Paola Bisignano

Stochastic steps in secondary active sugar transport

Significance The potential energy stored in ion gradients across cell membranes drives nutrients in and out of cells by cotransport proteins, e.g., uphill glucose accumulation in cells by sodium cotransporters. Insight into the mechanism of cotransport has been obtained from high-resolution atomic structures of the transporters, but further progress requires dynamic information about ion and substrate movements through the proteins. We have used multiple long molecular-dynamic simulations and electrophysiological assays to explore the dynamics of the transport cycle. Ligands bound to sodium-dependent glucose transporters are released to the cytoplasm stochastically, whereas release to the external solution is ordered with sugar first. The order of events is intimately tied to how the protein converts the energy stored in an ion gradient into a sugar gradient. Secondary active transporters, such as those that adopt the leucine-transporter fold, are found in all domains of life, and they have the unique capability of harnessing the energy stored in ion gradients to accumulate small molecules essential for life as well as expel toxic and harmful compounds. How these proteins couple ion binding and transport to the concomitant flow of substrates is a fundamental structural and biophysical question that is beginning to be answered at the atomistic level with the advent of high-resolution structures of transporters in different structural states. Nonetheless, the dynamic character of the transporters, such as ion/substrate binding order and how binding triggers conformational change, is not revealed from static structures, yet it is critical to understanding their function. Here, we report a series of molecular simulations carried out on the sugar transporter vSGLT that lend insight into how substrate and ions are released from the inward-facing state of the transporter. Our simulations reveal that the order of release is stochastic. Functional experiments were designed to test this prediction on the human homolog, hSGLT1, and we also found that cytoplasmic release is not ordered, but we confirmed that substrate and ion binding from the extracellular space is ordered. Our findings unify conflicting published results concerning cytoplasmic release of ions and substrate and hint at the possibility that other transporters in the superfamily may lack coordination between ions and substrate in the inward-facing state.

Development and Application of a Virtual Screening Protocol for the Identification of Multitarget Fragments

In this study, we report on a virtual ligand screening protocol optimized to identify fragments endowed with activity at multiple targets. Thanks to this protocol, we were able to identify a fragment that displays activity in the low‐micromolar range at both β‐secretase 1 (BACE‐1) and glycogen synthase kinase 3β (GSK‐3β). These two structurally and physiologically unrelated enzymes likely contribute, through different pathways, to the onset of Alzheimer′s disease (AD). Therefore, their simultaneous inhibition holds great potential in exerting a profound effect on AD. In perspective, the strategy outlined herein can be adapted to other target combinations.

Substrate-bound outward-open structure of a Na+-coupled sialic acid symporter reveals a new Na+site.

Many pathogenic bacteria utilise sialic acids as an energy source or use them as an external coating to evade immune detection. As such, bacteria that colonise sialylated environments deploy specific transporters to mediate import of scavenged sialic acids. Here, we report a substrate-bound 1.95 Å resolution structure and subsequent characterisation of SiaT, a sialic acid transporter from Proteus mirabilis. SiaT is a secondary active transporter of the sodium solute symporter (SSS) family, which use Na+ gradients to drive the uptake of extracellular substrates. SiaT adopts the LeuT-fold and is in an outward-open conformation in complex with the sialic acid N-acetylneuraminic acid and two Na+ ions. One Na+ binds to the conserved Na2 site, while the second Na+ binds to a new position, termed Na3, which is conserved in many SSS family members. Functional and molecular dynamics studies validate the substrate-binding site and demonstrate that both Na+ sites regulate N-acetylneuraminic acid transport.Sialic acid transporters (SiaT) are required for sialic acid uptake in a number of human pathogens and are of interest as targets for antimicrobial drug development. Here the authors present the substrate bound SiaT structure from the uropathogen Proteus mirabilis and provide insights into the mechanism of sialic acid transport.

