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GFP結(jié)構(gòu)的研究

The Molecular Structure of Green Fluorescent Protein

Fan Yang1, Larry G. Moss2, and George N. Phillips, Jr.1

1Department of Biochemistry and Cell Biology and the W.M. Keck Center for Computational Biology, Rice University, Houston, TX 77005-1892

and 2 Division of Endocrinology, Department of Medicine, Tufts University School of Medicine and the New England Medical Center, Boston, MA 02111

Introduction

Green fluorescent protein, GFP, is a spontaneously fluorescent protein isolated from coelenterates, such as the Pacific jellyfish, Aequoria victoria1. Its role is to transduce, by energy transfer, the blue chemiluminescence of another protein, aequorin, into green fluorescent light2. The molecular cloning of GFP cDNA3 and the demonstration by Chalfie that GFP can be expressed as a functional transgene4 have opened exciting new avenues of investigation in cell, developmental and molecular biology. Fluorescent GFP has been expressed in bacteria4, yeast5, slime mold6, plants7, 8, drosophila9, zebrafish10, and in mammalian cells11, 12. GFP can function as a protein tag, as it tolerates N- and C-terminal fusion to a broad variety of proteins many of which have been shown to retain native function.6, 13, 14 When expressed in mammalian cells fluorescence from wild type GFP is typically distributed throughout the cytoplasm and nucleus, but excluded from the nucleolus and vesicular organelles (reviewed by Cubitt et al.13, LG Moss unpublished observations). However, highly specific intracellular localization including the nucleus, mitochondria15, secretory pathway16, plasma membrane17 and cytoskeleton5 can be achieved via fusions both to whole proteins and individual targeting sequences. The enormous flexibility as a noninvasive marker in living cells allows for numerous other applications such as a cell lineage tracer, reporter of gene expression and as a potential measure of protein-protein interactions18.

Green fluorescent protein is comprised of 238 amino acids. Its wild-type absorbance/ excitation peak is at 395 nm with a minor peak at 475 nm with extinction coefficients of roughly 30,000 and 7,000 M-1 cm-1, respectively19. The emission peak is at 508 nm. Interestingly, excitation at 395 nm leads to decrease over time of the 395 nm excitation peak and a reciprocal increase in the 475 nm excitation band13. This presumed photoisomerization effect is especially evident with irradiation of GFP by UV light. Analysis of a hexapeptide derived by proteolysis of purified GFP led to the prediction that the fluorophore originates from an internal Ser-Tyr-Gly sequence which is post-translationally modified to a 4-(p-hydroxybenzylidene)- imidazolidin-5-one structure20. Studies of recombinant GFP expression in E. coli led to a proposed sequential mechanism initiated by a rapid cyclization between Ser65 and Gly67 to form a imidazolin-5-one intermediate followed by a much slower (hours) rate-limiting oxygenation of the Tyr66 side chain by O2 21. Combinatorial mutagenesis suggests that the Gly67 is required for formation of the fluorophore22. While no known co-factors or enzymatic components are required for this apparently auto-catalytic process, it is rather thermosensitive with the yield of fluorescently active to total GFP protein decreasing at temperatures greater than 30 C23. However, once produced GFP is quite thermostable.

Physical and chemical studies of purified GFP have identified several important characteristics. It is very resistant to denaturation requiring treatment with 6 M guanidine hydrochloride at 90 C or pH of <4.0 or >12.0. Partial to near total renaturation occurs within minutes following reversal of denaturing conditions by dialysis or neutralization24. Circular dichroism predicts significant amounts of -sheet structure that is subsequently lost on denaturation.24 Over a nondenaturing range of pH, increasing pH leads to a reduction in fluorescence by 395 nm excitation and an increased sensitivity to 475 nm excitation25. Reduction of purified GFP by sodium dithionite results in a rapid loss of fluorescence that slowly recovers in the presence of room air. While insensitive to sulfhydryl reagents such as 2-mercaptoethanol, treatment with the sulfhydral reagent dithiobisnitrobenzoic acid (DTNB) irreversibly eliminates fluorescence26.

