Septin C-terminal domain interactions: implications for filament stability and assembly.

Septins form a conserved family of filament forming GTP binding proteins found in a wide range of eukaryotic cells. They share a common structural architecture consisting of an N-terminal domain, a central GTP binding domain and a C-terminal domain, which is often predicted to adopt a coiled-coil conformation, at least in part. The crystal structure of the human SEPT2/SEPT6/SEPT7 heterocomplex has revealed the importance of the GTP binding domain in filament formation, but surprisingly no electron density was observed for the C-terminal domains and their function remains obscure. The dearth of structural information concerning the C-terminal region has motivated the present study in which the putative C-terminal domains of human SEPT2, SEPT6 and SEPT7 were expressed in E. coli and purified to homogeneity. The thermal stability and secondary structure content of the domains were studied by circular dichroism spectroscopy, and homo- and hetero-interactions were investigated by size exclusion chromatography, chemical cross-linking, analytical ultracentrifugation and surface plasmon resonance. Our results show that SEPT6-C and SEPT7-C are able to form both homo- and heterodimers with a high α-helical content in solution. The heterodimer is elongated and considerably more stable than the homodimers, with a K(D) of 15.8 nM. On the other hand, the homodimer SEPT2-C has a much lower affinity, with a K(D) of 4 µM, and a moderate α-helical content. Our findings present the first direct experimental evidence toward better understanding the biophysical properties and coiled-coil pairings of such domains and their potential role in filament assembly and stability.

Expression, purification and kinetic characterization of His-tagged glyceraldehyde-3-phosphate dehydrogenase from Trypanosoma cruzi.

Trypanosomes are flagellated protozoa responsible for serious parasitic diseases that have been classified by the World Health Organization as tropical sicknesses of major importance. One important drug target receiving considerable attention is the enzyme glyceraldehyde-3-phosphate dehydrogenase from the protozoan parasite Trypanosoma cruzi, the causative agent of Chagas disease (T. cruzi Glyceraldehyde-3-phosphate dehydrogenase (TcGAPDH); EC 1.2.1.12). TcGAPDH is a key enzyme in the glycolytic pathway of T. cruzi and catalyzes the oxidative phosphorylation of D-glyceraldehyde-3-phosphate (G3P) to 1,3-bisphosphoglycerate (1,3-BPG) coupled to the reduction of oxidized nicotinamide adenine dinucleotide, (NAD(+)) to NADH, the reduced form. Herein, we describe the cloning of the T. cruzi gene for TcGAPDH into the pET-28a(+) vector, its expression as a tagged protein in Escherichia coli, purification and kinetic characterization. The His(6)-tagged TcGAPDH was purified by affinity chromatography. Enzyme activity assays for the recombinant His(6)-TcGAPDH were carried out spectrophotometrically to determine the kinetic parameters. The apparent Michaelis-Menten constant (K(M)(app)) determined for D-glyceraldehyde-3-phosphate and NAD(+) were 352±21 and 272±25 µM, respectively, which were consistent with the values for the untagged enzyme reported in the literature. We have demonstrated by the use of Isothermal Titration Calorimetry (ITC) that this vector modification resulted in activity preserved for a higher period. We also report here the use of response surface methodology (RSM) to determine the region of optimal conditions for enzyme activity. A quadratic model was developed by RSM to describe the enzyme activity in terms of pH and temperature as independent variables. According to the RMS contour plots and variance analysis, the maximum enzyme activity was at 29.1°C and pH 8.6. Above 37°C, the enzyme activity starts to fall, which may be related to previous reports that the quaternary structure begins a process of disassembly.

The stability and aggregation properties of the GTPase domain from human SEPT4.

The septins are a family of conserved proteins involved in cytokinesis and cortical organization. An increasing amount of data implicates different septins in diverse pathological conditions including neurodegenerative disorders, neoplasia and infections. Human SEPT4 is a member of this family and its tissue-specific ectopic expression profile in colorectal and urologic cancer makes it a useful diagnostic biomarker. Thermal unfolding of the GTPase domain of SEPT4 (SEPT4-G) revealed an unfolding intermediate which rapidly aggregates into amyloid-like fibers under physiological conditions. In this study, we examined the effects of protein concentration, pH and metals ions on the aggregation process of recombinant SEPT4-G using a series of biophysical techniques, which were also employed to study chemical unfolding and stability. Divalent metal ions caused significant acceleration to the rate of SEPT4-G aggregation. Urea induced unfolding was shown to proceed via the formation of a partially unfolded intermediate state which unfolds further at higher urea concentrations. The intermediate is a compact dimer which is unable to bind GTP. At 1 M urea concentration, the intermediate state was plagued by irreversible aggregation at temperatures above 30 degrees C. However, higher urea concentration resulted in a marked decay of the aggregation, indicating that the partially folded structures may be necessary for the formation of these aggregates. The results presented here are consistent with the recently determined crystal structure of human septins and shed light on the aggregation properties of SEPT4 pertinent to its involvement in neurodegenerative disease.

