Protein engineering has created a palette of monomeric fluorescent proteins (FPs), but there remains an ~30 nm spectral gap between the most red-shifted useful Aequorea victoria green FP (GFP) variants and the most blue-shifted useful Discosoma sp. red FP (RFP) variants. To fill this gap, we have engineered a monomeric version of the yellow FP (YFP) from Zoanthus sp. coral. Our preferred variant, designated as mPapaya1, displays excellent fluorescent brightness, good photostability, and retains its monomeric character both in vitro and in living cells in the context of protein chimeras. We demonstrate that mPapaya1 can serve as a good Förster resonance energy transfer (FRET) acceptor when paired with an mTFP1 donor. mPapaya1 is a valuable addition to the palette of FP variants that are useful for multicolor imaging and FRET-based biosensing.
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