mSumireF a Monomeric Violet Fluorescent Protein.

The development of genetically encoded labels for multicolor experiments requires a diverse palette of fluorescent proteins that are well-behaved. Here, we report mSumireF, a monomeric variant of the violet fluorescent protein Sumire. On its own, the canonical monomerizing valine-to-lysine mutation at residue 206 causes a significant loss of brightness for the mSumire variant. We found that brightness is recovered upon the reversion of mSumire's tyrosine at position 165 back to phenylalanine as in its superfolder GFP grandparent. Importantly, this mSumireF variant exhibits significantly less oligomerization tendency in the organized smooth endoplasmic reticulum (OSER) assay in mammalian cells and in protein solution assays. Furthermore, we demonstrate that an mSumireF donor can be effectively paired with the fluorescent protein acceptors mCerulean3, mTurquoise2, and LSSmScarlet to generate FRET-based ATP biosensors. mSumireF thus provides an improved violet fluorescent protein to expand color options with reduced concern of unwanted dimerization.