Vladimir I. Martynov

GFP-like chromoproteins as a source of far-red fluorescent proteins.

We have employed a new approach to generate novel fluorescent proteins (FPs) from red absorbing chromoproteins. An identical single amino acid substitution converted novel chromoproteins from the species Anthozoa (Heteractis crispa, Condylactis gigantea, and Goniopora tenuidens) into far‐red FPs (emission λ max=615–640 nm). Moreover, coupled site‐directed and random mutagenesis of the chromoprotein from H. crispa resulted in a unique far‐red FP (HcRed) that exhibited bright emission at 645 nm. A clear red shift in fluorescence of HcRed, compared to drFP583 (by more than 60 nm), makes it an ideal additional color for multi‐color labeling. Importantly, HcRed is excitable by 600 nm dye laser, thus promoting new detection channels for multi‐color flow cytometry applications. In addition, we generated a dimeric mutant with similar maturation and spectral properties to tetrameric HcRed.

GFP Family: Structural Insights into Spectral Tuning

Proteins homologous to green fluorescent protein (GFP) span most of the visible spectrum, offering indispensable tools for live cell imaging. Structural transformations, such as posttranslational autocatalytic and photo-induced modifications, chromophore isomerization, and rearrangements in its environment underlie the unique capacity of these proteins to tune their own optical characteristics. A better understanding of optical self-tuning mechanisms would assist in the engineering of more precisely adapted variants and in expanding the palette of GFP-like proteins to the near-infrared region. The latest advances in this field shed light upon multiple features of protein posttranslational chemistry, and establish some important basic principles about the interplay of structure and spectral properties in the GFP family.

Structural Basis for Phototoxicity of the Genetically Encoded Photosensitizer KillerRed

KillerRed is the only known fluorescent protein that demonstrates notable phototoxicity, exceeding that of the other green and red fluorescent proteins by at least 1,000-fold. KillerRed could serve as an instrument to inactivate target proteins or to kill cell populations in photodynamic therapy. However, the nature of KillerRed phototoxicity has remained unclear, impeding the development of more phototoxic variants. Here we present the results of a high resolution crystallographic study of KillerRed in the active fluorescent and in the photobleached non-fluorescent states. A unique and striking feature of the structure is a water-filled channel reaching the chromophore area from the end cap of the β-barrel that is probably one of the key structural features responsible for phototoxicity. A study of the structure-function relationship of KillerRed, supported by structure-based, site-directed mutagenesis, has also revealed the key residues most likely responsible for the phototoxic effect. In particular, Glu68 and Ser119, located adjacent to the chromophore, have been assigned as the primary trigger of the reaction chain.

Photoconversion of the Chromophore of a Fluorescent Protein from Dendronephthya sp.

A green fluorescent protein from the coral Dendronephthya sp. (Dend FP) is characterized by an irreversible lightdependent conversion to a red-emitting form. The molecular basis of this phenomenon was studied in the present work. Upon UV-irradiation at 366 nm, the absorption maximum of the protein shifted from 494 nm (the green form) to 557 nm (the red form). Concurrently, in the fluorescence spectra the emission maximum shifted from 508 to 575 nm. The green form of native Dend FP was shown to be a dimer, and the oligomerization state of the protein did not change during its conversion to the red form. By contrast, UV-irradiation caused significant intramolecular changes. Unlike the green form, which migrates in SDS-polyacrylamide gels as a single band corresponding to a full-length 28-kD protein, the red form of Dend FP migrated as two fragments of 18- and 10-kD. To determine the chemical basis of these events, the denatured red form of Dend FP was subjected to proteolysis with trypsin. From the resulting hydrolyzate, a chromophore-containing peptide was isolated by HPLC. The structure of the chromophore from the Dend FP red form was established by methods of ESI, tandem mass spectrometry (ESI/MS/MS), and NMR-spectroscopy. The findings suggest that the light-dependent conversion of Dend FP is caused by generation of an additional double bond in the side chain of His65 and a resulting extension of the conjugated system of the green form chromophore. Thus, classified by the chromophore structure, Dend FP should be referred to the Kaede subfamily of GFP-like proteins.

Structural evidence for a dehydrated intermediate in green fluorescent protein chromophore biosynthesis.

The acGFPL is the first-identified member of a novel, colorless and non-fluorescent group of green fluorescent protein (GFP)-like proteins. Its mutant aceGFP, with Gly replacing the invariant catalytic Glu-222, demonstrates a relatively fast maturation rate and bright green fluorescence (λex = 480 nm, λem = 505 nm). The reverse G222E single mutation in aceGFP results in the immature, colorless variant aceGFP-G222E, which undergoes irreversible photoconversion to a green fluorescent state under UV light exposure. Here we present a high resolution crystallographic study of aceGFP and aceGFP-G222E in the immature and UV-photoconverted states. A unique and striking feature of the colorless aceGFP-G222E structure is the chromophore in the trapped intermediate state, where cyclization of the protein backbone has occurred, but Tyr-66 still stays in the native, non-oxidized form, with Cα and Cβ atoms in the sp3 hybridization. This experimentally observed immature aceGFP-G222E structure, characterized by the non-coplanar arrangement of the imidazolone and phenolic rings, has been attributed to one of the intermediate states in the GFP chromophore biosynthesis. The UV irradiation (λ = 250–300 nm) of aceGFP-G222E drives the chromophore maturation further to a green fluorescent state, characterized by the conventional coplanar bicyclic structure with the oxidized double Tyr-66 Cα=Cβ bond and the conjugated system of π-electrons. Structure-based site-directed mutagenesis has revealed a critical role of the proximal Tyr-220 in the observed effects. In particular, an alternative reaction pathway via Tyr-220 rather than conventional wild type Glu-222 has been proposed for aceGFP maturation.

