Late Onset of Primary Hemophagocytic Lymphohistiocytosis (HLH) with a Novel Constellation of Compound Heterozygosity Involving Two Missense Variants in the PRF1 Gene.

Hemophagocytic lymphohistiocytosis (HLH) is a rare but in most cases life-threatening immune-mediated disease of the hematopoietic system frequently associated with hematologic neoplasms. Here, we report on a case in which we detected a novel constellation of two missense variants affecting the PRF1 gene, leading to de novo primary HLH. Diagnostics included a comprehensive clinical work-up and standard methods of hematopathology as well as extended molecular genomics based on polymerase chain reaction (PCR) reactions and the calculation of three-dimensional molecule reconstructions of PRF1. Subsequently, a comprehensive review of the literature was performed, which showed that this compound heterozygosity has not been previously described. The patient was a 20-year-old female. Molecular diagnostics revealed two heterozygous missense variants in the PRF1 gene (A91V and R104C) on exon 2. Apart from the finding of two inconclusive genetic variants, all clinical criteria defined by the HLH study group of Histiocyte Society were met at initial presentation. The final diagnosis was made in cooperation with the Consortium of German HLH-reference centers. Here, chemotherapy did not lead to sufficient sustained disease control. Therefore, the decision for allogenic hematopoietic stem cell transplantation (alloHSCT) was made. Hitherto, the duration of response was 6 months. Due to severe and unmanageable hepatic graft-versus-host disease (GvHD), the patient died. We report on a novel constellation of a compound heterozygosity containing two missense variants on exon 2 of the PRF1 gene. To the authors' best knowledge, this is the first presentation of a primary HLH case harboring this genomic constellation with late-onset clinical manifestation.

Structures and General Transport Mechanisms by the Major Facilitator Superfamily (MFS).

The major facilitator superfamily (MFS) is the largest known superfamily of secondary active transporters. MFS transporters are responsible for transporting a broad spectrum of substrates, either down their concentration gradient or uphill using the energy stored in the electrochemical gradients. Over the last 10 years, more than a hundred different MFS transporter structures covering close to 40 members have provided an atomic framework for piecing together the molecular basis of their transport cycles. Here, we summarize the remarkable promiscuity of MFS members in terms of substrate recognition and proton coupling as well as the intricate gating mechanisms undergone in achieving substrate translocation. We outline studies that show how residues far from the substrate binding site can be just as important for fine-tuning substrate recognition and specificity as those residues directly coordinating the substrate, and how a number of MFS transporters have evolved to form unique complexes with chaperone and signaling functions. Through a deeper mechanistic description of glucose (GLUT) transporters and multidrug resistance (MDR) antiporters, we outline novel refinements to the rocker-switch alternating-access model, such as a latch mechanism for proton-coupled monosaccharide transport. We emphasize that a full understanding of transport requires an elucidation of MFS transporter dynamics, energy landscapes, and the determination of how rate transitions are modulated by lipids.

Hyperbolic Pressure-Temperature Phase Diagram of the Zinc-Finger Protein apoKti11 Detected by NMR Spectroscopy.

For a comprehensive understanding of the thermodynamic state functions describing the stability of a protein, the influence of the intensive properties of temperature and pressure has to be known. With the zinc-finger-containing Kti11, we found a suitable protein for this purpose because folding and unfolding transitions occur at an experimentally accessible temperature (280-330 °K) and pressure (0.1-240 MPa) range. We solved the crystal structure of the apo form of Kti11 to reveal two disulfide bonds at the metal-binding site, which seals off a cavity in the ß-barrel part of the protein. From a generally applicable proton NMR approach, we could determine the populations of folded and unfolded chains under all conditions, leading to a hyperbolic pressure-temperature phase diagram rarely observed for proteins. A global fit of a two-state model to all derived populations disclosed reliable values for the change in Gibbs free energy, volume, entropy, heat capacity, compressibility, and thermal expansion upon unfolding. The unfolded state of apoKti11 has a lower compressibility compared to the native state and a smaller volume at ambient pressure. Therefore, a pressure increase up to 200 MPa reduces the population of the native state, and above this value, the native population increases again. Pressure-induced chemical-shift changes in two-dimensional 1H-15N NMR spectra could be employed for a molecular interpretation of the thermodynamic properties of apoKti11.