The availability of E. coli clones expressing GFP has led to extensive mutational analysis of GFP function. Truncation of more than 7 amino acids from the C-terminus or more than the N-terminal Met lead to total loss of fluorescence27. All non-fluorescent mutants also failed to exhibit absorption spectra characteristic of the intact fluorophore, implying a possible defect in post-translational processing. Screens of random and directed point mutations for changes in fluorescent behavior have uncovered a number of informative amino acid substitutions. Mutation of Tyr66 in the fluorophore to His results in a shift of the excitation maximum to the UV (383 nm) with emission now in the blue at 448 nm21. A Tyr66Trp mutant is blue-shifted albeit to a lesser degree. Both changes are associated with a severe weakening of fluorescence intensity compared to wild type GFP. Mutation of Ser65 to Thr, Ala, Cys or Leu causes a loss of the 395 nm excitation peak with a major increase in blue excitation22, 28. When combined with Ser65 mutants , mutations at other sites near the fluorophore such as Val68Leu and Ser72Ala can further enhance the intensity of green fluorescence produced by excitation at 488 nm22, 29. However, amino acid substitutions significantly outside this region also affect the protein's spectral character. For example, Ser202Phe and Thr203Ile both cause the loss of excitation in the 475 nm region with preservation of 395 nm excitation4, 21, 30. Ile167 Thr results in a reversed ratio of 395 to 475 nm sensitivity13, while Glu222Gly is associated with the elimination of only the 395 nm excitation30. Another change, Val163Arg, not only enhances the magnitude of the Ser65Thr mutant, but also increases the temperature tolerance for functional GFP expression19. Molecular evolution techniques have been reported to improve GFP fluorescence31. Unfortunately, a roster of substitutions associated with complete loss of function has not been published.

Because GFP in crystallum exhibits a nearly identical fluorescence spectrum and lifetime to that for GFP in aqueous solution32 and fluorescence is not an inherent property of the isolated fluorophore, the elucidation of its three-dimensional structure will help provide an explanation for the generation of fluorescence in the mature protein, as well as the mechanism of autocatalytic fluorophore formation. Furthermore, the development of fluorescent proteins with additional emission and excitation characteristics would dramatically expand their biological applications. Color vision is based on the fact that spectral properties of a common fluorophore, cis-retinal, are altered as a function of protein environment within red, blue, or green opsins33. The GFP from the sea pansy, Renilla reniformis, which exhibits a single major excitation peak at 498 nm, apparently utilizes an identical core fluorophore to that of A. victoria GFP . These findings taken together with the spectral changes exerted by substitutions in amino acids over 100 residues from the GFP fluorophore suggest that a rational strategy to modify and expand the fluorescence behavior of GFP based on protein structure may be possible. Here we report the X-ray diffraction structure derived from a crystal of wild-type, recombinant A. victoria green fluorescent protein.

Results

The structure of GFP has been solved using seleniomethionyl-substituted protein and multi-wavelength anomalous dispersion (MAD) phasing methods. The electron density maps produced by the MAD phasing were very clear, revealing a dimer comprised of two quite regular -barrels with 11 strands on the outside of cylinders (Figure 1,2,3). These cylinders have a diameter of about 30 Å and a length of about 40 Å. Inspection of the density within the cylinders reveals modified tyrosine side chains as a part of an irregular -helical segment (Figure 4). Small sections of -helix also form caps on the ends of the cylinders. This motif, with a single -helix inside a very uniform cylinder of b-sheet structure, represents a new protein class, as it is not similar to any other known protein structure.