An intermediate structure in the thermal unfolding of the GTPase domain of human septin 4 (SEPT4/Bradeion-beta) forms amyloid-like filaments in vitro.

SEPT4 is a member of the mammalian septin family of GTPases. Mammalian septins are conserved proteins which form heteropolymers in vivo and which are implicated in a variety of cellular functions such as cytokinesis, exocytosis, and vesicle trafficking. However, their structural properties and modes of action are largely unknown. There is a limited, but as yet inconclusive, amount of experimental data suggesting that SEPT4 may accumulate in tau-based filamentous deposits and cytoplasmic inclusions in Alzheimer's and Parkinson's disease, respectively. Here we report an intermediate structure of the GTPase domain of human SEPT4 (SEPT4-G) during unfolding transitions induced by temperature. This partially unfolded intermediate, which is rich in beta-sheet and free of bound nucleotide, was plagued by irreversible aggregation. The aggregates have the ability to bind specific dyes such as Congo red and thioflavin-T, suggesting they are amyloid in nature. Under electron microscopy, fibers of variable diameter extending for several micrometers in length can be visualized. This is the first report of amyloid formation by a septin or domain thereof, and the capacity of SEPT4-G to form such fibrillar aggregates may shed some light on the current discussion concerning the formation of homo- and heteropolymers of septins in vitro.

Dissection of a human septin: definition and characterization of distinct domains within human SEPT4.

The septins are a conserved family of guanosine-5'-triphosphate (GTP)-binding proteins. In mammals they are involved in a variety of cellular processes, such as cytokinesis, exocytosis, and vesicle trafficking. Specifically, SEPT4 has also been shown to be expressed in both human colorectal cancer and malignant melanoma, as well as being involved in neurodegenerative disorders. However, many of the details of the modes of action of septins in general remain unclear, and little is known of their detailed molecular architecture. Here, we define explicitly and characterize the domains of human SEPT4. Regions corresponding to the N-terminal, GTPase, and C-terminal domains as well as the latter two together were successfully expressed in Escherichia coli in soluble form and purified by affinity and size-exclusion chromatographies. The purified domains were analyzed by circular dichroism spectroscopy, fluorescence spectroscopy, dynamic light scattering, and small-angle X-ray scattering, as well as with bioinformatics tools. Of the three major domains that comprise SEPT4, the N-terminal domain contains little regular secondary structure and may be intrinsically unstructured. The central GTPase domain is a mixed alpha/beta structure, probably based on an open beta sheet. As defined here, it is catalytically active and forms stable homodimers in vitro. The C-terminal domain also forms homodimers and can be divided into two regions, the second of which is alpha-helical and consistent with a coiled-coil structure. These studies should provide a useful basis for future biophysical studies of SEPT4, including the structural basis for their involvement in diseases such as cancer and neurodegenerative disorders.

In vitro monitoring of GTPase activity and enzyme kinetics studies using capillary electrophoresis.

A capillary electrophoresis (CE)-based method for the in vitro detection and monitoring of nucleotide-triphosphatase activity is described. This robust and reproducible method was used to investigate GTPase activity of a recombinant protein construct containing the catalytic domain of Human SEPT4/Bradeion beta (GST-rDGTPase). This example application demonstrates that the CE technique can replace classical radioactive methods for GTPase activity assays and may be used as a routine analytical tool. Enzyme kinetics of GST-rDGTPase was studied and yielded the following kinetic parameters: v(max) = 1.7 microM min(-1) +/- 0.1, Km = 1.0 mM +/- 0.3, and apKcat = 9 x 10(-3) s(-1). In addition the effect of co-factors such as Mg2+ and Mn2+ on the catalytic activity was investigated. The described analytical method was also shown to be useful to analyze diphosphated and triphosphated forms of other nucleotides.