07.2_13_lecture.pptx

Chapter 13 Cytoskeletal Systems Lectures by Kathleen Fitzpatrick © 2016 Pearson Education, Inc. Simon Fraser University 13.3 Microfilaments  Microfilaments are the smallest of the cytoskeletal filaments  They are best known for their role in muscle contraction  Development and maintenance of...

Chapter 13 Cytoskeletal Systems Lectures by Kathleen Fitzpatrick © 2016 Pearson Education, Inc. Simon Fraser University 13.3 Microfilaments  Microfilaments are the smallest of the cytoskeletal filaments  They are best known for their role in muscle contraction  Development and maintenance of cell shape (via microfilaments just beneath the plasma membrane at the cell cortex)  Structural core of microvilli © 2016 Pearson Education, Inc. Actin Is the Protein Building Block of Microfilaments  Actin is a very abundant protein in all eukaryotic cells  Once synthesized, it folds into a globular-shaped molecule that can bind ATP or ADP (G-actin; globular actin)  G-actin molecules polymerize to form microfilaments, F-actin © 2016 Pearson Education, Inc. G-Actin Monomers Polymerize into F-Actin Microfilaments  G-actin monomers can polymerize reversibly into filaments with a lag phase and elongation phase, similar to tubulin assembly  F-actin filaments are composed of two linear strands of polymerized G-actin wound into a helix  All the actin monomers in the filament have the same orientation © 2016 Pearson Education, Inc. Demonstration of Microfilament Polarity  Myosin subfragment 1 (S1) can be incubated with microfilaments (MFs)  S1 fragments bind and decorate the actin MFs in a distinctive arrowhead pattern  Because of this pattern, the plus end of an MF is called the barbed end, and the minus end is called the pointed end © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Actin binds ATP  Monomers in the cytosol bind ATP  When they complex ATP is converted to ADP  ADP becomes trapped Polarity of Microfilaments  The polarity of MFs is reflected in more rapid addition or loss of G-actin at the plus end than the minus end  After the G-actin monomers assemble onto a microfilament, the ATP bound to them is slowly hydrolyzed  So, the growing MF ends have ATP-actin, whereas most of the MF is composed of ADP-actin © 2016 Pearson Education, Inc. Cells Can Dynamically Assemble Actin into a Variety of Structures © 2016 Pearson Education, Inc. Actin-Binding Proteins Regulate the Polymerization, Length, and Organization of Actin  Cells can precisely control where actin assembles and the structure of the resulting network  They use a variety of actin-binding proteins to do so  Control occurs at the nucleation, elongation, and severing of MFs and at the association of MFs into networks © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Proteins That Regulate Monomers and Their Polymerization  If the concentration of ATP-bound G-actin is high, microfilaments will assemble until the G-actin is limiting  In the cell, a large amount of free G-actin is not available because it is bound by thymosin β4  Profilin competes with thymosin β4 for G-actin binding © 2016 Pearson Education, Inc. Proteins That Regulate Polymerization  ADF/cofilin is known to bind ADP-G-actin and F-actin and is thought to increase turnover of ADP-actin at the minus end of MFs  ADF/cofilin also severs filaments, creating new plus ends in the process © 2016 Pearson Education, Inc. Proteins That Cap Actin Filaments  Whether MFs can grow depends on whether their filament ends are capped  Capping proteins bind the ends of a filament to prevent further loss or addition of subunits  CapZ binds to plus ends to prevent addition of subunits there; tropomodulins bind to minus ends, preventing loss of subunits there © 2016 Pearson Education, Inc. Proteins That Crosslink Actin Filaments  Often, actin networks form as loose networks of crosslinked filaments  One of the proteins important in the formation of these networks is filamin  Filamin acts to “splice,” joining two MFs together where they intersect © 2016 Pearson Education, Inc. Proteins That Bundle Actin Filaments  Some actin-containing structures can be highly ordered  Actin may be bundled into tightly organized arrays, called focal contacts or focal adhesions  α-Actinin is a protein that is prominent in such structures  Fascin in filopodia keeps the actin tightly bundled © 2016 Pearson Education, Inc. Crosslinks  The MFs are connected to the plasma membrane by crosslinks made of myosin I and calmodulin  The MFs in the bundle are tightly bound together by crosslinking proteins fimbrin and villin © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Proteins That Link Actin to Membranes  MFs are connected to the plasma membrane and exert force on it during cell movement or cytokinesis  This (indirect) connection to the membrane requires one or more linking proteins  Examples of such proteins include band 4.1, ezrin, radixin, moesin, spectrin, and ankyrin © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Proteins That Promote Actin Branching and Growth  Actin can also form a dendritic (treelike) network  A complex of actin-related proteins, the Arp2/3 complex, nucleates new branches on the sides of filaments  Arp2/3 branching is activated by a family of proteins that includes WASP (Wiskott-Aldrich syndrome protein) and WAVE/Scar © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. 13.4 Intermediate Filaments  Intermediate filaments (IFs) are not found in cytosol of plant cells but are abundant in many animal cells  An abundant intermediate filament (IF) is keratin, an important component of structures that grow from skin in animals  IFs are the most stable and least soluble components of the cytoskeleton  They likely support the entire cytoskeleton © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Intermediate Filament Proteins Are Tissue Specific  IFs differ greatly in amino acid composition from tissue to tissue  They are grouped into six classes © 2016 Pearson Education, Inc. Classes of Intermediate Filament Proteins  Class I: acidic keratins  Class II: basic or neutral keratins  Proteins of classes I and II make up the keratins found in epithelial surfaces covering the body and lining its cavities © 2016 Pearson Education, Inc. Classes of Intermediate Filament Proteins (continued)  Class III: includes vimentin (connective tissue), desmin (muscle cells), and glial fibrillary acidic protein (GFAP) (glial cells)  Class IV: the neurofilament (NF) proteins found in neurofilaments of nerve cells © 2016 Pearson Education, Inc. Classes of Intermediate Filament Proteins (continued)  Class V: includes the nuclear lamins A, B, and C that form a network along the inner surface of the nuclear membrane  Class VI: nestin, the substance that makes up the neurofilaments in nerve cells of embryos  Animal cells can be distinguished based on the types of IF proteins they contain—a technique known as intermediate filament typing © 2016 Pearson Education, Inc. Intermediate Filaments Assemble from Fibrous Subunits  The fundamental subunits of IF proteins are dimers  IF proteins are fibrous rather than globular  Each has a homologous central rodlike domain of 310 to 318 amino acids in length  Flanking the central helical domain are N- and Cterminal domains that differ greatly among IF proteins © 2016 Pearson Education, Inc. One Model of Intermediate Filament Assembly  The basic structural unit consists of two IF polypeptides intertwined into a coiled-coil  The two polypeptides are aligned in parallel  Two such dimers align laterally to form a tetrameric protofilament  Protofilaments overlap to build up a filamentous structure about eight protofilaments thick © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Intermediate Filaments Confer Mechanical Strength on Tissues  Intermediate filaments are thought to play a tension-bearing role  In neurons, IFs are dynamically transported and remodeled  IFs are less susceptible to chemical attack than are MTs and microfilaments © 2016 Pearson Education, Inc. The Cytoskeleton Is a Mechanically Integrated Structure  Microtubules resist bending when a cell is compressed  Microfilaments serve as contractile elements that generate tension  Intermediate filaments are elastic and can withstand tensile forces © 2016 Pearson Education, Inc. Integration of Cytoskeletal Elements  Spectraplakins are linker proteins that connect intermediate filaments, microfilaments, and microtubules  One, called plectin, is found at sites where intermediate filaments connect to MFs and MTs © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc.