B. The Cytoskeletal System and Adhesion ______
• Red = Actin Microfilaments ______• Green = Tubulin Microtubules ______
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______Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ______
______3 Fibrous Components and Motors ______• Thin Filaments: Actin family of proteins ______• Microtubules: Tubulin family of proteins ______• Intermediate Filaments: Very cell‐specific family of proteins ______• The first two have molecular motors ______
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______1. Actin Microfilaments ______• Chains of few to many 42kDa monomers • Found in all eukaryotic cells (except nematode worm sperm) ______• Highly Conserved: Genes vary <20% from algae to humans; human primary protein sequence <5% ______• Three main isoforms: alpha, beta, gamma • Alpha is muscle actin, beta and gamma are found ______in nearly all cells ______
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ATP Binding ADP Binding ______Figure 16-12 Molecular Biology of the Cell (© Garland Science 2008)
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______Figure 16-74c,d Molecular Biology of the Cell (© Garland Science 2008)
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______Figure 16-47 Molecular Biology of the Cell (© Garland Science 2008)
______Fig. 6‐26 ______Microvillus
______Plasma membrane
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Microfilaments (actin filaments) ______
______Intermediate filaments
0.25 µm ______
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______Actin tracks allow specific movement between two points ______
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______Figure 16-69a Molecular Biology of the Cell (© Garland Science 2008) ______
______Remember..... ______
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______Figure 17-49a Molecular Biology of the Cell (© Garland Science 2008) ______The Key Structural Elements ______1. 70 Å filaments (f‐actin) ‐ polymers of globular monomers (g‐actin) formed as double helix ______
2. Structural polarity –sequences and structures ______unequally directed towards each of the ends ______
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“Dynamic instability” ______
a. Can form, crosslink, breakdown and reform rapidly
b. The cell can build new structures as its needs change ______
______Figure 16-7 Molecular Biology of the Cell (© Garland Science 2008) ______
______2. Tubulin Microtubules ______• Large hollow tubes of many 110kDa dimers • Three main isoforms: alpha, beta, gamma ______• Found in all eukaryotic cells • Alpha and beta found in microtubules, gamma ______found in microtubule organizing center (MTOC) • Highly Conserved: Primary sequences vary ______<10% from algae to humans ______
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GTP Binding GDP Binding ______Figure 16-11 Molecular Biology of the Cell (© Garland Science 2008) ______
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Large‐scale, long distance ______movement of cellular components, ______including organelles ______
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______Figure 16-30c Molecular Biology of the Cell (© Garland Science 2008) ______
Fig. 6‐22 Centrosome ______
Microtubule ______
Centrioles 0.25 µm ______
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Longitudinal section of Microtubules Cross section one centriole of the other centriole ______
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______Figure 16-30b Molecular Biology of the Cell (© Garland Science 2008) ______
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Projections of ______asters to move the chromosomes ______in cell division
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______Figure 16-85c Molecular Biology of the Cell (© Garland Science 2008) ______
______Directed transport along the neuronal axon ______
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______Figure 16-104 Molecular Biology of the Cell (© Garland Science 2008) ______
______Cilia and flagella ______
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Give direction ______to plant cell division and growth and ______dictate sites of plant wall synthesis ______
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______Plant microtubules have no obvious MTOC ______
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______The Key Structural Elements ______1. These are also polar – unequally directed towards each end
a. most: (–) end toward nucleus and (+) end to cell membrane ______2. 250 Å filaments formed as a hollow bundle of 13 protofilaments a. protofilaments are long polymers of tubulin dimers ______b. dimers are one alpha and one beta globular monomers c. gamma tubulin makes up the MTOC: centriole or basal body ______
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______Like actin, microtubules can be changed as the cells needs change
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______Figure 16-16c Molecular Biology of the Cell (© Garland Science 2008) ______3. Intermediate Filaments ______• Rope‐like filament structures formed from 8 protofilaments twisted together ______• Lamins found in nucleus, cytosolic IF’s found in some cells and not others ______• IF’s found only in some animals, including vertebrates, nematodes, molluscs ______• Plants and single‐celled organisms have no IF’s ______
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______Figure 16-19 Molecular Biology of the Cell (© Garland Science 2008) ______
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Keratins in the ______cytosol ______
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______Figure 16-20 Molecular Biology of the Cell (© Garland Science 2008) ______
Lamins in the Nucleus ______
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______Sarcomere Support ______
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______The Key Structural Elements ______1. Apolar – very stable, not structurally nor functionally ______oriented towards one end or the other ______2. No motors ______
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______4. Bacterial Homologs and Analogs ______• Homologs are genes and proteins that share common sequence ______
• Analogs are proteins that share common function ______
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______Actin homologs: MreB and Mbl ______
