Abstract
SummaryFluorescent proteins (FPs) are of vital importance to biomedical research. Many of the currently available fluorescent proteins do not fluoresce when expressed in non‐native environments, such as the bacterial periplasm. This strongly limits the options for applications that employ multiple FPs, such as multiplex imaging and Förster resonance energy transfer (FRET). To address this issue, we have engineered a new cyan fluorescent protein based on mTurquoise2 (mTq2). The new variant is dubbed superfolder turquoise2ox (sfTq2ox) and is able to withstand challenging, oxidizing environments. sfTq2ox has improved folding capabilities and can be expressed in the periplasm at higher concentrations without toxicity. This was tied to the replacement of native cysteines that may otherwise form promiscuous disulfide bonds. The improved sfTq2ox has the same spectroscopic properties as mTq2, that is, high fluorescence lifetime and quantum yield. The sfTq2ox‐mNeongreen FRET pair allows the detection of periplasmic protein‐protein interactions with energy transfer rates exceeding 40%. Employing the new FRET pair, we show the direct interaction of two essential periplasmic cell division proteins FtsL and FtsB and disrupt it by mutations, paving the way for in vivo antibiotic screening.SignificanceThe periplasmic space of Gram‐negative bacteria contains many regulatory, transport and cell wall‐maintaining proteins. A preferred method to investigate these proteins in vivo is by the detection of fluorescent protein fusions. This is challenging since most fluorescent proteins do not fluoresce in the oxidative environment of the periplasm. We assayed popular fluorescent proteins for periplasmic functionality and describe key factors responsible for periplasmic fluorescence. Using this knowledge, we engineered superfolder mTurquoise2ox (sfTq2ox), a new cyan fluorescent protein, capable of bright fluorescence in the periplasm. We show that our improvements come without a trade‐off from its parent mTurquoise2. Employing sfTq2ox as FRET donor, we show the direct in vivo interaction and disruption of unique periplasmic antibiotic targets FtsB and FtsL.
Highlights
In Gram-negative bacteria, the cytoplasm is enveloped by an inner membrane (IM) and an asymmetric outer membrane (OM)
Robust folding is the first prerequisite for periplasmic fluorescent proteins (FPs) fluorescence
Most FPs do not fluoresce in the Gramnegative periplasm due to hampered folding of their ß-barrel or maturation defects
Summary
In Gram-negative bacteria, the cytoplasm is enveloped by an inner membrane (IM) and an asymmetric outer membrane (OM). As much as 30% of Escherichia coli’s proteins are predicted to localize to the envelope and many essential processes function fully or partly within the periplasm (Weiner and Li, 2008). The protein interactions in the periplasm are of great interest for biotechnological and medical purposes like synthesis of exogenous proteins and antibiotic development (Harvey et al, 2004). The most direct way to observe these proteins in living cells is by fluorescence microscopy of genetically encoded fusions to fluorescent proteins (FPs). Fluorescence provides a means of detecting protein-protein interactions by Förster resonance energy transfer (FRET). The stringent distance dependence for FRET is ideal to detect direct protein-protein interactions as these occur in the nanometre range, whereas indirect protein interactions usually occur on a larger distance scale and are not detectable by FRET (Piston and Kremers, 2007)
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