Lecture 7: Cytoskeleton PDF

Lecture 7. Cytoskeleton  7. Cytoskeleton 1. Cytoskeleton. a. Function and Structure. b. Microtubules. Centrosome. Cilia. Flagella. c. Intermediate filaments. Nuclear Lamina. d. Actin filaments. Microvilli. Cell Crawling. Contractile...

Lecture 7. Cytoskeleton  7. Cytoskeleton 1. Cytoskeleton. a. Function and Structure. b. Microtubules. Centrosome. Cilia. Flagella. c. Intermediate filaments. Nuclear Lamina. d. Actin filaments. Microvilli. Cell Crawling. Contractile ring. Cell cortex. Muscle contraction. 7. Cytoskeleton o Network of protein filaments extending throughout the cytoplasm of all eukaryotic cells. Structural role  7. Cytoskeleton Function o Cell shape o Organization of the cytoplasm o Cell movements and cell motility o Organelle transport o Cell division (mitotic chromosomes) o Cytokinesis o Muscle contraction  7. Cytoskeleton Structure o Dynamic structure, continually reorganized as cells move and change shape (ex. cell division). o Composition (in order to their increasing diameter): o Actin filaments o Intermediate filaments o Microtubules  7. Cytoskeleton Actin filaments  7. Cytoskeleton Structure o Protein filaments are held together and linked to subcellular organelles and the plasma membrane by a variety of accessory proteins. o The structure and organization of each component of the cytoskeleton is different, as well as their roles in cell motility, organelle transport, cell division, and other types of cell movements.  7. Cytoskeleton Microtubules o Help determine cell shape o Intracellular transport of organelles (such as secretory vesicles) o Separation of chromosomes during cell division o Cell locomotion (cilia and flagella)  7. Cytoskeleton Microtubules o Composed mainly of the globular protein tubulin. o Tubulin is a dimer consisting of two closely related polypeptides, α-tubulin and β- tubulin. o A head-to-tail arrays of tubulin dimers is called protofilaments.  7. Cytoskeleton Microtubules o Microtubules consist of 13 linear protofilaments assembled around a hollow core. Protofilament  7. Cytoskeleton Microtubules o Microtubules are polar structures with + β-tubulina two distinct ends: a fast-growing plus α-tubulina end and a slow-growing minus end. -  7. Cytoskeleton Microtubules Centrosome: The major microtubule- organizing center  7. Cytoskeleton Microtubules In interphase cells, the centrosome is located near the nucleus and microtubules extend outward to the cell periphery. During mitosis, duplicated centrosomes separate and microtubules reorganize to form the mitotic spindle.  7. Cytoskeleton Microtubules o Colcemid is a commonly used experimental drug that binds tubulin and inhibits microtubule polymerization. o Another useful drug, taxol, stabilizes microtubules rather than inhibiting their assembly.  7. Cytoskeleton Microtubules o Treatment with colcemid to disassemble microtubules: When the drug is removed, the cells recover and new microtubules can be seen growing outward from the centrosome.  7. Cytoskeleton Microtubules o Microtubules show dynamic instability (treadmilling) o Tubulin dimers can depolymerize and polymerize. o Continual and rapid turnover of microtubules. o Growth of microtubules continues as long as there is a high concentration of tubulin bound to GTP.  7. Cytoskeleton Microtubules. Dynamic Instability.  7. Cytoskeleton Microtubules o Responsible of a variety of cell movements, including the intracellular transport and positioning of membrane vesicles and organelles. o The motor proteins: kinesins and dyneins are responsible for powering the variety of movements in which microtubules participate.  7. Cytoskeleton Microtubules o Kinesins move cargo toward the (+) end of microtubules (anterograde transport) o Dyneins transport cargo toward the (−) end (retrograde transport).  7. Cytoskeleton Microtubules o Cilia beat in a coordinated back-and-forth motion, which either moves the cell through fluid or moves fluid over the surface of the cell. Ciliated cells lining the respiratory tract, which clear mucus and dust from the respiratory passages.  7. Cytoskeleton Microtubules o Flagella differ from cilia: o In their length o In their wavelike pattern of beating o Only one or two flagella per cell. o Locomotion of protozoans (unicellular eukaryotic organisms) and sperm. o Cilia and flagella are very similar structures. Sperm  7. Cytoskeleton Microtubules  7. Cytoskeleton Intermediate filaments o Elaborate network in the cytoplasm of most cells, extending from a ring surrounding the nucleus to the plasma membrane. o Found in parts of cells subjected to mechanical stress; help to stabilize the position of organelles and help to attach cells to one another.  7. Cytoskeleton Intermediate filaments Diameter thinner than microtubules but thicker than actin filaments.  