Tag Archives: photostability

An excelent new tool for comparing fluorescent protein properties

Screenshot of the new tool

Screenshot of the new tool

Two very useful tools for visualizations of many fluorescent and photoswitchable proteins have been developed by Talley Lambert and Kurt Thorn (UCSF). Continue reading

Silicon nanocrystals – the next generation of fluorescent dyes?

Fluorescent microscopy is the only current method to follow biological structures and molecules in real-time in live specimens. Many advances were made but there are still a few problems with present fluorescent probes. Photobleaching (permanent disappearance of the fluorescent signal due to destruction of the fluorophore) is a major problem for all current fluorescent probes, be it chemical or biological. Photobleaching prevents prolong imaging.

A second problem is blinking of the fluorophore. Although blinking is actually useful in some super resolution techniques, it may still pose a problem for some applications.

Another problem is the size of some fluorescent tags such as fluorescent proteins and quantum dots. FP fusion to the studied protein can affect to protein function, simply by spatial interference.

Now, a new type of fluorophore was developed by the lab of Akihiro Kusumi from Kyoto university. Silicon nanocrystals (SiNCs) were apparently known for two decades as fluorescent, that are stable and can be used for biological applications. However, no serious effort was made to make SiNCs applicable in cell biology studies. This, he says, is due to the difficulty in producing SiNCs, manufacturing them in uniform size (and the size determines the fluorescent properties) and properties and conjugating them to biomolecules.

Kusumi developed a simplified protocol to produce mercaptosilane coated SiNC (mSiNC) that has a hydrodynamic diameter of 4.1 nm.  This was confirmed by three independent methods: gel filtration, FCS and dynamic light scattering (DLS). The hydrodynamic diameter of GFP is ~5.5nm and of Qdot655 – 19.5nm. They further measured by electron microscopy the actual size of the mSiNC core to be 3.2nm.

A & B - Elution traces of gel filtration experiments. The longer the molecule stays in the gel, the smaller it is. C- DLS analysis of mSiNC. D - FCS curves of the indicated molecules. E - image of mSiNCs obtained by electron microscopy (EM). F - distribution of measured diameter based on EM imaging.

A & B – Elution traces of gel filtration experiments. The longer the molecule stays in the gel, the smaller it is. C- DLS analysis of mSiNC. D – FCS curves of the indicated molecules. E – image of mSiNCs obtained by electron microscopy (EM). F – distribution of measured diameter based on EM imaging.

Characterization of the fluorescent properties showed that  the mSiNC fluorescent emission peak is at 655nm with a broad spectrum. Storage of dehydrated mSiNC did not affect the properties for over two months. However, fluorescence intensity decrease if stored in aqueous buffer at 37C (exponential decay time of 42 hours). Exposure of mSiNCs to excitation under fluorescent microscopy conditions showed that they are not blinking of photobleacing for over 300 minutes! Under similar conditions, GFP and the Cy3 dye photobleach within 3 seconds. the Qdot exhibited frequent blinking and fluctuations in fluorescent intensity compared to the mSiNC. mSiNCs fluorescence intensity is lower than Qdots. However, due to the instability of Qdot fluorescence, this difference diminishes over time.

After characterization came the actual test: they conjugated mSiNC to the transferrin protein (mSiNC-Tf) and applied it to cells, to follow the dynamics of cell surface transferrin receptors (TfR). whereas GFP-Tf allowed tracking for only 20 frames (0.67 sec), and Cy3-Tf allowed tracking of 150 frames (5sec), mSiNC-Tf allowed tracking for 3600 frames (120 seconds). This is quite an improvement.

A - a typical image of immobilized single mSiNC molecules. B - Distribution of fluorescence intensities of of single mSiNC spots (top) and single Qdot655 (bottom). C - Typical time-dependent changes of the fluorescence intensities of an mSiNC spot (top) and of a Qdot655 spot (bottom).

