Septin 9 Exhibits Polymorphic Binding to F-Actin and Inhibits Myosin and Cofilin Activity

J Mol Biol. 2015 Oct 9;427(20):3273-3284. doi: 10.1016/j.jmb.2015.07.026. Epub 2015 Aug 19.


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.

Keywords: 3D reconstruction; F-actin; cell motility; electron microscopy; septins.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Actin Depolymerizing Factors / antagonists & inhibitors*
  • Actins / metabolism*
  • Actomyosin / antagonists & inhibitors
  • Animals
  • Cytoskeleton / metabolism
  • Drosophila / metabolism*
  • Drosophila Proteins / metabolism*
  • Focal Adhesions / metabolism
  • Image Processing, Computer-Assisted
  • Microscopy, Electron
  • Myosins / antagonists & inhibitors*
  • Polymerization
  • Protein Binding
  • Protein Structure, Tertiary
  • Septins / metabolism*


  • Actin Depolymerizing Factors
  • Actins
  • Drosophila Proteins
  • septin 9 protein, Drosophila
  • Actomyosin
  • Septins
  • Myosins