7. Cytoskeleton Intermediate filaments o Formation of dimers in a coiled-coil structure. o The dimers then associate in a antiparallel fashion to form tetramers. o The final intermediate filament contains approximately eight protofilaments wound around each other in a ropelike structure.  7. Cytoskeleton Intermediate filaments  7. Cytoskeleton Intermediate filaments o Do not exhibit the dynamic behavior. o Frequently modified by phosphorylation, which can regulate their assembly and disassembly within the cell. Example: The phosphorylation of the nuclear lamins, which results in disassembly of the nuclear lamina and breakdown of the nuclear envelope during mitosis.  7. Cytoskeleton Intermediate filaments. Nuclear lamina o The intermediate filaments underlying the nuclear membrane. o The nuclear lamina is composed of fibrous proteins, lamins, which associate with each other to form filaments. o Provides mechanical support to the nuclear envelope.  7. Cytoskeleton Intermediate filaments. Nuclear lamina o Disassembly of the nuclear lamina results from phosphorylation of the lamins, which causes the filaments to break down into individual lamin dimers.  7. Cytoskeleton Intermediate filaments. o More than 50 different intermediate filament proteins have been identified and classified into six groups based on similarities between their amino acid sequences.  7. Cytoskeleton Actin filaments o Is the major cytoskeletal protein of most cells. o Actin filaments are particularly abundant beneath the plasma membrane, where they form a network.  7. Cytoskeleton Actin filaments o Functions: o Provides mechanical support o Determines cell shape o Allows movement of the cell surface o Enables cells to migrate o Engulfs particles o Involved in cell division o Implicated in muscle contraction  7. Cytoskeleton Actin filaments o Individual actin molecules are globular proteins. o Each actin monomer has tight binding sites that mediate head-to-tail interactions with two other actin monomers. o Actin monomers polymerize to form filaments.  7. Cytoskeleton Actin filaments o All the actin monomers are oriented in the same direction. o Actin filaments have a distinct polarity and their ends (called the plus and minus ends) are distinguishable from one another. o This polarity of actin filaments is important both in their assembly and in establishing a unique direction of movement.  7. Cytoskeleton Actin filaments o The plus end elongates five to ten times faster than the minus end. o Actin polymerization is reversible, filaments can depolymerize by the dissociation of actin subunits, allowing actin filaments to be broken down when necessary. Dynamic behavior of actin filaments (treadmilling)  7. Cytoskeleton Actin filaments o Individual actin filaments are assembled into two general types of structures, governed by a variety of actin-binding proteins that crosslink actin filaments in distinct patterns: Bundles  The actin filaments are crosslinked into closely packed parallel arrays. Networks The actin filaments are loosely crosslinked in orthogonal arrays that form three-dimensional structures with the properties of semisolid gels.  7. Cytoskeleton Actin filaments  7. Cytoskeleton Actin filaments Cellular structures (bundled architecture): Microvilli Stress fibers Filopodium Contractile ring  7. Cytoskeleton Actin filaments. Microvilli. o A bundle containing closely spaced actin filaments aligned in parallel, supports projections of the plasma membrane. o Numerous microvilli are present on the absorptive surface of intestinal epithelial cells, increasing the surface area for transport of nutrients.  7. Cytoskeleton Actin filaments. Contractile ring. o Contractile ring is composed of filaments that are more loosely spaced and are capable of contraction, and divides cells in two following mitosis.  7. Cytoskeleton Actin filaments. Cell cortex. o A three-dimensional network of actin filaments and associated actin-binding proteins beneath the plasma membrane. o It determines cell shape and is involved in a variety of cell surface activities, including movement. Cross-linking of actin filaments Unactivated blood platelet Actin filament growth and myosin contraction Activated platelet spreads out and contracts  7. Cytoskeleton Actin filaments. Cell cortex. o The actin filaments in networks are held together by large actin- binding proteins, such as filamin. o As a result, filamin forms cross-links between orthogonal actin filaments, creating a loose three-dimensional network.  7. Cytoskeleton Actin filaments. Myosin. o Actin filaments, usually in association with myosin, are responsible for many types of cell movements. o Different types: Myosin I (vesicular transport) and myosin II (muscle contraction), the most abundant and thoroughly studied of the myosin proteins, are present in nearly all eukaryotic cells.  