title: Septin 9 Exhibits Polymorphic Binding to F-Actin and Inhibits Myosin and Cofilin Activity.
abstract: Septins are a highly conserved family of proteins in eukaryotes that is recognized as a novel component of the cytoskeleton. Septin 9 (SEPT9) interacts directly with actin filaments and functions as an actin stress fiber cross-linking protein that promotes the maturation of nascent focal adhesions and cell migration. However, the molecular details of how SEPT9 interacts with F-actin remain unknown. Here, we use electron microscopy and image analysis to show that SEPT9 binds to F-actin in a highly polymorphic fashion. We demonstrate that the basic domain (B-domain) of the N-terminal tail of SEPT9 is responsible for actin cross-linking, while the GTP-binding domain (G-domain) does not bundle F-actin. We show that the B-domain of SEPT9 binds to three sites on F-actin, and the two of these sites overlap with the binding regions of myosin and cofilin. SEPT9 inhibits actin-dependent ATPase activity of myosin and competes with the weakly bound state of myosin for binding to F-actin. At the same time, SEPT9 significantly reduces the extent of F-actin depolymerization by cofilin. Taken together, these data suggest that SEPT9 protects actin filaments from depolymerization by cofilin and myosin and indicate a mechanism by which SEPT9 could maintain the integrity of growing and contracting actin filaments.
authors: Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507, USA.
authors: Clayton
authors: C
authors: Smith
authors: Department of Biology, Drexel University, Philadelphia, PA 19104, USA.
authors: Lee
authors: L
authors: Dolat
authors: Department of Biology, Drexel University, Philadelphia, PA 19104, USA.
authors: Dimitrios
authors: D
authors: Angelis
authors: Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507, USA.
authors: Eva
authors: E
authors: Forgacs
authors: Department of Biology, Drexel University, Philadelphia, PA 19104, USA. Electronic address: ets33@drexel.edu.
authors: Elias T
authors: ET
authors: Spiliotis
authors: Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507, USA. Electronic address: galkinve@evms.edu.
authors: Vitold E
authors: VE
authors: Galkin
mesh_terms: D051339:Actin Depolymerizing Factors / Q000037:antagonists & inhibitors*; D000199:Actins / Q000378:metabolism*; D000205:Actomyosin / Q000037:antagonists & inhibitors; D000818:Animals; D003599:Cytoskeleton / Q000378:metabolism; D004330:Drosophila / Q000378:metabolism*; D029721:Drosophila Proteins / Q000378:metabolism*; D022001:Focal Adhesions / Q000378:metabolism; D007091:Image Processing, Computer-Assisted; D008854:Microscopy, Electron; D009218:Myosins / Q000037:antagonists & inhibitors*; D058105:Polymerization; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D058112:Septins / Q000378:metabolism*
chemical_list: D051339:Actin Depolymerizing Factors; D000199:Actins; D029721:Drosophila Proteins; C000604588:septin 9 protein, Drosophila; D000205:Actomyosin; D058112:Septins; D009218:Myosins
keywords: 3D reconstruction; F-actin; cell motility; electron microscopy; septins