BIOL 3510 1st Edition Lecture 22 Outline of Last Lecture I. Three Main Cytoskeletal FilamentsII. Roles of Microtubules in a CellIII. Microtubule Growth and DisassemblyIV. Motor ProteinsOutline of Current Lecture I. Roles of Actin NetworksII. Microfilament Growth and DisassemblyIII. Actin in ActionIV. Resting and Contracting Muscle Cells Current LectureRoles of Actin Networks(A) Stable: microvilli(B) Stable: contractile bundles(C) Temporary: protrusions needed for cell movement(D) Temporary: cytokinetic contractile ringMicrofilaments (actin filaments) are composed of globular actin proteins aligned into two twisted protofilaments. Like MTs, microfilaments have a distinct polarity. Actin is more likely to be added to the plus end.Microfilament (MF) Growth and Disassembly- Actin is an ATPase- Actin-ATP is added to both ends, faster growth at plus end- After incorporation to a MF, ATP is hydrolyzed to ADP causing a conformation change that destabilizes the MFTreadmilling and dynamic instability are dependent on nucleotide hydrolysis.Some microfilaments are nucleated from actin nucleating proteins near the plasma membrane.Microfilament behavior is modified by actin-binding proteins. Actin in action: the three steps of cell crawling are actin dependent.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.#1 Actin polymerization at the leading edge of the cell results in the protrusion of lemellipodia and filopodia. Branched actin networks in lemellipodia are nucleated by ARP complexes.Growth of unbranched microfilaments in filopodia (and other structures) is promoted by formins. Formins promote the addition of actin monomers.#2 Integrins bind to the extracellular proteins and internal microfilaments.Integrins: transmembrane proteins that interact with the crawling cell’s environment, connected to actin filaments.#3 Internal contraction of the cell is mediated by myosin II, an actin-dependent motor protein.All actin-dependant motor proteins belong to the myosin family (plus end directed)- Two main types, myosin I and myosin II- Bind and hydrolyze ATPDifferent myosin I proteins bind to different cell componentsActin in action: How does a cell know what type of microfilament network to form?Cell signals converge on a family of Rho GTP binding proteins (GTPases)-Rho – promotes contractile bundles-Rac – promotes lamellipodia-Cdc42 – promotes filapodia Actin in action: Mucle Cell ContractionMyosin II filaments are critical for muscle cell contraction. Movement of myosin II along opposite oriented actin filaments creates a contractile force.Muscle cells contain myofibrils which are chains of repeating sarcomere units. Muscle contraction occurs when the myosin and actin filaments slide past each other.Transverse tubules convey the excitatory signal (action potential) from an activating neuron to the sarcoplasmic reticulum.In response to the action potential, Ca2+ is released from the sarcoplasmic reticulum.Resting muscle cell: tropomyosin covers MFs and prevents myosin from bindingContracting muscle cell: Ca2+ released from the sarcoplasmic reticulum bind to the troponin complex changing its conformation. This alters the tropomyosin – actin interaction allowing myosin to bind and initiating a
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