Figure 1.

Principle of super-resolution imaging of protein-protein interactions with BiFC of phototransformable fluorescent proteins (ptFPs). Protein A is fused to an N-terminal fragment of the ptFP (ptFP-N); and protein B is fused to the complementary C-terminal fragment (ptFP-C). When protein A and B interacts, ptFP-N and ptFP-C reconstitute. Following chromophore maturation, fluorescently labeled interaction loci can be excited and phototransformed for super-resolution detection with SOFI, PALM, or RESOLFT. The choice of SRM technique depends on photoactivation, photoconversion, or photoswitching property of FP used in each study.