The fluorophore is highly protected, located on the central helix within a couple of Ångstroms of the geometric center of the cylinder. The pocket containing the fluorophore has a surprising number of charged residues in the immediate environment (Figure 5 and Table 1). The environment around the fluorophore includes both apolar and polar amino acid side chains. Phe64 and Phe46 are near the fluorophore and separate the single tryptophan, Trp63 from direct contact with fluorophore (closest distance of 13 Å). A table of all atoms that come in contact with the fluorophore and their distances to the fluorophore is provided (Table 1).

The crystallographic contacts are all rather tenuous, consisting of a few amino acids side chains for each. The non-crystallographic symmetry is maintained by extensive contacts and thus is likely to be the source of the dimerization seen in solution studies (Figure 6). The dimer contacts are fairly tight and consist of a core of hydrophobic side chains from Ala206, Leu221, and Phe223 from each of the two monomers and a wealth of hydrophilic contacts (Figure 5), including Tyr39, Glu142, Asn144, Asn146, Ser147, Asn149, Tyr151, Arg168, Asn170, Glu172, Tyr200, Ser202, Gln204, and Ser208. Contacts with other crystallographic molecules are not extensive, and the salt-dependence of this dimer interface and/or the loose contacts with neighboring molecules may explain the difficulties with isomorphism in initial heavy atom phasing studies.

Mass spectrometry studies of the bacterially expressed wild-type and selenio- methionyl protein show masses of 26836.1 (±0.9) and 27069.3 (±1.4) g/mole, respectively. The masses calculated for the known pTu58 gene sequence, including the original inadvertent Gln80Arg PCR error during the cloning of the gene for GFP 4 and the cyclization and oxidation of the tyrosine are 26835.5 and 27070.0 for the seleniomethionine, respectively. The differences of 0.6 and -0.7 g/mole are small and the results are therefore consistent with essentially complete fluorophore formation, including the loss of water after cyclization. The error limits do not allow accurate determination of the degree of oxidation of the dehydrotyrosine, however. These results indicate the starting material for the crystallization was essentially fully formed GFP and the lack of difference density in Fo-Fc maps in this region shows that the crystal contains fully cyclized GFP.

Discussion

The remarkable cylindrical fold of the protein seems ideally suited for the function of the protein. The strands of -sheet are tightly fitted to each other like staves in a barrel, and form a regular pattern of hydrogen bonds. Together with the short -helices and loops on the ends, the 'can' structure forms a single compact domain and does not have obvious clefts for easy access of diffusable ligands to the fluorophore. This fold, taken with the observation that the fluorophore is near the geometric center of the molecule explains the observed protection of the fluorophore from collisional quenching by oxygen (Kbm < 0.004 M-1s-1)34 and hence reduction of the quantum yield. Perhaps more seriously, photochemical damage by the formation of singlet oxygen through intersystem crossing is reduced by the structure. The tightly constructed -can would appear to serve this role nicely, as well as provide overall stability and resistance to unfolding by heat and denaturants.

The location of certain amino acid side chains in the vicinity of the fluorophore also begins to explain the fluorescence and the behavior of certain mutants of the protein. At least two resonant forms of the fluorophore can be drawn, one with a partial negative charge on the benzyl oxygen of Tyr66, and one with the charge on the carbonyl oxygen of the imidazolidone ring. Interestingly, basic residues appear to form hydrogen bonds with each of these oxygen atoms, His148 with Tyr66 and Gln94 and Arg96 with the imidazolidone. These bases presumably act to stabilize and possibly further delocalize the charge on the fluorophore. Most of the other polar residues in the pocket form an apparent hydrogen-bonding network on the side of Tyr66 that requires abstraction of protons in the oxidation process. It is tempting to speculate that these residues help abstract the protons. As for the mutants, atoms in the side chains of Thr203, Glu222, and Ile167 are in van der Waals contact with Tyr66, so their mutation would have direct steric effects on the fluorophore and would also change its electrostatic environment if the charge were changed, as suggested previously30. A quantitative explanation will require further examination. It seems likely that other mutations of the residues identified to be near the fluorophore would also have effects on the absorption and/or emission spectra, and such experiments to change the electrostatic environment around the fluorophore are in progress. By virtue of their varied fluorophore environments and hence altered spectra, these mutants should lead to expanded uses of green fluorescent protein as gene markers, cell lineage markers, and encourage other uses in biotechnology.