Outward open conformation of a Major Facilitator Superfamily multidrug/H+ antiporter provides insights into switching mechanism.

Multidrug resistance (MDR) poses a major challenge to medicine. A principle cause of MDR is through active efflux by MDR transporters situated in the bacterial membrane. Here we present the crystal structure of the major facilitator superfamily (MFS) drug/H+ antiporter MdfA from Escherichia coli in an outward open conformation. Comparison with the inward facing (drug binding) state shows that, in addition to the expected change in relative orientations of the N- and C-terminal lobes of the antiporter, the conformation of TM5 is kinked and twisted. In vitro reconstitution experiments demonstrate the importance of selected residues for transport and molecular dynamics simulations are used to gain insights into antiporter switching. With the availability of structures of alternative conformational states, we anticipate that MdfA will serve as a model system for understanding drug efflux in MFS MDR antiporters.

Conformational Flexibility in the CD81-Binding Site of the Hepatitis C Virus Glycoprotein E2.

Numerous antibodies have been described that potently neutralize a broad range of hepatitis C virus (HCV) isolates and the majority of these antibodies target the binding site for the cellular receptor CD81 within the major HCV glycoprotein E2. A detailed understanding of the major antigenic determinants is crucial for the design of an efficient vaccine that elicits high levels of such antibodies. In the past 6 years, structural studies have shed additional light on the way the host's humoral immune system recognizes neutralization epitopes within the HCV glycoproteins. One of the most striking findings from these studies is that the same segments of the E2 polypeptide chain induce antibodies targeting distinct antigen conformations. This was demonstrated by several crystal structures of identical polypeptide segments bound to different antibodies, highlighting an unanticipated intrinsic structural flexibility that allows binding of antibodies with distinct paratope shapes following an "induced-fit" mechanism. This unprecedented flexibility extends to the entire binding site for the cellular receptor CD81, underlining the importance of dynamic analyses to understand (1) the interplay between HCV and the humoral immune system and (2) the relevance of this structural flexibility for virus entry. This review summarizes the current understanding how neutralizing antibodies target structurally flexible epitopes. We focus on differences and common features of the reported structures and discuss the implications of the observed structural flexibility for the viral replication cycle, the full scope of the interplay between the virus and the host immune system and-most importantly-informed vaccine design.

Generation of Conformation-Specific Antibody Fragments for Crystallization of the Multidrug Resistance Transporter MdfA.

A major hurdle in membrane protein crystallography is generating crystals diffracting sufficiently for structure determination. This is often attributed not only to the difficulty of obtaining functionally active protein in mg amounts but also to the intrinsic flexibility of its multiple conformations. The cocrystallization of membrane proteins with antibody fragments has been reported as an effective approach to improve the diffraction quality of membrane protein crystals by limiting the intrinsic flexibility. Isolating suitable antibody fragments recognizing a single conformation of a native membrane protein is not a straightforward task. However, by a systematic screening approach, the time to obtain suitable antibody fragments and consequently the chance of obtaining diffracting crystals can be reduced. In this chapter, we describe a protocol for the generation of Fab fragments recognizing the native conformation of a major facilitator superfamily (MFS)-type MDR transporter MdfA from Escherichia coli. We confirmed that the use of Fab fragments was efficient for stabilization of MdfA and improvement of its crystallization properties.

The multidrug-resistance transporter MdfA from Escherichia coli: crystallization and X-ray diffraction analysis.

The active efflux of antibiotics by multidrug-resistance (MDR) transporters is a major pathway of drug resistance and complicates the clinical treatment of bacterial infections. MdfA is a member of the major facilitator superfamily (MFS) from Escherichia coli and provides resistance to a wide variety of dissimilar toxic compounds, including neutral, cationic and zwitterionic substances. The 12-transmembrane-helix MdfA was expressed as a GFP-octahistidine fusion protein with a TEV protease cleavage site. Following tag removal, MdfA was purified using two chromatographic steps, complexed with a Fab fragment and further purified using size-exclusion chromatography. MdfA and MdfA-Fab complexes were subjected to both vapour-diffusion and lipidic cubic phase (LCP) crystallization techniques. Vapour-diffusion-grown crystals were of type II, with poor diffraction behaviour and weak crystal contacts. LCP lipid screening resulted in type I crystals that diffracted to 3.4 Šresolution and belonged to the hexagonal space group P6122.