A – a typical image of immobilized single mSiNC molecules. B – Distribution of fluorescence intensities of of single mSiNC spots (top) and single Qdot655 (bottom). C – Typical time-dependent changes of the fluorescence intensities of an mSiNC spot (top) and of a Qdot655 spot (bottom).

Usually, due to blinking, single-molecule trajectories have gaps in them. These trajectories are later reconstructed based on the assumption that these gaps are due to blinking. With mSiNC-Tf, no gaps were observed (even of a single frame) for the whole 120 second (3600 frames) tracking. Thus, using mSiNC-Tf, they could follow the dynamics of TfR for long periods of time.

So, the advantages of mSiNC are very clear – they are extremely photostable, which allows for prolong imaging without loss of fluorescence. They are also small, allowing single molecule imaging with minimal perturbation. They can be conjugated to proteins.

Here are links to videos showing the lack of blinking; mSiNC-Tf on cell membrane (exhibiting Brownian motion) and what seems to be receptor internalization.

What are the disadvantages?

Well, first, the protocol to produce them, and verify their uniformity, though simplified, is yet not simple.

Second, the very wide emission spectra limits the use for multicolor imaging.

Last, similar to other dyes, it is excellent for live imaging of outer-membrane molecules, but may be more difficult for intra-cellular tagging for live imaging.

ResearchBlogging.orgNishimura H, Ritchie K, Kasai RS, Goto M, Morone N, Sugimura H, Tanaka K, Sase I, Yoshimura A, Nakano Y, Fujiwara TK, & Kusumi A (2013). Biocompatible fluorescent silicon nanocrystals for single-molecule tracking and fluorescence imaging. The Journal of cell biology, 202 (6), 967-83 PMID: 24043702

New and improved – the next generation of GFP?

A new and improved green fluorescent protein, named mNeonGreen, was developed.

It was engineered from a Yellow fluorescent protein (LanYFP) that was isolated from the cephalochordate Branchiostoma lanceolatum. Therefore, LanYFP is genetically unrelated to the commonly used Aequorea victoria GFP.

LanYFP has a high quantum yield (0.95) and extinction coefficient (~150,000 M−1 cm−1) – making it a very bright protein.  LanYFP is a tetramer – not useful for most applications.

Directed evolution to make it a monomer produced the new, monomeric protein, mNeonGreen. The ex/em of mNeonGreen is slightly shifted to the yellow compared to EGFP (509/516 compared to 488/509), making it a better choice to separate from CFP emission.

Though slightly less brighter than its parent protein LanYFP, mNeonGFP is 2-3 times brighter than EGFP and actually brighter than most green & yellow proteins.

Another great advantage of this new protein is that is it fast folding – the authors claim it is <10 min at 37C. This is fairly close to the superfolder GFP.

It is also very photostable (comparable to EGFP), performs well as a fusion construct at N & C termini many tested proteins, performs 4-times better that EGFP in stochastic single-molecule superresolution imaging and is a better FRET partner (both as acceptor and donor) than other proteins.

mNeonGreen fused to histone H2B shows the different stages of the chromosomes during cell division. Source: Shaner et al., (2013) Nature Methods 10:407-409.

mNeonGreen fused to histone H2B shows the different stages of the chromosomes during cell division. Source: Shaner et al., (2013) Nature Methods 10:407-409.

In short, this may very well be the “next generation” of fluorescent proteins. It has all the good qualities, and seems to have none of the bad ones. It performs better than most, if not all fluorescent proteins in every tested parameter.

Only its name is rather plain. I would call it something like wonderGFP or GreenLantern (hey, it even has the Lan from the animal they developed it from).

(Update: see here for details on how to get your hands on this protein).

ResearchBlogging.orgShaner NC, Lambert GG, Chammas A, Ni Y, Cranfill PJ, Baird MA, Sell BR, Allen JR, Day RN, Israelsson M, Davidson MW, & Wang J (2013). A bright monomeric green fluorescent protein derived from Branchiostoma lanceolatum. Nature methods, 10, 407-409 PMID: 23524392

The red color dilemma: how to choose the right fluorescent protein

This post continues the previous post.

