Presentation on theme: "Fundamentals of Cell Biology"— Presentation transcript:
1Fundamentals of Cell Biology Chapter 5: The Cytoskeleton and Cellular Architecture
2Chapter Summary: The Big Picture (1) Chapter foci:Cytoskeletal proteins form a skeleton inside the cellIntermediate filaments provide the cell with mechanical strengthMicrotubules are associated with cellular traffickingActin is responsible for large-scale movementsEukaryotic cytoskeletal proteins evolved from early prokaryotes
3Chapter Summary: The Big Picture (2) Section topics:The cytoskeleton is represented by three functional classes of proteinsIntermediate filaments are the strongest, most stable elements of the cytoskeletonMicrotubules organize movement inside a cellActin filaments control the movement of cellsEukaryotic cytoskeletal proteins arose from prokaryotic ancestors
4The cytoskeleton is represented by three functional classes of proteins Key Concepts:The cytoskeleton is a complex mixture of 3 different types of proteins that are responsible for providing mechanical strength to cells and supporting movement of cellular contents.The most visible form of cytoskeletal proteins are long filaments found in the cytosol, but these proteins also form smaller shapes that are equally important for cellular function.The structural differences between the 3 protein types underscores their 4 different functions in cells.
5Cytoskeletonoccupies large portion of cytosol and appears to link organelles to each other and to plasma membrane3 elements: IFsMTsActinElements do not form mixed polymersFigure 05.01: The cytoskeleton forms an interconnected network of filaments in the cytosol of animal cells.
6IFs are the strongest, most stable elements of the cytoskeleton Key Concepts (1):Intermediate filaments are highly stable polymers that have great mechanical strength.Intermediate filament polymers are composed of tetramers of individual intermediate filament proteins.Several different genes encode intermediate filament proteins, and their expression is often cell- and tissue-specific.
7IFs are the strongest, most stable elements of the cytoskeleton Key Concepts (2):Intermediate filament assembly and disassembly are controlled by posttranslational modification of individual intermediate filament proteins.Specialized intermediate-filament-containing structures protect the nucleus, support strong adhesion by epithelial cells, and provide muscle cells with great mechanical strength.
8IFs provide mechanical strength to cells Figure 05.02: Two types of intermediate filaments.Figure 05.03: Intermediate filaments have the most mechanical strength of the cytoskeletal proteins.
10The primary building block of IFs is a filamentous subunit α-helices in the central rod domainFigure 05.04: The central rod domain of intermediate filament proteins forms an alpha helix. The head (and tail) regions form globular shapes.
11IF subunits form coiled-coil dimers Figure 05.05: A model for intermediate filament assembly. The coiled coil formed by the dimer formed the structural basis for the strength of intermediate filaments.
12Heterodimers overlap to form filamentous tetramers Coiled-coils align to form antiparallel staggered structures
13Assembly of a mature IF from tetramers occurs in 3 stages Figure 05.05: A model for intermediate filament assembly. The coiled coil formed by the dimer formed the structural basis for the strength of intermediate filaments.
14Posttranslational modifications control the shape of intermediate filaments Chemical modification of IF controls their shape and functionPhosphorylation-dephosphorylationGlycosylationFarnesylationTransglutamination of head and tail domains
15IFs form specialized structures Keratins in epitheliumCostameresFigure 05.06: Keratin expression patterns vary in different epithelial tissues.Figure 05.07: Costameres link the contractile apparatus of muscle cells to the plasma membrane and extracellular matrix.
16Microtubules (MT) organize movement inside a cell Key Concepts (1):MTs are hollow, tube-shaped polymers comprised of proteins called tubulins.MTs serve as “roads” or tracks that guide the intracellular movement of cellular contents.MT formation is initiated at specific sties in the cytosol called MT-organizing centers. The basic building block of a MT is a dimer of two different tubulin proteins.
17MTs organize movement inside a cell Key Concepts (2):MTs have structural polarity, which determines the direction of the molecular transport they support. This polarity is caused by the binding orientation of the proteins in the tubulin dimer.The stability of MTs is determined, at least in part, by the type of guanine nucleotides bound by the tubulin dimers within it.Dynamic instability is caused by the rapid growth and shrinkage of MTs at one end, which permits cells to rapidly reorganize their MTs.
18MTs organize movement inside a cell Key Concepts (3):MT-binding proteins play numerous roles in controlling the location, stability, and function of microtubules.Dyneins and kinesins are the motor proteins that use ATP energy to transport molecular “cargo” along MTs.Cilia and flagella are specialized MT-based structures responsible for motility in some cells.
19MT cytoskeleton is a network of "roads" for molecules "pass to and fro"
20MT assembly begins at a MT-organizing center (MTOC) Figure 05.08: The distribution of microtubules in a human epithelial cell. The microtubules are stained green and the DNA is stained red.
21Figure 05.09: The structure and location of the centrosome. The MTOC contains the gamma tubulin ring complex (γTuRC) that nucleates MT formationCentriolesPericentriolar materialgamma (γ ) tubulin Figure 05.09: The structure and location of the centrosome.
22The primary building block of MTs is an alpha-beta tubulin dimer α - and β -tubulin bind together to form stable dimerIf purified α-β tubulin dimers bound to GTP are concentrated enough (critical concentration), they spontaneously form MTs Figure 05.10: A three dimensional model of the dimer formed by α- and β-tubulin.Figure 05.11: In vitro assembly of microtubules is spontaneous and GTP-dependent. The graph represents the turbidity of a solution of α-β tubulin dimers over time.
23MTs are hollow "tubes" composed of 13 protofilaments Polymers of dimers sheet composed of 13 protofilaments folds into a tubeGTP binding and hydrolysis regulate MT polymerization and disassemblyFigure 05.12: A simple model of microtubule assembly.