Mutations in regions of the sequence adjacent to the fluorophore, i.e. in the range of positions 65-67, have been systematically explored22, some having significant wavelength shifts and most suffer a loss of fluorescence intensity. For example, mutation of the central Tyr to Phe or His shifts the excitation bands but there is an overall loss of intensity. Secondary mutations to compensate for the deleterious intensity effects should also now be possible. The Ser65Thr mutant is particularly interesting because of its reported increase in fluorescence intensity21, 28. The mechanism for increased fluorescence may be reduced collisional quenching, as the additional methyl group may make for better packing in the interior of the protein. On the other hand, the effect has been suggested to be through improved conversion of the tyrosine to dehydrotyrosine. However, the fact that we see significantly altered structure relative to standard protein conformations in the wild-type argues against a dramatic increase in cyclization and/or oxidation. This effect is most likely produced by increased expression and/or folding of the protein. The crystal structure of the Ser65Thr mutant has also been solved 35, and it will be interesting to compare the two structures for clues about the fluorescence and other differences. The report of improvements in GFP by DNA shuffling31, comprising mutations Phe100Ser, Met154Thr and Val164Ala are difficult to explain based on the structure. Positions 154 and 164 are on the surface of the protein and may exert their effects through improved solubility and/or reduced aggregation. The Phe-Ser mutation at first glance would appear to destabilize the core of the protein and we have no idea how it would improve the system.

The mechanism of activation of the fluorophore from ordinary protein structure is consistent with a non-enzymatic cyclization mechanism like that of Asn-Gly deamidation 36 followed by oxidation of the tyrosine to dehydrotyrosine, as previously suggested. The role of molecular oxygen in this mechanism and in GFP fluorescence is paradoxical, however. Molecular oxygen is proposed to be needed for formation of the double bond between C and C on the tyrosine to form an extended aromatic system, but oxygen must also be excluded from regular interactions with the fluorophore or else collisional quenching of the fluorescence or damaging photochemistry will occur. The low bimolecular quenching rate suggests that the protein's design sacrifices efficient fluorophore formation for stability and higher quantum yields once fully formed.

The excited state dynamics of GFP have been studied using Stark, steady state, and time-resolved fluorescence spectroscopies37(and Youvan and Michel-Beyerle, personal communication) . The results suggest that proton transfer is involved in interconversions within two ground and two excited states. The extended set of polar interactions around the flourophore could easily accomodate proton rearrangements, with the most likely direct effects being associated with the His148 with the hydroxyl of Tyr66, Arg96 interactions with the imidazolidone, or Glu222 interactions with the hydroxyl of Ser65. Since the Ser65/Glu222 mutants have both lost their native interactions together with their ~400 nm absorption bands, one possibility is that the 400nm band arises from the abstraction of the Ser65 hydroxyl proton by Glu222. This is speculation however, but similar spectroscopic studies on mutants at these positions may be able to differentiate the roles of these sites in excited state dynamics.

The N- and C-termini truncation studies27 and the fluorescent fusion products6, 13, 14 are now understandable, given the structure of the protein. Since the C-terminus loops back outside the cylinder and the last seven or so amino acids are disordered it shouldn't be critical to have them present and further addition would seem to be easily tolerated. These residues do not form a stave of the barrel. The role of the N-terminus is a little less clear, as the first strand in the barrel does not begin until amino acid 10 or 11 Thus barrel formation does not require the N-terminal region. The N-terminal segment, is however, an integral part of the 'cap' on one end of the protein, and may be essential in folding events or in protecting the fluorophore. Again, extensions at the N-terminus would not disrupt the motif structure of the protein.

