Folding and maturation, or how to evolve your own GFP (part 2)

In part 1 I discussed the directed evolution of fast-folding GFPs. These were developed for specific purposes of improving the solubility, stability and folding of the protein. Now, I will discuss the maturation step and how it was measured for a variety of GFPs.

As mentioned in the first part, maturation is the final step in the transformation of GFP from a chain of amino acids to a fluorescent protein – the creation of the chromofore. The chromofore has a very long name (p-hydroxybenzylideneimidazolidone) which in wild type  GFP is formed from amino acids S65, Y66 and G67. In this process, the amide nitrogen of G67 backbone performs a nucleophilic attack on the carbonyl carbon of S65. Oxidation with atmospheric O2 and dehydration reactions create the imidazolinone ring, which is conjugated to the side chain of Y66.

In most papers that measure GFP maturation, the authors do not distinguish folding from maturation. In many cases, the experiments involve complete denaturation of the protein in vitro (e.g. by urea or guanidinium chloride) and then measuring fluorescence following re-folding. In vivo, it would be difficult to assess folding kinetics (because you do not really have an exact time zero for when the protein is being translated).  [In our lab, we are trying to develop a system to visualize translation in vivo in real time by imaging techniques.  If it works, it would be useful for determining exact folding & maturation rates in vivo of a single fluorescent protein].

In any case, it is easier to measure the kinetics of the maturation step, both in vivo and in vitro, simply by removing and adding oxygen. I mentioned in part 1 that this is how the maturation rate of GFP-S65 was calculated in vivo.  A new paper from 2011 from the lab of Funatsu analyzed the maturation rates of several GFPs in vitro. Their idea was simple – using a commercial kit, the performed in vitro transcription-translation reaction of the FPs at anaerobic conditions (they added catalase and glucose oxidase to get 0.1mg/l of oxygen). They then stopped the reaction and added oxygen, then followed fluorescence.

The proteins they assayed were: wild-type GFP, GFP-S65T, GFP-S65T/S147P, EGFP, sfGFP, Emerald, GFPm, GFPmut2, GFPmut3, sgGFP, “cycle 3”(a.k.a GFPuv), frGFP (a.k.a GFPuv3), GFPuv4 and 5 and two yellow FPs: EYFP and Venus..

Their results are interesting. As expected, the S65T mutation improved the maturation rate by 3.2 fold. However, EGFP matured only 20% faster than the S65T alone. The S65T/S147P is a variant that is stable under a range of temperatures. This mutant matured faster than EGFP, at a rate 40% faster than S65T alone. GFPmut2 and GFPmut3 (S65G/S72A) both showed a 7-fold faster maturation than WT GFP (>2-fold the S65T alone). sgGFP (SuperGloGFP, F64L/S65C/I167T, from Qbiogene) showed improved maturation rate compared to S65T (70%  faster).  Emerald, which contains 4 mutations on top of the EGFP mutations showed only a slight increase in maturation rate (similar to S65T/S147P). “cycle 3” was only slightly better than WT GFP, frGFP (which is cycle 3 +EGFP mutations combined) showed a maturation rate which was 10% less than EGFP.  Addition of the I167T mutation to create GFPuv5 increased the maturation rate by ~70% (just like in sgGFP).

Most interesting is the super-folder GFP (sfGFP), which showed a maturation rate of only 2.3-fold over the WT (that is ~70% of that of S65T). Thus, though this protein may be more stable and may fold very fast compared to other variants, the important step of maturation is the slowest among all variants tested (except the WT). Since folding assays measure fluorescence as the output for a mature protein, it means that the folding step (prior to maturation) is much faster than previously appreciated.

GFPm is a weird case.  GFPm, developed by David Tirrell, is a variant with mutations from cycle 3 and GFPmut3. However, Tirrell tried something unique –  to replace the leucines in the protein with 5,5,5- tri-fluoroleucine (tfl). The fluoreinated form proved to be insoluble, and fluorescent of the cells (E. coli) was 500-fold less than the regular protein. They then went on with mutagenesis, developing new variants which were brighter (up to 650-fold over GFPm). Personally, I do not understand how this can be of much use, since using tfl will probably have major effects on every aspect of cell biology we are interested in. Anyway, the maturation rate of GFPm (not fluoreinated) is itself pretty high – almost 3-fold over S65T and is actually the fastest maturing GFP variant tested. Oddly, they do not discuss this result anywhere in the text. Perhaps there is a technical issue they wanted to avoid?

Last but not least, the two YFPs showed maturation rates which are similar to WT GFP (Venus) or ~2.3-fold better (EYFP).

So how does all this information help us?

First, if we know which mutations enhance maturation and which slow it down, we can design faster-maturing proteins.

