CYTOSKELETON (I) Actin filaments Cell Biology Lecture 9

Readings and Objectives Russell : Chapter 1 (not complete information) Cooper: Chapter 12 Objectives Actin and actin filament dynamism Actin bundles and network Actin and myosin: role in contractile assemblies Role in cellular movement

Introduction: Cytoskeleton A network of protein filaments and tubules, extends from the nucleus to the plasma membrane Structural framework cell shape, localize organelles, general organization of the cytoplasm Movement Cell movement, internal transport of organelles, muscle contraction Dynamic structures, continually reorganized Composed of three main types of protein filaments: Actin filaments (7 nm) Intermediate filaments (8-11 nm) Microtubules ( 25 nm)

Structure and Organization of Actin Filaments Actin filaments (microfilaments): Polymer of Actin, flexible fibers 7 nm in diameter, several µm in length Actin first isolated from muscle cells in 1942 Abundant in all types of eukaryotic cells Mammals have 6 actin genes: 4 are expressed in muscle cells and 2 in nonmuscle cells Highly conserved Prokaryotic ancestor is MreB

Actin and Actin Filament 3-D structure determined in 1990 Actin: globular (G-actin), 375 aa (43 kD), barbed and pointed ends, binds head-tail to nucleate a trimer Filamentous (F-actin): monomers added to both end Filament is polar pointed end vs barbed end

G-Actin and Actin Filament Polymerization is reversible The rate at which monomers are added to filaments is proportional to their concentration ATP bound actin binds to barbed end with high affinity ADP-actin has low affinity to the pointed ends when ATP hydrolyses to ADP ADP-actin dissociates from filaments more readily than ATP-actin Therefore, the critical concentration of actin monomers is higher for addition to the pointed end than to the barbed end of actin filaments

Treadmilling: polarity of F-actin growth At cellular actin concentrations Barbed end of a filament grows 5–10 times faster than the pointed end ADP-actin dissociates from pointed end Exchange of ATP for ADP added to barbed-end Process is called Treadmilling Dynamic growth Direction? PointedBarbed

Actin Binding Proteins Actin Binding Proteins (ABP): modulate the Assembly and disassembly of actin filaments ABD/Actin interaction has diverse functionality Contribute to the cellular role of actin filaments

Actin Remodeling Some actin-binding proteins bind along the length of actin filaments, stabilizing them or cross-linking them to one another Others stablize by capping the ends and preventing dissociation Others promote dissociation, while others regulate the exchange of ATP for ADP.

Initiation of Actin Filament polymerization Nucleation is the rate-limiting step Formin and the Arp2/3 complex determine where filaments are formed by facilitating nucleation Formins nucleate long unbranched actin filaments

Initiation of Actin Filament polymerization actin filaments actively turn over and branch extensively These filaments are nucleated by the Arp2/3 (Actin Related Protein) complex, which binds actin/ATP near the barbed ends

Actin Filament Depolymerization The ADF/cofilin (Actin Depolymerizing Factor) family modifies existing filaments enhance the rate of dissociation of actin/ADP monomers from the pointed end, and remain bound to the monomers, preventing their reincorporation

Actin Filament Severing ADF/cofilin can also bind to and sever actin filaments Profilin reverses the ADF/Cofilin effect Stimulate exchange of bound ADP for ATP and dissociating the actin/ATP monomers from cofilin Become available for reassembly in two filaments

Summary: Assembly of an Actin Filament

Actin filament higher order assemblies Actin bundles—cross-linked into closely packed parallel arrays Actin networks—cross-linked in arrays that form 3-D meshworks

Actin Bundles Two types of actin bundles: Parallel filaments cross linked by actin-bundling proteins Have two domains to bind actin and align the filaments Two types of actin bundles: Non contractile filaments (14 nm apart) aligned in parallel, same polarity, barbed ends adjacent to the plasma membrane Fimbrin: a 68 kD proteins, cross links by its two actin binding domains (ABD)

Actin bundles: Intenstinal Microvilli Actin bundles take part in avariety of cell surface protrusions cell movement phagocytosis absorption of nutrients Intenstinal microvilli: Membrane projections, increase absorption surface

Actin bundles: Intenstinal Microvilli closely packed parallel bundles of 20 to 30 actin filaments Relatively permanent The filaments are cross-linked in part by fimbrin and villlin The actin bundles are attached to the plasma membrane by the calcium-binding protein calmodulin in association with myosin I At the base attach to actin cortex

Actin bundles: membrane protrusions Other surface protrusions are transient and form in response to environmental stimuli Pseudopodia- responsible for phagocytosis and the movement of amoebas Lamellipodia- broad, sheetlike extensions at the leading edge of fibroblasts Filopodia- thin projections of the plasma membrane in migrating cells

Actin Bundles: Contractile Contractile bundles: more widely-spaced filaments (40 nm), cross-linked by α-actinin α-actinin: a 102 kD protein with single ABD and an α-helical spacer Interacts with actin as a dimer Increased spacing allows actin interaction with motor protein myosin II Important in muscle fiber contraction

Higher order Actin assembly: Actin Network Filamin (280 kD) form flexible cross-links Filamin dimer: flexible V-shaped molecule actin-binding domains at the end of each arm dimerization domain Β-sheet spacer Binds actin orthogonally, form 3-D network beneath the plasma membrane network (cell cortex) determines cell shape, and cell movement

Actin Network and cell cortex Red blood cells as mode lack other cytoskeletal components, so the cortical cytoskeleton is the principal determinant of cell shape Spectrin, major actin-binding cortex protein tetramer of two polypeptide chains, α and β ends of the spectrin tetramers bind actin filaments, resulting in the spectrin-actin network

Actin Network and Cell Cortex Ankyrin links the spectrin-actin network and the plasma membrane Protein 4.1 is another link that binds spectrin-actin junctions and the transmembrane protein glycophorin

Association of Actin filaments with Motor proteins Brings higher level of functional complexity to cells Cellular or organismal movement Intracellular cargo transportation, cell division Association with motor protein myosin Myosin is a molecular motor: converts chemical energy (ATP) to mechanical energy  force and movement. Muscle contraction: model for understanding actin-myosin interactions and the motor activity of myosin molecules

Actin-Myosin and Muscle Contraction muscle fibers, large cells (50 µm in diameter and up to several centimeters in length) Cytoplasm consists of myofibrilsmyosin filaments and thin actin filaments sarcomeres, myofibril units of skeletal and cardiac muscle actin filaments attached at their barbed ends to the Z disc

Sacromere: a structural and contractile unit Titin is extremely large protein; extend from the M line to the Z disc keep myosin II filaments centered in the sarcomere maintain the resting tension that allows a muscle to snap back if overextended Nebulin, associated with actin, regulate assembly of actin filaments

Sliding Filament Model was proposed in 1954 Myosin slides on actin filament Sarcomere shortens, bringing the Z discs closer There is no change in the width of the A band, but the I bands and H zone almost disappear

Sliding Filament Model Tropomyosin binds along actin filaments, also bound to troponin No Ca2+, tropomyosin-troponin block binding of myosin to actin nerve impulses, stimulate release of Ca2+ from the sarcoplasmic reticulum Ca2+ binds troponin C, shifts the complex Allows myosin binding to actin

Sliding Filament Model Myosin II (the type in muscle), large protein with two heavy chains and two pairs of light chains heavy chains have a globular head region and a long α-helical tail Tails twist around in a coiled-coil globular heads bind actin myosin moves the head groups along the actin filament in the direction of the barbed end

Sliding Filament Model ATP hydrolysis is required Binding of ATP dissociates myosin from actin ATP hydrolysis induces a conformational change that displaces the myosin head group

Sliding Filament Model myosin head binds to a new position on the actin filament and Pi is released The “power stroke”: Myosin head returns to its original conformation, which drives actin filament sliding, and ADP is released

Muscle contraction and the role of Actin thin filaments

Actin and myosin in cell divison Cytokinesis—division of a cell following mitosis A contractile ring of actin and myosin II is assembled underneath the plasma membrane Contraction of the ring pinches the cell in two

Actin and myosin: vesicular transport Myosin I: much smaller than myosin II,contains a globular head group, acts as a molecular motor Short tails bind to other structures Movement of myosin I along actin filament transport cargo, such as a vesicle