Molecular modeling of human neutral sphingomyelinase provides insight into its molecular interactions

The neutral sphingomyelinase (N-SMase) is considered a major candidate for mediating the stress-induced production of ceramide, and it plays an important role in cell-cycle arrest, apoptosis, inflammation, and eukaryotic stress responses. Recent studies have identified a small region at the very N-terminus of the 55 kDa tumour necrosis factor receptor (TNF-R55), designated the neutral sphingomyelinase activating domain (NSD) that is responsible for the TNF-induced activation of N-SMase. There is no direct association between TNF-R55 NSD and N-SMase; instead, a protein named factor associated with N-SMase activation (FAN) has been reported to couple the TNF-R55 NSD to N-SMase. Since the three-dimensional fold of N-SMase is still unknown, we have modeled the structure using the protein fold recognition and threading method. Moreover, we propose models for the TNF-R55 NSD as well as the FAN protein in order to study the structural basis of N-SMase activation and regulation. Protein-protein interaction studies suggest that FAN is crucially involved in mediating TNF-induced activation of the N-SMase pathway, which in turn regulates mitogenic and proinflammatory responses. Inhibition of N-SMase may lead to reduction of ceramide levels and hence may provide a novel therapeutic strategy for inflammation and autoimmune diseases. Molecular dynamics (MD) simulations were performed to check the stability of the predicted model and protein-protein complex; indeed, stable RMS deviations were obtained throughout the simulation. Furthermore, in silico docking of low molecular mass ligands into the active site of N-SMase suggests that His135, Glu48, Asp177, and Asn179 residues play crucial roles in this interaction. Based on our results, these ligands are proposed to be potent and selective N-SMase inhibitors, which may ultimately prove useful as lead compounds for drug development

TWISTED DWARF1 mediates the action of auxin transport inhibitors on actin cytoskeleton dynamics

Plant growth and architecture is regulated by the polar distribution of the hormone auxin. Polarity and flexibility of this process is provided by constant cycling of auxin transporter vesicles along actin filaments, coordinated by a positive auxin-actin feedback loop. Both polar auxin transport and vesicle cycling are inhibited by synthetic auxin transport inhibitors, such as 1-N-naphthylphthalamic acid (NPA), counteracting the effect of auxin; however, underlying targets and mechanisms are unclear. Using NMR, we map the NPA binding surface on the Arabidopsis thaliana ABCB chaperone TWISTED DWARF1 (TWD1). We identify ACTIN7 as a relevant, although likely indirect, TWD1 interactor, and show TWD1-dependent regulation of actin filament organization and dynamics and that TWD1 is required for NPA-mediated actin cytoskeleton remodeling. The TWD1-ACTIN7 axis controls plasma membrane presence of efflux transporters, and as a consequence act7 and twd1 share developmental and physiological phenotypes indicative of defects in auxin transport. These can be phenocopied by NPA treatment or by chemical actin (de)stabilization. We provide evidence that TWD1 determines downstream locations of auxin efflux transporters by adjusting actin filament debundling and dynamizing processes and mediating NPA action on the latter. This function appears to be evolutionary conserved since TWD1 expression in budding yeast alters actin polarization and cell polarity and provides NPA sensitivity

Trading off stability against activity in extremophilic aldolases

Understanding enzyme stability and activity in extremophilic organisms is of great biotechnological interest, but many questions are still unsolved. Using 2-deoxy-D-ribose-5-phosphate aldolase (DERA) as model enzyme, we have evaluated structural and functional characteristics of different orthologs from psychrophilic, mesophilic and hyperthermophilic organisms. We present the first crystal structures of psychrophilic DERAs, revealing a dimeric organization resembling their mesophilic but not their thermophilic counterparts. Conversion into monomeric proteins showed that the native dimer interface contributes to stability only in the hyperthermophilic enzymes. Nevertheless, introduction of a disulfide bridge in the interface of a psychrophilic DERA did confer increased thermostability, suggesting a strategy for rational design of more durable enzyme variants. Constraint network analysis revealed particularly sparse interactions between the substrate pocket and its surrounding α-helices in psychrophilic DERAs, which indicates that a more flexible active center underlies their high turnover numbers