I encountered a serious dilemma in choosing the right protein. This is also a great opportunity to learn about the many properties of fluorescent proteins.

Let us start with the obvious: excitation and emission maxima and spectra.

We all know that EGFP has an excitation maximum at 488nm. That is, EGFP protein that is excited with photons at 488nm will give its maximum emission intensity at its emission maximum, 509nm.

However, we must remember that this is the maximum emission. The emission spectra is much wider, and for GFP it goes from ~470nm up to ~630nm. Figure 1 shows the EGFP excitation (dashed line) and emission (full green) spectra.

Fig. 1: GFP specrum with 488 excitation

And now, what if we want to look simultaneously at two colors, EGFP and dTomato?

Figure 2 (upper panel) shows you that excitation at 488nm (the max ex. of EGFP) also excite dTomato, leading to almost 30% emission at its peak of 581nm. Therefore, if we would collect all the emitted photons, we would detect the combined excitation of EGFP and dTomato.

Fig. 2: GFP dTomato spectra with 488 excitation. Lower panel: with filters.

We therefore use filters with a narrow band of wavelength (fig. 2, bottom panel). Thus, if we use the 510/20 filter, we would detect only photons emitted at the band, in this case only from EGFP. If we use the 580/30 filter, we would detect photons coming from dTomato and EGFP.

Now what happens if we add the long-stokes shift protein LSSmKate1 to the system? Figure  3 shows LSSmKate1 is maximally excited at 463nm (red dashed), and has max. emission at 624nm (full red) [note- I drew the LSSmKate spectra, based on prior publications]. However, 460nm also excites EGFP and dTomato. The EGFP signal, at 624nm is negligible. However, dTomato gives emission at 8-9% of its maximal emission. Is this negligible? We will soon learn.

Fig. 3: GFP dTomato LSSmkate1 spetra with 460 excitation

Now let’s look at mCherry. With excitation laser at 460, we have virtually no excitation of mCherry. We would therefore prefer to use mCherry in our 3-color system, instead of dTomato.

Fig. 4: GFP mCherry LSSmkate1 spectra with 460 excitation

This table summarizes the excitation & emission maxima:

FP Max ex. Max em.
EGFP 488 509
dTomato 554 581
LSSmKate1 463 624
LSSmKate2 460 605
mCherry 587 610

This table shows the relative emission of the different FPs with different excitations (the LSS data is estimated based on publications, since the BD

spectrum viewer does not include LSSmKates in its database):

  % from max emission % from max emission

Em   (nm):

509 580 605 610 624 509 580 605 624
EGFP Ex.488 100% 8% <5% Ex.460 65% 5%
dTomato 27% 20% 19% 15% 15% 11% 9%
LSSmKate1 50%? 100
LSSmKate2 50%? 100
mCherry 8%

However, excitation & emission spectra are not the only considerations. There are other parameters to consider.

Brightness, QY and EC

Different fluorescent proteins have different brightness: some are very bright, some are dim. As a matter of history, we usually consider the relative brightness, compared to EGFP (which is set to 1.00). You can see in the table below the relative brightness of the proteins discussed above.

The brightness is determined by two parameters: quantum yield (QY) and Extinction coefficient (EC).

QY is simply the ration between the number of photons absorbed to the number of photons emitted. If QY=1, then for each absorbed photon you get one emitted photon.  However, we never get 100% efficiency. The protein with the highest QY that I encountered is called ZsGreen with QY=0.91.

EC (or ε) relates to the formula A = εcl in which A is the absorbance, l is the path length (in cm, usually) and c the concentration in Molar units. ε, in M-1cm-1 units, is a measurement of the capability of a certain fluorescent protein to absorb light at a certain wavelength.

