eqFP611 is a basic (constitutively fluorescent) red fluorescent protein published in 2002, derived from Entacmaea quadricolor. It is reported to be a very slowly-maturing tetramer with very low acid sensitivity.
The protein has a reduced tendency to oligomerize, as shown by its monomeric appearance in SDS/PAGE analysis and single-molecule experiments. However, it forms tetramers at higher concentrations in the absence of detergent.
Together, eqFP578 and eqFP611 represent a separate group of red fluorescent proteins, which are characterized by high fluorescence brightness, complete chromophore maturation and, notably, a reduced tendency to oligomerize. These features make E. quadricolor fluorescent proteins an attractive starting point for the generation of bright monomeric red fluorescent proteins.
The precursor to mRuby, eqFP611 dimer, readily undergoes reversible photobleaching due to isomerization from a bright trans form to a dim cis form, and mutations near the chromophore affect isomerization