7. Cytoskeleton Actin filaments. Myosin. o Myosin is a motor protein  it converts chemical energy in the form of ATP to mechanical energy, thus generating force and movement. Allowing: muscle contraction, movements of non muscle cells, cell division.  7. Cytoskeleton Actin filaments. Cell crawling. o It is a basic form of cell locomotion, employed by a wide variety of different kinds of cells. o Amoebas o Migration of embryonic cells during development o Invasion of tissues by white blood cells to fight infection o The spread of cancer cells during the metastasis of malignant tumors  7. Cytoskeleton Actin filaments. Cell crawling. Involves a coordinated cycle of movements, which can be viewed in three stages: 1) Protrusions: pseudopodia or lamellipodia must be extended from the leading edge of the cell. 2) Attachment: these extensions must attach to the substratum across which the cell is migrating. 3) Retraction: the trailing edge of the cell must dissociate from the substratum and retract into the cell body.  7. Cytoskeleton Actin filaments. Cell crawling. 1) Extension of the leading edge involves the polymerization and crosslinking of actin filaments. 2) Attachment involves the formation of focal adhesions. 3) Role for myosin II in contracting the actin cortex and generating the force required for retraction of the trailing edge.  7. Cytoskeleton Cell crawling.  7. Cytoskeleton Actin filaments. Muscle contraction. Types of muscle cells: o Skeletal muscle  which is responsible for all voluntary movements. o Cardiac muscle  which pumps blood from the heart.  7. Cytoskeleton Actin filaments. Muscle contraction. o Smooth muscle  which is responsible for involuntary movements of organs such as the stomach, intestine, uterus, and blood vessels. o Myoepithelial cells  they form the dilator muscle of the eye’s iris and serve to expel saliva, sweat and milk from the corresponding glands.  7. Cytoskeleton Actin filaments. Muscle contraction. o Skeletal muscles are bundles of muscle fibers, which are single large cells formed by the fusion of many individual cells during development. o Most of the cytoplasm consists of myofibrils, the contractile organelles of skeletal muscle.  7. Cytoskeleton Actin filaments. Muscle contraction. Muscle Fiber  7. Cytoskeleton Actin filaments. Muscle contraction. o Myofibrils are cylindrical bundles of two types of filaments: thick filaments of myosin and thin filaments of actin. o Each myofibril is organized as a chain of contractile units called sarcomeres, which are responsible for the striated appearance of skeletal and cardiac muscle. 7. Cytoskeleton  7. Cytoskeleton Actin filaments. Muscle contraction.  7. Cytoskeleton Actin filaments. Muscle contraction. During muscle contraction, each sarcomere shortens, bringing the Z discs closer together.  7. Cytoskeleton Actin filaments. Muscle contraction. o The actin filaments slide past the myosin filaments toward the middle of the sarcomere. The result is shortening of the sarcomere without any change in filament length. o Myosin is the motor that drives filament sliding. o The relative orientation of myosin and actin filaments is the same on both halves of the sarcomere.  7. Cytoskeleton Actin filaments. Muscle contraction. 7. Cytoskeleton 7. Cytoskeleton  7. Cytoskeleton Actin filaments. Muscle contraction. o The contraction of skeletal muscle is triggered by nerve impulses. o The sarcoplasmic reticulum is a specialized network of internal membranes that stores high concentrations of Ca2+ ions. o The release of Ca2+  muscle contraction 7. Cytoskeleton o When a nerve impulse reaches a skeletal muscle cell, it causes a change in the electric potential across the plasma membrane (depolarization). o The invaginations of the plasma membrane, called T (transverse) tubules, terminate next to the SR. This system brings the membrane depolarization signal into the cytosol, where it stimulates the SR to release stored calcium into the cytosol through Ca2+ channels in the SR membrane. 7. Cytoskeleton Depolarization of a muscle cell (step 1) induces the release of Ca2+ ions stored in the SR via Ca2+ release proteins in the SR membrane (step 2). Subsequently, Ca2+ATPases in the SR membrane pump Ca2+ ions from the cytosol back into the SR, restoring the cytosolic Ca2+ concentration to its resting level within about 30 milliseconds (step 3).  7. Cytoskeleton Actin filaments. Muscle contraction. Regulatory proteins of muscle contraction: o Tropomyosin, a fibrous protein that binds to actin filaments. o Troponin, bounds to tropomyosin and Ca2+.  7. Cytoskeleton Actin filaments. Muscle contraction. o Low Ca2+  troponin/tropomyosin complex blocks the interaction of actin and myosin, so the muscle does not contract. o High Ca2+  calcium binding to troponin shifts the position of the complex, relieving the inhibition and allowing contraction to proceed.  7. Cytoskeleton Actin filaments. Muscle contraction.  7. Cytoskeleton Muscle contraction.