The formula ε*QY gives a measure of the brightness, so for EGFP, the brightness is 55,000*0.60=33,000

For ZsGreen, the EC is only 43,000 so the relative brightness is only 1.18. This example shows that a high QY does not necessarily mean a very high brightness.

This table summarizes al the brightness data:

FP QY EC(M-1 cm-1) Brightness Relative brightness
EGFP 0.60 55,000 33,000 1.00
dTomato 0.69 69,000 47,610 1.44
LSSmKate1 0.08 26,000 2080 0.06
LSSmKate2 0.17 31,200 5304 0.16
mCherry 0.22 72,000 15,840 0.48

Let’s go back to our dilemma: dTomato or mCherry?

We now know that with a laser excitation of 460nm, we get 100% emission from LSSmKat1 at 624nm, and only 9% emission from dTomato at 624nm.

However, dTomato is 24-fold brighter than LSSmKate1.  If we look only on the QY data, at 8% emission, dTomato will produce ~6 photons for each 100 photons (100*0.69*9%), whereas LSSmKat1 will produce 8 photons (100*0.08*100%). In other words, although we get very low emission of dTomato, relative to its maximum, it is almost as high as the LSSmKate1 emission in that wavelength. Since eventually, we just “see” the photons with no knowledge of their source, the signal that we will get will be ~40% from dTomato. This will create a problem if we wish to detect changes in fluorescent intensity over time due to changes in protein localization (or other changes) because we will not be able to know if the change is due to dTomato or LssmKate1.

That is why mCherry is a better choice than dTomato for this experiment.

Photostability

Another parameter to take into consideration is the protein’s photostability.

Photostability is a measure (in seconds) of how long it takes for half of the number of proteins to bleach at the maximum excitation.  This matter is important for any experiment involving fluorescent proteins (or dyes) but is crucial when doing live imaging, particularly for time-lapse experiments.

LSSmKate1 is more stable than LSSmKate2 (see table below), and therefore, it might be a better choice than LSSmKate2 for my time-lapse experiment.

FP photostability
EGFP 174 sec
dTomato 98 sec
LSSmKate1 60 sec
LSSmKate2 44 sec
mCherry 96 sec

There are other parameters to take into account, some of them I discussed in earlier posts.

These include:

pH stability (and the effect of pH on the absorption and emission spectra).

Maturation rate – how long does it take your fluorescent protein to fold correctly and create the chromophore? If you are using your protein to measure expression rate, and the maturation time is longer than the time frame of your experiment, then you will not get the information you are looking for. Maturation rate  depends on protein characteristics, as well as oxygen levels and temperature. Several fast-folding FPs were developed in recent years.

Oligomerization state – many FPs are naturally monomeric. However some FPs particularly in the orange & red range) are dimeric or tetrameric.  Since in many cases the FP is genetically encoded as a fusion with a protein of interest, fusing a dimeric FP may cause your protein of interest to dimerize, through the FP, thus altering its biology. If your protein is naturally oligomeric, fusing it to a dimeric or tetrameric FP may create large protein aggregates. One solution for dimeric FPs is to create a tandem fusion proteins (i.e. two FPs are fused one to the other and to the your protein).

Generally, the letter m at the beginning of the FP name indicates that it is monomeric (e.g. mCherry, mKate), d indicates it is dimeric (dTomato) and td indicates tandem dimer (tdTomato). However, there are many FPs with no indication of their oligomeric state in their name (e.g. TurboRFP is dimeric). EGFP and its many derivatives are monomeric (except at high concentrations when they form weak dimers).

Phototoxicity  – although the FPs themselves do not emit harmful radiation, the excitation light can be harmful to the cell. For instance, UV light, that is used to excite BFPs, as well as some LSS FPs and photoactivatable FPs, can cause direct DNA damage, create reactive oxygen species that will cause an oxidative stress etc…  This is one reason why BFPs are the least popular FPs, particularly for live imaging.