title: Dissection of a human septin: definition and characterization of distinct domains within human SEPT4.
abstract: 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.
authors.affiliation: Centro de Biotecnologia Molecular e Estrutural (CBME), Instituto de Física de São Carlos (IFSC), Universidade de São Paulo (USP), Av. Trabalhador São Carlense 400, centro, Box 369, São Carlos, SP, 13560-970, Brazil. wanius@if.sc.usp.br
authors.forename: Wanius
authors.initials: W
authors.lastname: Garcia
authors.forename: Ana Paula Ulian
authors.initials: AP
authors.lastname: de Araújo
authors.forename: Mario de Oliveira
authors.initials: Mde O
authors.lastname: Neto
authors.forename: Michel R M
authors.initials: MR
authors.lastname: Ballestero
authors.forename: Igor
authors.initials: I
authors.lastname: Polikarpov
authors.forename: Manami
authors.initials: M
authors.lastname: Tanaka
authors.forename: Tomoo
authors.initials: T
authors.lastname: Tanaka
authors.forename: Richard Charles
authors.initials: RC
authors.lastname: Garratt
mesh_terms: D000595:Amino Acid Sequence; D002846:Chromatography, Affinity; D002850:Chromatography, Gel; D002942:Circular Dichroism; D003598:Cytoskeletal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D020558:GTP Phosphohydrolases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D058112:Septins; D017386:Sequence Homology, Amino Acid; D013050:Spectrometry, Fluorescence
chemical_list: D003598:Cytoskeletal Proteins; D020558:GTP Phosphohydrolases; C492027:SEPTIN4 protein, human; D058112:Septins