The chemical modification studies26 using sulfhydral reagents can be partially explained. Reaction of one of the cysteines near one end of the cylinder, Cys70, would appear to disrupt the packing of the cap on that end, and hence allow quenching of the fluorophore. Significant fluorescence intensity effects by the modification of Cys48 on the exterior of the protein would not be expected, a priori. The structure determination of the dithionite-reduced, non-fluorescent species has not yet been studied, but should provide additional data on the nature of the fluorophore. The pH dependence of the excitation bands at 395 nm and 475 nm25 is almost certainly due to His148, whose N atom is 3.3 Å from the Tyr66 hydroxyl oxygen atom of the fluorophore, although NMR pKa measurements or mutagenesis studies would be needed for confirmation.

The dimer we see as the asymmetric unit in the crystal is likely to be the same one formed in solution, since the ionic strength of the crystallization buffer is low, and we see dimers at low (<100 mM) ionic strengths in solution. Thus, it is not surprising to us to see the large number of hydrophilic dimer contacts. The smaller hydrophobic patch could conceivably be involved in physiological interactions with aequorin, as there would be a natural advantage to close proximity for efficient energy transfer. It is not known at present whether dimers form in physiological circumstances, or what the effect of dimerization is on energy transfer, aside from the circumstantial inferences on the excitation spectra previously reported on the native protein25. The dimer contacts should now be able to be modified in such a way to disrupt the formation of dimers without affecting stability and folding. Other nearby residues could also be converted to hydrophobic residues to enhance dimer stability if desired. Control of the dimerization will be important for fluorescence resonance energy transfer (FRET) studies of protein-protein interactions using GFP, as one would not want to induce association and hence resonance energy transfer between the differently colored GFP proteins by mechanisms other that of the target protein interactions. Mutants may also be developed for reduction of aggregation during expression and hence fewer problems with inclusion bodies.

Thus the three-dimensional structure of GFP has provided a physico-chemical basis of many observed features of the protein, including its stability, protection of it fluorophore, behavior of mutants, and dimerization properties. The structure will also allow directed mutation studies to complement random and combinatorial approaches.

Experimental protocols

Green fluorescent protein was purified from E. coli strain BL21(DE3)pLysS (Novagen) containing plasmid pTu58, bearing the wild-type Aequorea victoria green fluorescent protein4. For the seleniomethionine protein, the plasmid was moved to E. coli methionine auxotroph strain B834(DE3)pLysS (Novagen). The purification involved cell lysis, centrifugation of cell debris, and four column chromatography steps: DEAE anion exchange column (Sigma, CL-6B) with a zero to 1M NaCl gradient in 10mM phosphate, 2mM EDTA, 2mM DTT, pH 7; a hydrophobic interaction column (Sigma, CL-4B) with a 0.1 to zero M phosphate gradient in 2mM EDTA, 2mM DTT, pH 7; an HPLC anion exchange column (Bio-Rad, Bio-Gel DEAE-5PW) with a zero to 1M NaCl gradient in 10 mM phosphate, 2 mM EDTA, 2mM DTT, pH 7; and an HPLC gel filtration column (Bio-Rad, Bio-Gel SEC-125) with 0.1 M phosphate, 2mM EDTA, 2mM DTT, pH 7. Gel filtration columns run at 10mM phosphate showed predominately a 2-fold higher molecular weight species. Matrix-assisted laser desorption ionization mass spectrometry was performed by the University of Texas Health Sciences Center analytical chemistry service.

The protein was crystallized in sitting drop vapor diffusion wells (Hampton Research) at room temperature using 58% 2-methyl-2,4-pentanediol (Aldrich), 50mM morpholino ethane sulfonic acid, 0.1% sodium azide at pH 6.8. The protein concentration varied, but was typically 20-30 mg/ml. Crystals grew as green fluorescent square bipyramids up to 0.5 mm on a side. The space group was determined to be P41212 or its enantiomorph, with a=b=87.15 Å and c=119.85 Å at cryogenic temperatures, and a=b= 89.23 Å and c= 119.78 Å at room temperature. The unit cell also varies with changes in ionic strength, and this effect thwarted solution by multiple isomorphous replacement. Packing density calculations suggested that there were probably two molecules per asymmetric unit.

