title: Endogenous species of mammalian nonmuscle myosin IIA and IIB include activated monomers and heteropolymers.
abstract: BACKGROUND
  Class II myosins generate contractile forces in cells by polymerizing into bipolar filaments and pulling on anchored actin filaments. Nonmuscle myosin II (NMII) plays central roles during cell adhesion, migration, cytokinesis, and tissue morphogenesis. NMII is present in virtually all mammalian cell types as tissue-specific combinations of NMIIA, NMIIB, and NMIIC isoforms. It remains poorly understood how the highly dynamic NMII-actin contractile system begins to assemble at new cellular locations during cell migration and how incorporation of different NMII isoforms into this system is coordinated.
  
  
  RESULTS
  Using platinum replica electron microscopy in combination with immunogold labeling, we demonstrate that individual activated (phosphorylated on the regulatory light chain and unfolded) NMIIA and NMIIB molecules represent a functional form of NMII in motile cells and that NMIIA and NMIIB copolymerize into nascent bipolar filaments during contractile system assembly. Using subdiffraction stimulated emission depletion microscopy together with a pharmacological block-and-release approach, we report that NMIIA and NMIIB simultaneously incorporate into the cytoskeleton during initiation of contractile system assembly, whereas the characteristic rearward shift of NMIIB relative to NMIIA is established later in the course of NMII turnover.
  
  
  CONCLUSIONS
  We show existence of activated NMII monomers in cells, copolymerization of endogenous NMIIA and NMIIB molecules, and contribution of both isoforms, rather than only NMIIA, to early stages of the contractile system assembly. These data change the current paradigms about dynamics and functions of NMII and provide new conceptual insights into the organization and dynamics of the ubiquitous cellular machinery for contraction that acts in multiple cellular contexts.
authors.affiliation: Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA.
authors.forename: Maria S
authors.initials: MS
authors.lastname: Shutova
authors.affiliation: Department of Pathology and Laboratory Medicine, The Children's Hospital of Philadelphia, University of Pennsylvania, Philadelphia, PA 19104, USA.
authors.forename: Waldo A
authors.initials: WA
authors.lastname: Spessott
authors.affiliation: Department of Pathology and Laboratory Medicine, The Children's Hospital of Philadelphia, University of Pennsylvania, Philadelphia, PA 19104, USA.
authors.forename: Claudio G
authors.initials: CG
authors.lastname: Giraudo
authors.affiliation: Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA. Electronic address: svitkina@sas.upenn.edu.
authors.forename: Tatyana
authors.initials: T
authors.lastname: Svitkina
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism*; D000818:Animals; D002448:Cell Adhesion; D002478:Cells, Cultured; D004622:Embryo, Mammalian; D024507:Nonmuscle Myosin Type IIA / Q000378:metabolism*; D024501:Nonmuscle Myosin Type IIB / Q000378:metabolism*; D010766:Phosphorylation; D020033:Protein Isoforms / Q000378:metabolism; D051381:Rats
chemical_list: D020033:Protein Isoforms; D024507:Nonmuscle Myosin Type IIA; D024501:Nonmuscle Myosin Type IIB