07.1_13_lecture.pptx

Chapter 13 Cytoskeletal Systems Lectures by Kathleen Fitzpatrick © 2016 Pearson Education, Inc. Simon Fraser University Cytoskeletal Systems  The interior of a cell is highly structured  The cytoskeleton is a network of interconnected filaments & tubules  Plays a roles in cell movement & divi...

Chapter 13 Cytoskeletal Systems Lectures by Kathleen Fitzpatrick © 2016 Pearson Education, Inc. Simon Fraser University Cytoskeletal Systems  The interior of a cell is highly structured  The cytoskeleton is a network of interconnected filaments & tubules  Plays a roles in cell movement & division  Dynamic & changeable © 2016 Pearson Education, Inc. 13.1 Major Structural Elements of the Cytoskeleton  The major structural elements of the cytoskeleton are MICROFILAMENTS Strength © 2016 Pearson Education, Inc. Movement Movement Eukaryotes Have Three Basic Types of Cytoskeletal Elements  Microtubules are composed of tubulin subunits  Microfilaments are composed of actin subunits  Intermediate filaments have variable composition © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Bacteria Have Cytoskeletal Systems That Are Structurally Similar to Those in Eukaryotes  Bacteria have polymer systems similarly cytoskeletal elements  Actin-like MreB- involved in DNA segregation & cell shape  Tubulin-like FtsZ- involved in regulating division  Crescentin regulator of cell shape © 2016 Pearson Education, Inc. The Cytoskeleton Is Dynamically Assembled and Disassembled  Microfilaments are essential components of muscle fibrils,  Microtubules are structural elements of cilia and flagella  Research has shown that the cytoskeleton is dynamically assembled and disassembled © 2016 Pearson Education, Inc. 13.2 Microtubules  Microtubules (MTs) are the largest structural elements of the cytoskeleton  They are involved in a variety of functions in the cell © 2016 Pearson Education, Inc. Two Types of Microtubules Are Responsible for Many Functions in the Cell  Cytoplasmic microtubules are located in the cytosol & are responsible for a variety of functions  Maintaining axons  Formation of mitotic and meiotic spindles  Maintaining or altering cell shape  Placement and movement of vesicles © 2016 Pearson Education, Inc. Tubulin Heterodimers Are the Protein Building Blocks of Microtubules  Microtubules are straight, hollow cylinders of varied length that consist of (usually 13) longitudinal arrays of polymers called protofilaments  The basic subunit of a protofilament is a heterodimer of tubulin, one α-tubulin and one β-tubulin  These bind noncovalently to form an αβ-heterodimer, which does not normally dissociate © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Subunit Structure  α and β subunits have nearly identical threedimensional structure, but only 40% amino acid identity  All the dimers in the MT are oriented the same way © 2016 Pearson Education, Inc. Microtubules Polarity and Isoforms  Because of dimer orientation, protofilaments have an inherent polarity  The two ends differ both chemically and structurally  the plus end and the minus end © 2016 Pearson Education, Inc. Microtubules Form by the Addition of Tubulin Dimers at Their Ends  Microtubules form by the reversible polymerization of tubulin dimers in the presence of GTP and Mg2+  Dimers aggregate into oligomers, which serve as “seeds” from which new microtubules grow  This process is called nucleation; the addition of more subunits at either end is called elongation © 2016 Pearson Education, Inc. Microtubule Assembly  MT formation is slow at first because the process of nucleation is slow; this period is known as the lag phase  The elongation phase is much faster  When the mass of MTs reaches a point where the amount of free tubulin is diminished, the assembly is balanced by disassembly; this is known as the plateau phase © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Critical Concentration  Microtubule assembly in vitro depends on concentration of tubulin dimers  The tubulin concentration at which MT assembly is exactly balanced by disassembly is called the critical concentration  MTs grow when the tubulin concentration exceeds the critical concentration and vice versa © 2016 Pearson Education, Inc. Addition of Tubulin Dimers Occurs More Quickly at the Plus Ends of Microtubules  The two ends of an MT differ chemically, and one can grow or shrink much faster than the other  The rapidly growing MT end is the plus end, and the other is the minus end © 2016 Pearson Education, Inc. Microtubule Treadmilling  The plus and minus ends of microtubules have different critical concentrations  If the free tubulin concentration is above the critical concentration for the plus end but below that of the minus end, treadmilling will occur  Treadmilling: addition of subunits at the plus end, and removal from the minus end © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. GTP Hydrolysis Contributes to the Dynamic Instability of Microtubules  Each tubulin heterodimer binds two GTP molecules; α-tubulin binds one, and β-tubulin binds a second  The GTP bound to the β-subunit is hydrolyzed to GDP after the heterodimer is added to the MT  GTP is needed to promote heterodimer interactions and addition to MTs, but its hydrolysis is not required for MT assembly © 2016 Pearson Education, Inc. Dynamic Instability  Dynamic instability model: one population of MTs grows by polymerization at the plus ends, whereas another population shrinks by depolymerization  Growing MTs have GTP at the plus ends, and shrinking MTs have GDP  The GTP cap at the plus end prevents subunit removal © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. GTP-Tubulin and Dynamic Instability  If GTP-tubulin is high, it is added to an MT quickly, creating a large GTP-tubulin cap  If the concentration falls, the rate of tubulin addition decreases  At a sufficiently low GTP-tubulin, the rate of GTP hydrolysis exceeds the rate of subunit addition, and the cap shrinks © 2016 Pearson Education, Inc. Catastrophe and Rescue  If the GTP cap disappears altogether, the MT becomes unstable, and loss of GDP-bound subunits is favored  Individual MTs can go through periods of growth and shrinkage; a switch from growth to shrinkage is called microtubule catastrophe  A sudden switch back to growth phase is called microtubule rescue © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Microtubules Originate from Microtubule-Organizing Centers Within the Cell  MTs originate from a microtubuleorganizing center (MTOC)  Many cells have an MTOC called a centrosome near the nucleus © 2016 Pearson Education, Inc. γ-Tubulin  Centrosomes have large ring-shaped protein complexes in them; these contain γ-tubulin  γ-tubulin is found only in centrosomes  γ-tubulin ring complexes (γ-TuRCs) nucleate the assembly of new MTs away from the centrosome  Loss of γ-TuRCs prevents a cell from nucleating MTs © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. MTOCs Organize and Polarize the Microtubules Within Cells  MTOCs nucleate and anchor MTs  MTs grow outward from the MTOC with a fixed polarity—the minus ends are anchored in the MTOC  Because of this, dynamic growth and shrinkage of MTs occurs at the plus ends, near the cell periphery © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Microtubule Stability Is Tightly Regulated in Cells by a Variety of Microtubule-Binding Proteins  Cells regulate MTs with great precision  Some MT-binding proteins use ATP to drive vesicle or organelle transport or to generate sliding forces between MTs  Others regulate MT structure © 2016 Pearson Education, Inc. Microtubule-Stabilizing/Bundling Proteins  MAPs, microtubule-associated proteins, bind at regular intervals along a microtubule wall, allowing for interaction with other cellular structures and filaments © 2016 Pearson Education, Inc. Microtubule-Destabilizing/Severing Proteins  Some proteins promote depolarization of MTs  Stathmin/Op18 binds to tubulin heterodimers and prevents their polymerization  Catastrophins act at the ends of MTs and promote the peeling of subunits from the ends  Proteins such as katanin sever MTs © 2016 Pearson Education, Inc.