Alteration of fluorescent protein spectroscopic properties upon cryoprotection

From femtoseconds to minutes: time-resolved macromolecular crystallography at XFELs and synchrotrons

Over the last decade, the development of time-resolved serial crystallography (TR-SX) at X-ray free-electron lasers (XFELs) and synchrotrons has allowed researchers to study phenomena occurring in proteins on the femtosecond-to-minute timescale, taking advantage of many technical and methodological breakthroughs. Protein crystals of various sizes are presented to the X-ray beam in either a static or a moving medium. Photoactive proteins were naturally the initial systems to be studied in TR-SX experiments using pump-probe schemes, where the pump is a pulse of visible light. Other reaction initiations through small-molecule diffusion are gaining momentum. Here, selected examples of XFEL and synchrotron time-resolved crystallography studies will be used to highlight the specificities of the various instruments and methods with respect to time resolution, and are compared with cryo-trapping studies.

Deciphering Structural Photophysics of Fluorescent Proteins by Kinetic Crystallography

Because they enable labeling of biological samples in a genetically-encoded manner, Fluorescent Proteins (FPs) have revolutionized life sciences. Photo-transformable fluorescent proteins (PTFPs), in particular, recently attracted wide interest, as their fluorescence state can be actively modulated by light, a property central to the emergence of super-resolution microscopy. PTFPs, however, exhibit highly complex photophysical behaviours that are still poorly understood, hampering the rational engineering of variants with improved performances. We show that kinetic crystallography combined with in crystallo optical spectroscopy, modeling approaches and single-molecule measurements constitutes a powerful tool to decipher processes such as photoactivation, photoconversion, photoswitching, photoblinking and photobleaching. Besides potential applications for the design of enhanced PTFPs, these investigations provide fundamental insight into photoactivated protein dynamics.

In crystallo optical spectroscopy (icOS) as a complementary tool on the macromolecular crystallography beamlines of the ESRF

The analysis of structural data obtained by X-ray crystallography benefits from information obtained from complementary techniques, especially as applied to the crystals themselves. As a consequence, optical spectroscopies in structural biology have become instrumental in assessing the relevance and context of many crystallographic results. Since the year 2000, it has been possible to record such data adjacent to, or directly on, the Structural Biology Group beamlines of the ESRF. A core laboratory featuring various spectrometers, named the Cryobench, is now in its third version and houses portable devices that can be directly mounted on beamlines. This paper reports the current status of the Cryobench, which is now located on the MAD beamline ID29 and is thus called the ID29S-Cryobench (where S stands for `spectroscopy'). It also reviews the diverse experiments that can be performed at the Cryobench, highlighting the various scientific questions that can be addressed.

Towards a high-throughput system for high-pressure cooling of cryoprotectant-free biological crystals

A prototype of a high-pressure cooling apparatus dedicated to macromolecular crystallography on synchrotrons is reported. The system allows cooling of biological crystals without the addition of penetrating or nonpenetrating exogenous cryoprotectant by transforming the aqueous solvent into high-density amorphous ice at a pressure of 200 MPa. The samples are directly fished from crystallization trays with cryopins specifically designed for the pressurizing device and which are compatible with robotized sample changers on synchrotron beamlines. Optionally, the system allows noble gas derivatization during the high-pressure cooling procedure. Some technical details of the equipment and of the method are described in this article. A representative series of test crystals shows that the system is capable of successfully cooling samples that normally require a wide variety of cryoprotection conditions. The last section focuses on pressure-induced structural modifications of these proteins, which are shown to be few but nevertheless of interest.

The 1.6 Å resolution structure of a FRET-optimized Cerulean fluorescent protein

Genetically encoded cyan fluorescent proteins (CFPs) bearing a tryptophan-derived chromophore are commonly used as energy-donor probes in Förster resonance energy transfer (FRET) experiments useful in live cell-imaging applications. In recent years, significant effort has been expended on eliminating the structural and excited-state heterogeneity of these proteins, which has been linked to undesirable photophysical properties. Recently, mCerulean3, a descendant of enhanced CFP, was introduced as an optimized FRET donor protein with a superior quantum yield of 0.87. Here, the 1.6 Å resolution X-ray structure of mCerulean3 is reported. The chromophore is shown to adopt a planar trans configuration at low pH values, indicating that the acid-induced isomerization of Cerulean has been eliminated. β-Strand 7 appears to be well ordered in a single conformation, indicating a loss of conformational heterogeneity in the vicinity of the chromophore. Although the side chains of Ile146 and Leu167 appear to exist in two rotamer states, they are found to be well packed against the indole group of the chromophore. The Ser65 reversion mutation allows improved side-chain packing of Leu220. A structural comparison with mTurquoise2 is presented and additional engineering strategies are discussed.