24The growth and shrinkage of MTs is called dynamic instability Some microtubules rapidly grow and shrink in cells = dynamic instabilityElongation is at the+ end by GTP-bound dimersFigure 05.13: Growth and shrinkage of microtubules in a living cell. The microtubules have been tagged with a fluorescent molecule, and recorded by video over time.Figure 05.14: The growth of microtubules begins at the gamma tubulin ring and continues as long as the plus end contains GTP-bound tubulin dimers.
25Figure 05.15: Two fates of the plus ends of microtubules. Catastrophe?What happens when the supply of GTP-bound tubulin dimers runs out?1) MT depolymerizes at the + endOR2) Capping proteins prevent depolymerizationFigure 05.15: Two fates of the plus ends of microtubules.
26Some MTs exhibit treadmilling In cases where neither end of MT is stabilized, tubulin dimers are added to the + end and lost from the - endOverall length of these MTs remains fairly constant, but the dimers are always in fluxFigure 05.16: Treadmilling in microtubules.
27Benefits of dynamic instability Allows cells to haveflexibility with trafficking during cell movementability to exert force by bonding with cargo moleculesFigure 05.17: Microtubules exert enough force to move cargo by dynamic instability.Figure 05.18: Longitudinal and lateral bonds make microtubules strong.
28MT-associated proteins regulate the stability and function of MTs “MAPs” = capping proteins, rescue-associated proteins, and proteins that govern the motionmotor protein = special type of MAP that transports organelles/vesiclesDyneins and kinesins
29MotorsFigure 05.19: The structure of dynein and kinesis, the two most common motor proteins that bind to microtubules.Figure 05.20: How a microtubule motor protein moves along a microtubule.
30Cilia and Flagella Axoneme Sliding dynein = whip movement Figure 05.24: The coordinated motion of a cilium and a flagellum.Figure 05.23: The structure of an axoneme.
31Actin filaments control the movement of cells Key Concepts (1):Actin filaments are thin polymers of actin proteins.Actin filaments are responsible for large-scale changes in cell shape, including most cell movement.Actin filament polymerization is initiated at numerous sites in the cytosol by actin-nucleating proteins.Actin filaments have structural polarity, which determines the direction that force is exerted on them by myosin motor proteins.
32Actin filaments control the movement of cells Key Concepts (2):The stability of actin filaments is deteremined by the type of adenine nucleotides bound by the actin proteins within them.Actin-binding proteins play numerous roles in controlling the location, stability, and function of actin filaments.Cell migration is a complex process, requiring assembly and disassembly of different types of actin filament networks.
33The building block of actin filaments is the actin monomer Smallest diameter of cytoskeletal filaments – 7nm “microfilament”Great tensile strengthStructural polarity+ end – barbed end- end – pointed endOften bound to myosinFigure 05.25: The general structure of an actin filament. The lateral and longitudinal bonds holding actin monomers together are indicated at right.Figure 05.26: An electron micrograph of an actin filament partially coated with mysoin proteins.
34Actin found in wide variety of locations and configurations Figure 05.27: A number of different actin filament-based structures in cells.
35ATP binding/hydrolysis regulate actin filament polymerization and disassembly + ATP = polymerizationATPADP = depolymerizationFigure 05.28: The structure of an actin monomer. A ribbon model, derived from a crystalized form of the protein.
36Actin polymerization occurs in 3 stages Figure 05.29: The three stages of actin filament assembly in vitro.
37Actin filaments have structural polarity Actin filaments undergo treadmillingFigure 05.30: Treadmilling in actin filaments. Note the similarity of this treadmilling with that shown for microtubules in Figure 5-16.
386 classes of proteins bind to actin to control its polymerization/organization Monomer-binding proteins regulate actin polymerizationNucleating proteins regulate actin polymerizationFigure 05.31: The structure and function of profilin, an actin monomer-binding protein.Figure 05.32: ARP2/3 nucleates the formation of a new actin filament off the side of an existing filament.
396 classes of proteins bind to actin to control its polymerization/organization 3. Capping proteins affect the length and stability of actin filaments 4&5. Severing and depolymerizing proteins control actin filament disassembly 6. Cross-linking proteins organize actin filaments into bundles and networksFigure 05.34: Three forms of crosslinked actin filaments created by different crosslinking proteins.
40Cell MigrationActin-binding motor proteins exert force on actin filaments to induce cell movementCell migration is a complex, dynamic reorganization of an entire cellMigrating cells produce three characteristic forms of actin filaments: filopodia, lamellopodia, and contractile filaments
41FilopodiaFigure 05.35: Different forms of actin in stationary and migrating cells.
42Myosins are a family of actin-binding motor proteins myosins = multisubunit proteins organized into 3 structural domainsMotorRegulatoryTailFigure 05.36: Myosin proteins contain three funtional domains
43Figure 05.37: The contractile cycle of myosin. Myosins move towards one end of the actin filamentsmyosin V crawls towards the - end, all other myosins crawl towards the + endAllows for movement of cellFigure 05.37: The contractile cycle of myosin.
44Striated muscle contraction is a well-studied example of cell movement Figure 05.38: The anatomy of a skeletal muscle. The sarcomere contains actin and myosin arranged in parallel bundles.
45Eukkaryotic cytoskeletal proteins arose from prokaryotic ancestors Modern prokaryotic cells express a number of cytoskeletal proteins that are homologous to eukaryotic cytoskeletal proteins and behave similarlyVimentin (IF)FtsZ (MT)MreB and ParM (actin)Shared properties seem to include protection of DNA, compartmentalization and motility.