title: INF2-mediated severing through actin filament encirclement and disruption.
abstract: BACKGROUND
  INF2 is a formin protein with the unique ability to accelerate both actin polymerization and depolymerization, the latter requiring filament severing. Mutations in INF2 lead to the kidney disease focal segmental glomerulosclerosis (FSGS) and the neurological disorder Charcot-Marie Tooth disease (CMTD).
  
  
  RESULTS
  Here, we compare the severing mechanism of INF2 with that of the well-studied severing protein cofilin. INF2, like cofilin, binds stoichiometrically to filament sides and severs in a manner that requires phosphate release from the filament. In contrast to cofilin, however, INF2 binds ADP and ADP-Pi filaments equally well. Furthermore, two-color total internal reflection fluorescence (TIRF) microscopy reveals that a low number of INF2 molecules, as few as a single INF2 dimer, are capable of severing, while measurable cofilin-mediated severing requires more extensive binding. Hence, INF2 is a more potent severing protein than cofilin. While a construct containing the FH1 and FH2 domains alone has some severing activity, addition of the C-terminal region increases severing potency by 40-fold, and we show that the WH2-resembling DAD motif is responsible for this increase. Helical 3D reconstruction from electron micrographs at 20 Å resolution provides a structure of filament-bound INF2, showing that the FH2 domain encircles the filament.
  
  
  CONCLUSIONS
  We propose a severing model in which FH2 binding and phosphate release causes local filament deformation, allowing the DAD to bind adjacent actin protomers, further disrupting filament structure.
authors.affiliation: Department of Biochemistry, Geisel School of Medicine at Dartmouth, Hanover, NH 03755, USA.
authors.forename: Pinar S
authors.initials: PS
authors.lastname: Gurel
authors.affiliation: Electron Imaging Center for NanoMachines and Department of Microbiology, Immunology, and Molecular Genetics, UCLA School of Medicine, Los Angeles, CA 90095, USA.
authors.forename: Peng
authors.initials: P
authors.lastname: Ge
authors.affiliation: Department of Chemistry and Biochemistry, UCLA, Los Angeles, CA 90095, USA.
authors.forename: Elena E
authors.initials: EE
authors.lastname: Grintsevich
authors.affiliation: Department of Biochemistry, Geisel School of Medicine at Dartmouth, Hanover, NH 03755, USA.
authors.forename: Rui
authors.initials: R
authors.lastname: Shu
authors.affiliation: CEA, iRTSV, Laboratoire Physiologie Cellulaire & Végétale, CNRS, UMR 5168, Université Joseph Fourier-Grenoble 1, 38054 Grenoble, France.
authors.forename: Laurent
authors.initials: L
authors.lastname: Blanchoin
authors.affiliation: Electron Imaging Center for NanoMachines and Department of Microbiology, Immunology, and Molecular Genetics, UCLA School of Medicine, Los Angeles, CA 90095, USA; Molecular Biology Institute, UCLA, Los Angeles, CA 90095, USA.
authors.forename: Z Hong
authors.initials: ZH
authors.lastname: Zhou
authors.affiliation: Department of Chemistry and Biochemistry, UCLA, Los Angeles, CA 90095, USA; Molecular Biology Institute, UCLA, Los Angeles, CA 90095, USA.
authors.forename: Emil
authors.initials: E
authors.lastname: Reisler
authors.affiliation: Department of Biochemistry, Geisel School of Medicine at Dartmouth, Hanover, NH 03755, USA. Electronic address: henry.higgs@dartmouth.edu.
authors.forename: Henry N
authors.initials: HN
authors.lastname: Higgs
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism* / Q000648:ultrastructure; D000818:Animals; D008840:Microfilament Proteins / Q000737:chemistry / Q000378:metabolism*; D008854:Microscopy, Electron; D008958:Models, Molecular; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D011817:Rabbits
chemical_list: D008840:Microfilament Proteins