Wannier, T. M. et al. Proc. Natl. Acad. Sci. USA 115, E11294–E11301 (2018).

Commonly used ‘monomeric’ red fluorescent proteins (RFPs) are engineered from naturally oligomeric precursors. This monomerization is crucial for RFPs’ performance as protein tags, as fusion to a dimeric or tetrameric fluorescent protein will lead to obligate oligomerization of the protein of interest, which can cause artifacts. However, monomeric variants of fluorescent proteins are often substantially dimmer than their precursors owing to extensive mutagenesis. Wannier et al. have developed a comprehensive approach for generating true monomers from oligomeric RFPs that treats separately the problems of protein stabilization, core optimization, and surface engineering. To demonstrate their approach, they developed monomeric variants of two far-red fluorescent proteins, mCardinal and HcRed. One of the developed variants, mKelly1, represents a promising far-red tag for biological applications.