Lelek, M., et al. Proc. Natl. Acad. Sci. USA 109, 8564–8569 (2012).

The binding of fluorescein arsenical helix binder (FlAsH) to small tetracysteine tags in proteins has not quite lived up to its early promises of nonperturbative fluorescence labeling of proteins in living cells, but it has proven useful for labeling proteins that mediate bacterial and viral infection. These proteins are sensitive to fusion tags, and immunolabeling is unsuitable for crucial live-cell imaging studies. Now Lelek et al. show that FlAsH can be converted between excited and dark states for switching-based super-resolution imaging. They labeled the integrase enzyme of human immunodeficiency virus (HIV) with FlAsH and used its switching ability to image HIV intracellular complexes at ∼30-nanometer resolution during infection. This allowed them to measure the size and shape of the viral complex as it transited the cell. They observed the presence of intact capsids in the cytoplasm and much smaller complexes in the nucleus.