protein of interest and further specifically and covalently tagged with a suitable ligand, such as a fluorescent dye. Since its introduction, SNAP-tag has found numerous applications in biochemistry and for the investigation of the function and localisation of proteins and enzymesin living cells. Compared to the current standard labelling methods used in fluorescence microscopy, the use of SNAP-tag presents significant advantages.
Applications
Cell biology utilizes tools that allow manipulation and visualization of proteins in living cells. An important example is the use of fluorescent proteins, such as the green fluorescent protein or yellow fluorescent protein. Molecular biology methods allow these fluorescent proteins to be introduced and expressed in living cells as fusion proteins. However, the photo-physical properties of the fluorescent proteins are generally not suited for single-molecule spectroscopy. Fluorescent proteins have, in comparison to commercially available dyes, a much lower fluorescence quantum yield and are quickly destroyed upon excitation with a focused laser beam. The SNAP-tag protein is an engineered version of the ubiquitous mammalian enzyme AGT, encoded in humans by the O-6-methylguanine-DNA methyltransferase gene. SNAP-tag was obtained using a directed evolution strategy, leading to a hAGT variant that accepts O6-benzylguanine derivatives instead of repairing alkylated guanine derivatives in damaged DNA. An orthogonal tag, called CLIP-tag, was further engineered from SNAP-tag to accept O2-benzylcytosine derivatives as substrates, instead of O6-benzylguanine. A split-SNAP-tag version suitable for protein complementation assay and protein-protein interaction studies was later developed. Apart from fluorescence microscopy, SNAP-tag and CLIP-tag have proven useful in the elucidation of numerous biological processes, including the identification of multiprotein complexes using various approaches such as FRET, cross-linking, proximity ligation assay. Other application include the measurement of protein half-lives in vivo, and small molecule-protein interactions.
