Monomerization of Far-Red Fluorescent Proteins

Anthozoa class red fluorescent proteins (RFPs) are frequently used as biological markers, with far-red emitting variants (λem ~ 600 – 900 nm) sought for whole animal imaging because biological tissues are permeable to light in this range. A barrier to the use of naturally occurring RFP variants as molecular markers is that all are tetrameric, which is not ideal for cell biological applications. Efforts to engineer monomeric RFPs have usually produced dimmer and blue-shifted variants, as the chromophore is sensitive to small structural perturbations. In fact, despite much effort, only four native RFPs have been successfully monomerized, leaving the vast majority of RFP biodiversity untapped in biomarker development. Here we report the generation of monomeric variants of HcRed and mCardinal, both far-red dimers, and describe a comprehensive methodology for the rapid monomerization of novel red-shifted oligomeric RFPs. Among the resultant variants, is mKelly1 (emission maximum: λem = 656 nm), which along with the recently reported mGarnet2 ([1][1]), forms a new class of bright, monomeric, far-red FPs. [1]: #ref-1