Multi-wavelength anomalous dispersion (MAD) data were taken at Brookhaven beam line X4A at four wavelengths. The wavelengths to be used were determined by reference and crystal absorption scans. The data were taken at liquid nitrogen temperature using inverse-beam geometry in wedges of four degrees and processed using DENZO38. Native and selenio-methionine data sets were also taken in the laboratory on an R-AXIS IIC detector with CuK radiation. The native data set used in the refinement had a Rmerge of 7.7% to 1.9 Å resolution (99+% complete in all shells with 5-fold average redundancy). Selenium atoms were located initially by standard difference Patterson maps between selenium-substituted and native protein using SHELX9639 and HEAVY40 and confirmed by Patterson maps using the MAD data. MADSYS software41 was used to give the anomalous diffraction differences shown in Table 2. and to extract Fa, Ft, and phase information.

The resulting MAD-phased map was solvent flattened and two-fold averaged based on the selenium sites using CCP442, skeletonized using the program O43, and immediately revealed two 11-stranded cylindrical -barrels. The polypeptide chain was traced for one of the barrels beginning from the seleniomethionines and extending the structure in each direction, helped by the recognition of the modified tyrosine in the middle of the barrel as Tyr66, the nucleus of the fluorophore. The correct enantiomorph is space group P41212, as confirmed by the handedness of the b-barrel and the a-helices. Refinement has been started using the program X-Plor44 using the native data collected at room temperature; the current R-factor at 1.9 Å is 0.21 with an R-free of 0.26, with good geometry (rms bond and angle deviations from ideality of 0.013 Å and 1.8, respectively) and tight restraint of the non-crystallographic symmetry. All measured data were included in the refinement. Coordinates and structure factors have been deposited at the Brookhaven Protein Data Bank under accession numbers 1GFL and R1GFLSF, respectively.

Acknowledgments

We would like to thank J. Sobelewski and L. Moitoso-deVargas for initial purification of GFP, Prof. Dan Leahy for procedures for growing the seleniomethionine auxotroph and helpful suggestions, Drs. Mike Berry, Frank Whitby, Mike Soltis, Henry Bellamy, Michael Stowell, Craig Ogata for help with MAD data collection, Prof. Frank Prendergast for suggesting the name b-can, the Howard Hughes Medical Institute and Brookhaven National Laboratory (beamline X4A) and Stanford's SSRL (beamline X1-5) for synchrotron time, and the W. M. Keck Foundation, Robert A. Welch Foundation, and NIH AR40252 (GNP), GRASP Center (DK34928) and DK34447 (LGM) for financial support.

Table 1. List of amino acid side chains with close contacts (less than 5 Å) to the fluorophore. The fluorophore is defined as the 7 atoms of the phenol of Tyr66, the 6 atoms of the imidazolidone, and the bridging methylene between the rings. The following amino acid side chains would be expected to have the most direct effects on fluorescence and perhaps fluorophore formation. The atom names are taken from the Brookhaven Protein Data Bank nomenclature.
    
