Single mRNA imaging in live cells is a useful technique to elucidate its precise localization and dynamics. We developed a method for visualizing endogenous mRNAs in living cells with single molecule sensitivity using genetically encoded probes. An RNA-binding protein of human PUMILIO1 (PUM-HD) was used for recognizing base sequences of a target mRNA, β-actin mRNA. Two PUM-HDs were modified by amino acid mutations to bind specifically to tandem 8-base sequences of the target mRNA. Because each PUM-HD was connected with amino- and carboxyl-terminal fragments of enhanced green fluorescent protein (EGFP), the probes emit fluorescence by reconstitution of EGFP fragments upon binding to β-actin mRNAs. The EGFP reconstituted on the mRNAs was monitored with a total internal reflection fluorescence microscope. Results show that each fluorescent spot in live cells represented a single β-actin mRNA and that distinct spatial and temporal movement of the individual β-actin mRNAs was visualized. We also estimated the average velocity of the movement of the single mRNAs along microtubules in live cells. This method is widely applicable to tracking various mRNAs of interest in the native state of living cells with single-mRNA sensitivity.

Channelrhodopsin-2 (ChR2) from the green alga Chlamydomonas reinhardtii functions as a light-gated cation channel that has been developed as an optogenetic tool to stimulate specific nerve cells in animals and control their behavior by illumination. The molecular mechanism of ChR2 has been extensively studied by a variety of spectroscopic methods, including light-induced difference Fourier transform infrared (FTIR) spectroscopy, which is sensitive to structural changes in the protein upon light activation. An atomic structure of channelrhodopsin was recently determined by x-ray crystallography using a chimera of channelrhodopsin-1 (ChR1) and ChR2. Electrophysiological studies have shown that ChR1/ChR2 chimeras are less desensitized upon continuous illumination than native ChR2, implying that there are some structural differences between ChR2 and chimeras. In this study, we applied light-induced difference FTIR spectroscopy to ChR2 and ChR1/ChR2 chimeras to determine the molecular basis underlying these functional differences. Upon continuous illumination, ChR1/ChR2 chimeras exhibited structural changes distinct from those in ChR2. In particular, the protonation state of a glutamate residue, Glu-129 (Glu-90 in ChR2 numbering), in the ChR chimeras is not changed as dramatically as in ChR2. Moreover, using mutants stabilizing particular photointermediates as well as time-resolved measurements, we identified some differences between the major photointermediates of ChR2 and ChR1/ChR2 chimeras. Taken together, our data indicate that the gating and desensitizing processes in ChR1/ChR2 chimeras are different from those in ChR2 and that these differences should be considered in the rational design of new optogenetic tools based on channelrhodopsins.

Subcellular localization and dynamics of mRNAs control various physiological functions in living cells. A novel technique for visualizing endogenous mRNAs in living cells is necessary for investigation of the spatiotemporal movement of mRNAs. A pumilio homology domain of human pumilio 1 (PUM-HD) is a useful RNA binding protein as a tool for mRNA recognition because the domain can be modified to bind a specific 8-base sequence of target mRNA. In this study, we designed PUM-HD to match the sequence of β-actin mRNA and developed an mRNA probe consisting of two PUM-HD mutants flanking full-length enhanced green fluorescent protein (EGFP). Fluorescence microscopy with the probe in living cells revealed that the probe was labeled precisely with the β-actin mRNA in cytosol. Fluorescent spots from the probe were colocalized with microtubules and moved directionally in living cells. The PUM-HD mutants conjugated with full-length EGFP can enable visualization of β-actin mRNA localization and dynamics in living cells.