Characterization of Split Fluorescent Protein Variants and Quantitative Analyses of Their Self-Assembly Process

Many biotechniques use complementary split-fluorescent protein (sFPs) fragments to visualize protein-protein interactions, image cells by ensemble or single molecule fluorescence microscopy, or assemble nanomaterials and protein superstructures. Yet, the reassembly mechanisms of sFPs, including fragment binding rates, folding, chromophore maturation and overall photophysics remain poorly characterized. Here, we evolved asymmetric and self-complementing green, yellow and cyan sFPs together with their full-length equivalents (flFPs) and described their biochemical and photophysical properties in vitro and in cells. While re-assembled sFPs have spectral properties similar to flFPs, they display slightly reduced quantum yields and fluorescence lifetimes due to a less sturdy β-barrel structure. The complementation of recombinant sFPs expressed in vitro follows a conformational selection mechanism whereby the larger sFP fragments exist in a monomer-dimer equilibrium and only monomers are competent for fluorescence complementation. This bimolecular fragment interaction involves a slow and irreversible binding step, followed by chromophore maturation at a rate similar to that of flFPs. When expressed as fusion tags in cells, sFPs behave as monomers directly activated with synthetic complementary fragments. This study resulted in the development of sFP color variants having improved maturation kinetics, brightness, and photophysics for fluorescence microscopy imaging of cellular processes, including single molecule detection.