Protein Fluorophore  Distance
    
residue atom residue atom  (Å) 
     
Arg 96 NH2 Tyr 66  O 2.7 
Gln 94 NE2 Tyr 66  O 3.0 
His 148 ND1 Tyr 66  OH 3.3 
Gln 69 CD Tyr 66  O 3.4 
Glu 222 OE2 Tyr 66  CE2 3.5 
Val 150 CG2 Tyr 66  CE1 3.6 
Phe 165 CE1 Tyr 66  CD1 3.6 
Thr 203 CG2 Tyr 66  CE2 3.6 
Ile 167 CD1 Tyr 66  OH 3.7 
Thr 62 CG2 Tyr 66  CG 3.7 
Tyr 145 CE2 Tyr 66  OH 3.7 
Ser 205 OG Tyr 66  CE2 4.0 
Val 61 CG1 Tyr 66  CE2 4.4 
Gln 183 NE2 Tyr 66  O 4.8 
Val 68 CG2 Ser 65  C 4.9 
    

Table 2. Anomalous diffraction differences and scattering factors for seleniomethionyl GFP at the four wavelengths used. Following the format used by Yang et al.41, Bijvoet differences ratios are given in diagonal elements with centric values in parentheses, and dispersive differences are given in off-diagonal elements. Scattering factors were chosen to minimize the cumulative errors in Bijvoet and dispersive terms.
         
Wave-
length
 30 > d > 3.4 (Å)  3.4 > d > 2.7 (Å)  2.7 > d > 2.2 (Å)  scattering
factors (e)
 
        
(Å) 0.9879  0.9794 0.9792  0.9686 0.9879  0.9794 0.9792  0.9686 0.9879  0.9794 0.9792  0.9686 f'  f''
              
0.9879 0.026  0.042 0.030  0.020 0.037  0.046 0.037  0.032 0.064  0.071 0.068  0.064 -4.0  1.1
 (0.026)     (0.035)     (0.052)      
0.9794  0.050 0.026  0.045  0.058 0.035  0.049  0.088 0.065  0.077 -10.5  3.9
  (0.029)     (0.039)     (0.060)     
0.9792   0.067  0.032   0.074  0.040   0.101  0.071 -7.9  5.5
   (0.031)    (0.042)    (0.062)   
0.9686    0.049    0.059    0.089  -3.4 3.9 
    (0.027)     (0.039)     (0.064)   
             

Figure Legends

Figure 1. The overall shape of the protein and its association into dimers. Eleven strands of -sheet (green) form the walls of a cylinder. Short segments of -helices (blue) cap the top and bottom of the 'b-can' and also provide a scaffold for the fluorophore which is near geometric center of the can. This folding motif, with -sheet outside and helix inside, represents a new class of proteins. Two monomers are associated into a dimer in the crystal and in solution at low ionic strengths. This view is directly down the two-fold axis of the non-crystallographic symmetry. Figures 1 and 6 were produced with Ribbons45.

Figure 2. Stereo view of a monomer, with colors that vary slowly as a function of the distance along the polypeptide chain. The termini and C atoms of every 20th amino acid are marked just to the upper right of each atom. Figure produced by RasMol46.

Figure 3. A topology diagram of the folding pattern in GFP. The -sheet strands are shown in light green, a-helices in blue, and connecting loops in yellow. The positions in the sequence that begin and end each major secondary structure element are also given. The anti-parallel strands (except for the interactions between stands 1 and 6) make a tightly formed barrel.

Figure 4. Model of the fluorophore and its environment superposed on the MAD-phased electron density map at 2.2 Å resolution. The clear definition throughout the map allowed the chain to be traced and side chains to be well placed. The density for Ser65, Tyr66 and Gly67 is quite consistent with the dehydrotyrosine - imidazolidone structure proposed for the fluorophore. Many of the side chains adjacent to the fluorophore are labeled. Figures 4 and 5 were produced with O43.

Figure 5. Stereo view of the fluorophore and its environment. His148, Gln94 and Arg96can be seen on opposite ends of the fluorophore and probably stabilize resonant forms of the fluorophore. Charged, polar, and non-polar side chains all contact the fluorophore in some way.

Figure 6. The dimer contact region. The two polypeptide chains associate over a broad area, with a small hydrophobic patch (in the yellow box) and numerous hydrophilic contacts. The two-fold symmetry axis is in the plane of the page, and is marked by the red arrow. The polar residues are marked with red atoms for oxygen and blue for nitrogen.

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