Three-dimensional structure of yellow fluorescent protein zYFP538 from Zoanthus sp. at the resolution 1.8 Å

The three-dimensional structure of yellow fluorescent proteins zYFP538 (zFP538) from the button polyp Zoanthus sp. was determined at a resolution of 1.8 Å by X-ray analysis. The monomer of zYFP538 adopts a structure characteristic of the green fluorescent protein (GFP) family, a β-barrel formed from 11 antiparallel β segments and one internal α helix with a chromophore embedded into it. Like the TurboGFP, the β-barrel of zYFP538 contains a water-filled pore leading to the chromophore Tyr67 residue, which presumably provides access of molecular oxygen necessary for the maturation process. The post-translational modification of the chromophore-forming triad Lys66-Tyr67-Gly68 results in a tricyclic structure consisting of a five-membered imidazolinone ring, a phenol ring of the Tyr67 residue, and an additional six-membered tetrahydropyridine ring. The chromophore formation is completed by cleavage of the protein backbone at the Cα-N bond of Lys66. It was suggested that the energy conflict between the buried positive charge of the intact Lys66 side chain in the hydrophobic pocket formed by the Ile44, Leu46, Phe65, Leu204 and Leu219 side chains is the most probable trigger that induces the transformation of the bicyclic green form to the tricyclic yellow form. A stereochemical analysis of the contacting surfaces at the intratetramer interfaces helped reveal a group of conserved key residues responsible for the oligomerization. Along with others, these residues should be taken into account in designing monomeric forms suitable for practical application as markers of proteins and cell organelles.

Structure of a red fluorescent protein from Zoanthus, zRFP574, reveals a novel chromophore

The three-dimensional structure of the red fluorescent protein (RFP) zRFP574 from the button polyp Zoanthus sp. (two dimers per asymmetric unit, 231 x 4 amino acids) has been determined at 2.4 A resolution in space group C222(1). The crystal structure, refined to a crystallographic R factor of 0.203 (R(free) = 0.249), adopts the beta-barrel fold composed of 11 strands similar to that of the yellow fluorescent protein zYFP538. The zRFP574 chromophore, originating from the protein sequence Asp66-Tyr67-Gly68, has a two-ring structure typical of GFP-like proteins. The bond geometry of residue 66 shows the strong tendency of the corresponding C(alpha) atom to sp(2) hybridization as a consequence of N-acylimine bond formation. The zRFP574 chromophore contains the 65-66 cis-peptide bond characteristic of red fluorescent proteins. The chromophore phenolic ring adopts a cis conformation coplanar with the imidazolinone ring. The crystallographic study has revealed an unexpected chemical feature of the internal chromophore. A decarboxylated side chain of the chromophore-forming residue Asp66 has been observed in the structure. This additional post-translational modification is likely to play a key role in the bathochromic shift of the zRFP574 spectrum.

Probing the structural determinants of yellow fluorescence of a protein from Phialidium sp.

Fluorescent proteins homologous to green fluorescent protein (avGFP) display pronounced spectral variability due to different chromophore structures and variable chromophore interactions with the surrounding amino acids. To gain insight into the structural basis for yellow emission, the 3D structure of phiYFP (λ(em)=537 nm), a protein from the sea medusa Phialidium sp., was built by a combined homology modeling - mass spectrometry approach. Mass spectrometry of the isolated chromophore-bearing peptide reveals that the chromophore of phiYFP is chemically identical to that of avGFP (λ(em)=508 nm). The experimentally acquired chromophore structure was combined with the homology-based model of phiYFP, and the proposed 3D structure was used as a starting point for identification of the structural features responsible for yellow fluorescence. Mutagenesis of residues in the local chromophore environment of phiYFP suggests that multiple factors cooperate to establish the longest-wavelength emission maximum among fluorescent proteins with an unmodified GFP-like chromophore.

Posttranslational Chemistry of Proteins of the GFP Family

This review focuses on the current knowledge about posttranslational chemistry underlying the diverse optical properties of GFP-like proteins.

pH-sensor properties of a fluorescent protein from Dendronephthya sp.

Genetically encoded fluorescent protein-based biosensors are widely used for pH monitoring in live cells. In this work, we have shown that fluorescent protein from Dendronephthya sp. (DendFP) has pronounced pH sensitivity. In contrast to the majority of the known genetically encoded pH-sensors, for this protein, the acidification of the environment does not lead to fluorescence quenching but just shifts it emission maximum from red to green. For this reason, it appears possible to quantitatively measure pH by the ratio of emission intensity in the red and green range, which sets DendFP apart from other pH-sensors.