CrystalDirect™: a novel approach for automated crystal harvesting based on photoablation of thin films

In situ serial crystallography facilitates 96-well plate structural analysis at low symmetry

The advent of serial crystallography has rejuvenated and popularized room-temperature X-ray crystal structure determination. Structures determined at physiological temperature reveal protein flexibility and dynamics. In addition, challenging samples (e.g. large complexes, membrane proteins and viruses) form fragile crystals that are often difficult to harvest for cryo-crystallography. Moreover, a typical serial crystallography experiment requires a large number of microcrystals, mainly achievable through batch crystallization. Many medically relevant samples are expressed in mammalian cell lines, producing a meager quantity of protein that is incompatible with batch crystallization. This can limit the scope of serial crystallography approaches. Direct in situ data collection from a 96-well crystallization plate enables not only the identification of the best diffracting crystallization condition but also the possibility for structure determination under ambient conditions. Here, we describe an in situ serial crystallography (iSX) approach, facilitating direct measurement from crystallization plates mounted on a rapidly exchangeable universal plate holder deployed at a microfocus beamline, ID23-2, at the European Synchrotron Radiation Facility. We applied our iSX approach on a challenging project, autotaxin, a therapeutic target expressed in a stable human cell line, to determine the structure in the lowest-symmetry P1 space group at 3.0 Å resolution. Our in situ data collection strategy provided a complete dataset for structure determination while screening various crystallization conditions. Our data analysis reveals that the iSX approach is highly efficient at a microfocus beamline, improving throughput and demonstrating how crystallization plates can be routinely used as an alternative method of presenting samples for serial crystallography experiments at synchrotrons.

X-ray structure and enzymatic study of a bacterial NADPH oxidase highlight the activation mechanism of eukaryotic NOX

NADPH oxidases (NOX) are transmembrane proteins, widely spread in eukaryotes and prokaryotes, that produce reactive oxygen species (ROS). Eukaryotes use the ROS products for innate immune defense and signaling in critical (patho)physiological processes. Despite the recent structures of human NOX isoforms, the activation of electron transfer remains incompletely understood. SpNOX, a homolog from Streptococcus pneumoniae, can serves as a robust model for exploring electron transfers in the NOX family thanks to its constitutive activity. Crystal structures of SpNOX full-length and dehydrogenase (DH) domain constructs are revealed here. The isolated DH domain acts as a flavin reductase, and both constructs use either NADPH or NADH as substrate. Our findings suggest that hydride transfer from NAD(P)H to FAD is the rate-limiting step in electron transfer. We identify significance of F397 in nicotinamide access to flavin isoalloxazine and confirm flavin binding contributions from both DH and Transmembrane (TM) domains. Comparison with related enzymes suggests that distal access to heme may influence the final electron acceptor, while the relative position of DH and TM does not necessarily correlate with activity, contrary to previous suggestions. It rather suggests requirement of an internal rearrangement, within the DH domain, to switch from a resting to an active state. Thus, SpNOX appears to be a good model of active NOX2, which allows us to propose an explanation for NOX2's requirement for activation.

Crystal structure of the phospholipase A and acyltransferase 4 (PLAAT4) catalytic domain

Phospholipase A and Acyltransferase 4 (PLAAT4) is a class II tumor suppressor, that also plays a role as a restrictor of intracellular Toxoplasma gondii infection through restriction of parasitic vacuole size. The catalytic N-terminal domain (NTD) interacts with the C-terminal domain (CTD), which is important for sub-cellular targeting and enzymatic function. The dynamics of the NTD main (L1) loop and the L2(B6) loop adjacent to the active site, have been shown to be important regulators of enzymatic activity. Here, we present the crystal structure of PLAAT4 NTD, determined from severely intergrown crystals using automated, laser-based crystal harvesting and data reduction technologies. The structure showed the L1 loop in two distinct conformations, highlighting a complex network of interactions likely influencing its conformational flexibility. Ensemble refinement of the crystal structure recapitulates the major correlated motions observed in solution by NMR. Our analysis offers useful insights on millisecond dynamics based on the crystal structure, complementing NMR studies which preclude structural information at this time scale.

An automated platform for structural analysis of membrane proteins through serial crystallography

Membrane proteins are central to many pathophysiological processes, yet remain very difficult to analyze structurally. Moreover, high-throughput structure-based drug discovery has not yet been exploited for membrane proteins because of lack of automation. Here, we present a facile and versatile platform for in meso membrane protein crystallization, enabling rapid atomic structure determination at both cryogenic and room temperatures. We apply this approach to human integral membrane proteins, which allowed us to identify different conformational states of intramembrane enzyme-product complexes and analyze by molecular dynamics simulations the structural dynamics of the ADIPOR2 integral membrane protein. Finally, we demonstrate an automated pipeline combining high-throughput microcrystal soaking, automated laser-based harvesting, and serial crystallography, enabling screening of small-molecule libraries with membrane protein crystals grown in meso. This approach brings needed automation to this important class of drug targets and enables high-throughput structure-based ligand discovery with membrane proteins.

Fragment-Based Discovery of Non-bisphosphonate Binders of Trypanosoma brucei Farnesyl Pyrophosphate Synthase

Trypanosoma brucei is the causative agent of human African trypanosomiasis (HAT). Nitrogen-containing bisphosphonates, a current treatment for bone diseases, have been shown to block the growth of the T. brucei parasites by inhibiting farnesyl pyrophosphate synthase (FPPS); however, due to their poor pharmacokinetic properties, they are not well suited for antiparasitic therapy. Recently, an allosteric binding pocket was discovered on human FPPS, but its existence on trypanosomal FPPS was unclear. We applied NMR and X-ray fragment screening to T. brucei FPPS and report herein on four fragments bound to this previously unknown allosteric site. Surprisingly, non-bisphosphonate active-site binders were also identified. Moreover, fragment screening revealed a number of additional binding sites. In an early structure-activity relationship (SAR) study, an analogue of an active-site binder was unexpectedly shown to bind to the allosteric site. Overlaying identified fragment binders of a parallel T. cruzi FPPS fragment screen with the T. brucei FPPS structure, and medicinal chemistry optimisation based on two binders revealed another example of fragment "pocket hopping". The discovery of binders with new chemotypes sets the framework for developing advanced compounds with pharmacokinetic properties suitable for the treatment of parasitic infections by inhibition of FPPS in T. brucei parasites.

Automated harvesting and processing of protein crystals through laser photoablation

Currently, macromolecular crystallography projects often require the use of highly automated facilities for crystallization and X-ray data collection. However, crystal harvesting and processing largely depend on manual operations. Here, a series of new methods are presented based on the use of a low X-ray-background film as a crystallization support and a photoablation laser that enable the automation of major operations required for the preparation of crystals for X-ray diffraction experiments. In this approach, the controlled removal of the mother liquor before crystal mounting simplifies the cryocooling process, in many cases eliminating the use of cryoprotectant agents, while crystal-soaking experiments are performed through diffusion, precluding the need for repeated sample-recovery and transfer operations. Moreover, the high-precision laser enables new mounting strategies that are not accessible through other methods. This approach bridges an important gap in automation and can contribute to expanding the capabilities of modern macromolecular crystallography facilities.