Second, we can use this data to estimate translation or translocation rates in vivo. However, we should remember that the data obtained in vitro (at 37C) does not neccessarily agree with actual maturation rates in vivo in every cell type. for instance, yeast or fly grow in colder temperatures which may affect maturation. Oxygen levels in tissue culture dish are differnt than in entire animals, and also differnt in differnt organs.  Also, if the GFP is fused to another protein, it may also affect folding as we learned in part 1, as well as maturation. Finally, the in vitro environment in the tube lacks many biomolecules (proteins other than used for translation, small molecules, ions, oxidizing molecules, antioxidants etc) which can affect oxygen availability in the immediate environment of the newly translated GFP.

Third, we can use such data to design cool experiments. So… stay tuned for part 3, which I will dedicate to biological timers.

ResearchBlogging.orgIizuka R, Yamagishi-Shirasaki M, & Funatsu T (2011). Kinetic study of de novo chromophore maturation of fluorescent proteins. Analytical biochemistry, 414 (2), 173-8 PMID: 21459075
Yoo TH, Link AJ, & Tirrell DA (2007). Evolution of a fluorinated green fluorescent protein. Proceedings of the National Academy of Sciences of the United States of America, 104 (35), 13887-90 PMID: 17717085

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11 responses to “Folding and maturation, or how to evolve your own GFP (part 2)

  1. Hi,
    Thanks for this very interesting series of posts! I’m interested in studying maturation of a few different GFP variants. I’d like to make the assay as easy as possible, so I would like to do things in vivo.

    Would the following make sense:
    1) grow up cells expressing GFP variants anaerobically in liquid culture.
    2) Dilute them into aerobic liquid media containing chloramphenicol to stop protein synthesis (at the same time, using a multichannel pipette, so they receive oxygen simultaneously)
    3) measure maturation in a fluorescence plate reader

    Like

    • Hi Tim,
      Glad you like it.
      It sounds like a good plan. But before the experiment, I would do some controls:
      1. Measure the fluorescent parameters (especially brightness), expression level and oligomeric state (including aggregation into inclusion bodies) of each variant (in vivo and in vitro). These parameters must be normalized so you can compare the fluorescence in your maturation assay.
      2. chloramphenicol will stop translation, but may induce protein degradation. I suggest a preliminary experiment just to see how stable your GFP variants are (at least for the duration of your maturation assay) when translation is inhibited.
      3. Of course have the proper indicators so you could follow O2 levels.

      Good luck!

      Like

  2. Hi,
    I need to correct a misconception. You seem to state the kinetics of the refolding experiments of sfGFP don’t account for maturation…and that the real refolding rate is even “faster than previously appreciated”. It seems as if you think the chromphore decyclizes when the protein shell is unfolded. In fact it doesn’t. In the refolding experiments in Waldo & co-workers, the chromophore had already fully matured prior to the unfolding. Even when subsequently unfolded, the chromophore remains, eliminating the need to go through the maturation step during subsequent refolding. Monitoring appearance of fluorescence effectively measures the forward folding rate, unaffected by maturation.

    Like

  3. Hi,
    I just read your post and I am very interested in the subject.
    Iizuka et al. 2011 claimed that they calculated maturation rate constants by measuring fluorescence from the time when they added oxygen. Their conclusion was basically that the maturation might be faster than previously reported.
    Considering that two reactions that are part of the fluorophore maturation process (cyclization and dehydration) happen before oxydation, I am confused as to why they thought they were looking at the maturation process as a whole. Weren’t they only following the oxydation reaction?

    Regards,

    Ivan
    USyd

    Like

    • yes, they only looked at the oxidation step.
      there’s a mix up of definitions: folding, maturation etc which adds to the confusion.
      anyway, from my limited understanding, the claim is that because there wasn’t a major difference at the ‘oxidation’ step (sfGFP compared to S65T) but we know the total maturation is faster, it suggests that the folding, cyclization etc… are faster than appreciated.

      Like

  4. Hej
    Thanks a lot for this amazing and summarising input! I am still a little bit puzzled to what I have already read in literature: Now having folding and maturation times in mind – out of your knowledge:

    What would be the actual GFP variant of choice to use for fusion proteins to be transported as fusion partner in the periplasm.

    Meaning which GFP variant folds “slow” enough to not hamper the transport of a fused periplasmic protein but which matures reasonably well under periplasmic conditions?
    Thanks a lot!
    Best regards,
    Tonja

    Like

    • Geoffrey Waldo

      Our superfolder GFP (Pedelacq…Waldo & coworkers nature biotech 24(1) 2006) transits to the periplasm very nicely and works in t. thermophilus at 70-75C fused to PhoA. Several other labs have published using it in periplasm.

      Like

    • Geoffrey Waldo

      Our superfolder GFP (Pedelacq…Waldo & coworkers nature biotech 24(1) 2006) transits to the periplasm very nicely and works in t. thermophilus at 70-75C fused to PhoA. Several other labs have published using it in periplasm. If you are going to fuse things to GFP, then superfolder is the way to go because it has been evolved to fold independently of fusions.

      Liked by 1 person

  5. Pingback: Design guidlines for tandem fluorescent timers | greenfluorescentblog

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