Conformational transitions of the sodium-dependent sugar transporter, vSGLT

Significance Transporters isomerize between conformations to shuttle cargo across membranes, but the mechanism is not understood. Double electron–electron resonance measurements on the sodium-dependent sugar transporter (vSGLT) were used to explore the conformational state of the transporter under specific ligand conditions. Although sugar transport by vSGLT is driven by sodium gradients, vSGLT adopts an inward-open conformation irrespective of the presence of sodium. In the presence of sodium and galactose, the transporter transitions to an occluded conformation. We propose that the cell’s negative membrane potential aids in driving vSGLT toward the outward-facing state to bind sugar and begin the transport cycle. These findings could be applicable to other transporters whereby the inherent cellular membrane potential is integrated into the transport cycle. Sodium-dependent transporters couple the flow of Na+ ions down their electrochemical potential gradient to the uphill transport of various ligands. Many of these transporters share a common core structure composed of a five-helix inverted repeat and deliver their cargo utilizing an alternating-access mechanism. A detailed characterization of inward-facing conformations of the Na+-dependent sugar transporter from Vibrio parahaemolyticus (vSGLT) has previously been reported, but structural details on additional conformations and on how Na+ and ligand influence the equilibrium between other states remains unknown. Here, double electron–electron resonance spectroscopy, structural modeling, and molecular dynamics are utilized to deduce ligand-dependent equilibria shifts of vSGLT in micelles. In the absence and presence of saturating amounts of Na+, vSGLT favors an inward-facing conformation. Upon binding both Na+ and sugar, the equilibrium shifts toward either an outward-facing or occluded conformation. While Na+ alone does not stabilize the outward-facing state, gating charge calculations together with a kinetic model of transport suggest that the resting negative membrane potential of the cell, absent in detergent-solubilized samples, may stabilize vSGLT in an outward-open conformation where it is poised for binding external sugars. In total, these findings provide insights into ligand-induced conformational selection and delineate the transport cycle of vSGLT.

Protonation state of glutamate 73 regulates the formation of a specific dimeric association of mVDAC1

Significance Cellular acidification occurs in response to many cellular events, including ischemia and apoptosis. The voltage-dependent anion channel (VDAC) is the most abundant protein in the outer mitochondrial membrane and has been implicated in mitochondrial regulation, cell death, and assembling protein complexes with Bcl2 family of apoptotic regulating proteins. During apoptosis, VDAC has demonstrated rapid oligomerization followed by cytochrome c release into the cytoplasm. We report a new dimeric model of VDAC induced by minor acidification with a pKa of 7.4, which is regulated by the protonation state of transmembrane residue E73. This study links a specific dimer formation with cellular acidification, which may be a salient feature of the molecular mechanism(s) involved in mitochondrial regulation. The voltage-dependent anion channel (VDAC) is the most abundant protein in the outer mitochondrial membrane and constitutes the primary pathway for the exchange of ions and metabolites between the cytosol and the mitochondria. There is accumulating evidence supporting VDAC’s role in mitochondrial metabolic regulation and apoptosis, where VDAC oligomerization has been implicated with these processes. Herein, we report a specific pH-dependent dimerization of murine VDAC1 (mVDAC1) identified by double electron–electron resonance and native mass spectrometry. Intermolecular distances on four singly spin-labeled mVDAC1 mutants were used to generate a model of the low-pH dimer, establishing the presence of residue E73 at the interface. This dimer arrangement is different from any oligomeric state previously described, and it forms as a steep function of pH with an apparent pKa of 7.4. Moreover, the monomer–dimer equilibrium affinity constant was determined using native MS, revealing a nearly eightfold enhancement in dimerization affinity at low pH. Mutation of E73 to either alanine or glutamine severely reduces oligomerization, demonstrating the role of protonated E73 in enhancing dimer formation. Based on these results, and the known importance of E73 in VDAC physiology, VDAC dimerization likely plays a significant role in mitochondrial metabolic regulation and apoptosis in response to cytosolic acidification during cellular stress.

Cilia-Associated Oxysterols Activate Smoothened

Primary cilia are required for Smoothened to transduce vertebrate Hedgehog signals, but how Smoothened accumulates in cilia and is activated is incompletely understood. Here, we identify cilia-associated oxysterols that promote Smoothened accumulation in cilia and activate the Hedgehog pathway. Our data reveal that cilia-associated oxysterols bind to two distinct Smoothened domains to modulate Smoothened accumulation in cilia and tune the intensity of Hedgehog pathway activation. We find that the oxysterol synthase HSD11β2 participates in the production of Smoothened-activating oxysterols and promotes Hedgehog pathway activity. Inhibiting oxysterol biosynthesis impedes oncogenic Hedgehog pathway activation and attenuates the growth of Hedgehog pathway-associated medulloblastoma, suggesting that targeted inhibition of Smoothened-activating oxysterol production may be therapeutically useful for patients with Hedgehog-associated cancers.