Membrane proteins are challenging to study given their hydrophobic nature, generally low native abundance and intrinsic instability (1,2).  Regardless, half of all protein drug targets are membrane proteins.  For imaging, most fluorescent proteins (i.e. GFP) cannot specifically visualize cell surface subpopulations.  

The SNAP-tag® system is based on a DNA repair enzyme, O⁶-alkylguanine-DNA alkyltransferase (AGT). It allows for multiple substrate options to enable color changes. It is highly temperature and fixation stable and can be used in vitro or in vivo. The substrate consists of two parts: the benzylguanine group and the functional group which can be a fluorophore, biotin or bead. During the labeling reaction the substituted benzyl group covalently attaches to the SNAP-tag releasing guanine. Once the fluorophore is coupled to the desired protein, the label fluorescesces permitting visualization in living or fixed cells.

SNAP-tag, CLIP-tag™ and cell surface-specific ACP/MCP-tag systems can specifically label subpopulations of target proteins expressed on the cell surface using non-cell permeable substrates (3). This approach permits discrimination of different populations of a cell surface protein: those properly translocated to the plasma membrane from those retained in the secretory pathway or already internalized (e.g. upon ligand binding).

SNAP-tag® is a registered trademark of New England Biolabs, Inc.
CLIP-tag™ is a trademark of New England Biolabs, Inc.

References

1. Lacapère J-J, Pebay-Peyroula E, Neumann J-M, Etchebest C. (2007) Trends Biochem Sci. 32, 259–270. PMID: 17481903
2. von Heijne G. (2007) J Intern Med. 261, 543–557. PMID: 17547710
3. Keppler, A., Pick, H., Arrivoli, C. et al. (2004) Proc. Natl. Acad. Sci. USA, 101, 9955. PMID: 15226507