Monomeric fluorescent timers that change color from blue to red report on cellular trafficking

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Abstract
Based on the mechanism for chromophore formation in red fluorescent proteins, we developed three mCherry-derived monomeric variants, called fluorescent timers (FTs), that change their fluorescence from the blue to red over time. These variants exhibit distinctive fast, medium and slow blue-to-red chromophore maturation rates that depend on the temperature. At 37 °C, the maxima of the blue fluorescence are observed at 0.25, 1.2 and 9.8 h for the purified fast-FT, medium-FT and slow-FT, respectively. The half-maxima of the red fluorescence are reached at 7.1, 3.9 and 28 h, respectively. The FTs show similar timing behavior in bacteria, insect and mammalian cells. Medium-FT allowed for tracking of the intracellular dynamics of the lysosome-associated membrane protein type 2A (LAMP-2A) and determination of its age in the targeted compartments. The results indicate that LAMP-2A transport through the plasma membrane and early or recycling endosomes to lysosomes is a major pathway for LAMP-2A trafficking.