Characterization of an improved donor fluorescent protein for Forster resonance energy transfer microscopy

Abstract

The genetically encoded fluorescent proteins (FP), used in combination with Forster resonance energy transfer (FRET) microscopy, provide the tools necessary for the direct visualization of protein interactions inside living cells. Typically, the Cerulean and Venus variants of the cyan and yellow FPs are used for FRET studies, but there are limitations to their use. Here, Cerulean and the newly developed monomeric Teal FP (mTFP) are compared as FRET donors for Venus using spectral and fluorescence lifetime measurements from living cells. The results demonstrate that when compared to Cerulean, mTFP has increased brightness, optimal excitation using the standard 458-nm laser line, increased photostability, and improved spectral overlap with Venus. In addition, the two-photon excitation and fluorescence lifetime characteristics are determined for mTFP. Together, these measurements indicate that mTFP is an excellent donor fluorophore for FRET studies, and that its use may improve the detection of interactions involving proteins that are difficult to express, or that need to be produced at low levels in cells.

Fig. 1

The spectral overlap of Cerulean or mTFP with Venus is compared. The excitation and emission spectra for (a) CFP or (b) mTFP in combination with Venus are shown, illustrating the spectral overlap (gray shaded area). The 458-nm laser line and the FRET emission channel (535 to 590 nm) are shown, with DSBT into the FRET channel indicated by cross-hatching. (c) The efficiency of energy transfer E_FRET is plotted as a function of the separation distance (r) for both the Cerulean-Venus and mTFP-Venus fluorophore pairs, and was determined according to Eq. (1) (see Sec. 3.1). The R₀ value was determined from the overlap integral J_λ (see Table 1). The separation distance spanning the range of 0.5 R₀ to 1.5 R₀ is shaded. (d) The difference in E_FRET for the Cerulean-Venus and mTFP-Venus pairs is plotted as a function of separation distance.

Fig. 2

The photobleaching characteristics of Cerulean or mTFP when illuminated with the 458-nm laser line are compared. Cells that expressed either Cerulean or mTFP were illuminated with the 458-nm laser line using 1.8-μW laser power at the specimen plane, and a single scan was acquired. Cells with approximately 2000 gray-level intensity were selected and exposed to 80 scan cycles using the same laser power. The mean normalized gray-level intensity (±SD) for each consecutive scan was determined for three to five cells.

Fig. 3

The emission spectra were acquired from cells that expressed the Cerulean, mTFP, or Venus FPs. Spectral imaging with the 32-channel detector was used to collect lambda stacks at a bandwidth of 10.7 nm from cells that expressed either (a) Cerulean, (b) mTFP, or (c) Venus; the calibration bar indicates 10 μm. The lambda stacks were used to generate the reference spectra shown for each fluorophore. (c) The Venus emission spectrum was acquired with excitation using either the 514-nm (solid line) or 458-nm (dotted line) laser line. These reference spectra were used for spectral unmixing of the component signals from cells that expressed the Cerulean-Venus or mTFP-Venus fusion proteins.

Fig. 4

The spectral measurements of the signal from cells expressing either (a) Cerulean or (b) mTFP directly coupled to Venus. The calibration bar indicates 10 μm. The FRET standards (Cer-5aa-Venus and mTFP-5aa-Venus) show a strong acceptor signal relative to the donor signal, which is consistent with FRET. In contrast, the Cer-TRAF-Venus and mTFP-TRAF-Venus show reduced acceptor signal relative to the donor, consistent with low FRET efficiency. Spectral imaging was used to collect lambda stacks from cells that expressed the fusion proteins, and the reference spectra in Fig. 3 were used for spectral unmixing of the component signals from cells that expressed the different fusion proteins to determine the mean FRET efficiency for each fusion protein (Table 2).

Fig. 5

The measurement of the changes in the donor signals from either the (a) mTFP-5aa-Venus or (b) mTFP-TRAF-Venus fusion proteins after acceptor photobleaching. Spectral measurements were acquired from cells expressing the indicated fusion proteins; the calibration bar indicates 10 μm. The linked Venus fluorophore was then photobleached by more than 70% using the 514-nm laser line. The spectral measurements were then reacquired under identical conditions to the first, and changes in the donor signal were measured. The dashed line in (a) indicates the change in the donor signal for the mTFP-5aa-Venus fusion protein. In contrast, there was little change in the donor signal for (b) mTFP-TRAF-Venus following acceptor photobleaching.

Fig. 6

The 2p-excitation spectrum for mTFP and donor lifetime measurements for the fusion proteins consisting of Cerulean or mTFP linked to Venus. (a) Cells expressing mTFP alone were illuminated at the indicated wavelength, and the normalize gray-level intensity (±SD) was determined. (b) Cells expressing Cerulean or mTFP alone, or the FRET standards Cer-5aa-Venus or mTFP-5aa-Venus, were used to acquire fluorescence lifetime measurements as described in the text. The fluorescence lifetime decay kinetics for donor fluorophores alone or in the presence of Venus were determined by fitting the data to a double exponential decay. The lifetime distributions for representative cells expressing (c) Cerulean (the calibration bar indicates 10 μm), (d) mTFP, (e) Cer-5aa-Venus, or (f) mTFP-5aa-Venus are shown. The results of the fluorescence lifetime analysis are summarized in Tables 2 and 3.