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Mbl+ Mbl‐
Helical proteins give cell its shape by distributing themselves ______throughout the cell and directing sites of cell wall synthesis. Analogous to plant tubulin! ______Figure 16-26 Molecular Biology of the Cell (© Garland Science 2008) ______
______Actin homolog: ParM ______
______Analogous to tubulin! ______
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______Figure 16-27a Molecular Biology of the Cell (© Garland Science 2008) ______Tubulin homolog: FtsZ ______
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______As a bacterium divides: after the two chromosome copies have been segregated, the Z‐ring forms and then contracts to split the daughter cells. Two rings reform in the daughters. ______
This is analogous to actin! ______Figure 16-25a Molecular Biology of the Cell (© Garland Science 2008)
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______Intermediate filament homolog: Crescentin ______
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Creates the curved shape by forming a cortical band ______Figure 16-28 Molecular Biology of the Cell (© Garland Science 2008)
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______b. Molecular motors allow filament‐ based intracellular transport ______The three basic types
1. Myosins: move along microfilaments towards the ______positive end (mostly), Some myosin gene products move toward negative end ______2. Kinesins: move along microtubules towards the positive end (away from the MTOC)
3. Dyneins: move along microtubules towards the negative ______end (toward the MTOC) ______
______Molecular Motors are ATPases that Couple Hydrolysis with Movement ______
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Globular heads on fibrous tails: ______a. Heads have ATPase activity, dictate filament track, direction and speed b. Tails have “cargo” binding and myosin thick filament organizing activity ______Figure 16-54a Molecular Biology of the Cell (© Garland Science 2008)
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Kinesins and Myosins are Evolutionarily Related ______1. Both have single head (type I ) and double head (type II) designs 2. Little sequence similarity but remarkable similarities in tertiary structures ______
Dyneins are Something Else ______1. They are an ancient family of proteins found in all eukaryotes
a. Cytosolic dyneins move most structures, ciliar dyneins move cilia and flagella ______2. Generally similar structure to the others –can have trimer heads ______
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______The motor cargo complex grabs onto ______meshwork and transmembrane proteins. ______
______Figure 16-67 Molecular Biology of the Cell (© Garland Science 2008)
______“Power Stroke” mechanism to couple movement to ATP metabolism ______1. At the end of one cycle the motor is bound to actin at 45o
2. ATP binding Filament Release ______
3. ATP Hydrolysis Conformational Relaxation to 90o ______4. Release of Pi Filament Binding 5. Release of ADP Conformational Stroke to 45o ______6. The shift in the motor back to 45o moves the cargo forward ______
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______2. Regulation of Cell Function via Control of the Cytoskeleton ______
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______a. Actin filaments are assembled and disassembled rapidly and thus are able to help regulate long‐term activities, change and movement ______
Assembly of specific actin structures and cellular functions is dependent on the expression and activity of microfilament ______associated proteins (MFAP’s) in specific cellular locations ______
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______Actin fiber assembly is spontaneous if only actin and ATP are present and the bonds between subunits are non‐covalent ______
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______Nucleation is the rate‐limiting step in assembly and the first control ______Figure 16-10 Molecular Biology of the Cell (© Garland Science 2008) ______
______To make a dense actin network, or gel, the cell must express ______and activate ARP complexes at that site ______
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Activating factor can follow internal or external signals ______Figure 16-34c Molecular Biology of the Cell (© Garland Science 2008) ______
______Formin facilitates nucleation and causes rapid extension ______
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______Expressed with ARP complexes, leads to very rapid assemblage and branching ______Figure 16-36 Molecular Biology of the Cell (© Garland Science 2008) ______Thymosin and Profilin Regulate Speed ______
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______Caps mean slow growth and stabilized fibers ______
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______Figure 16-43 Molecular Biology of the Cell (© Garland Science 2008)
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______Lateral association of double helical fibers by bundling proteins ______
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______Figure 16-49a Molecular Biology of the Cell (© Garland Science 2008)
______Stabilization of gel structures ______
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______Figure 16-51 Molecular Biology of the Cell (© Garland Science 2008)
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______Microvilli assemblage ______
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______Figure 16-50a Molecular Biology of the Cell (© Garland Science 2008)
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Many changing cell functions are regulated by ______actin fiber reorganization, which requires coordinated disassembly of existing fibers ______followed by nucleation and polymerization at a new location ______
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______Rapid disassembly of fibers ______
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______Severing Depolymerization ______
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______Combine disassembly with localized assembly ______
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Profilin promotes ATP exchange and ARP nucleation ______
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______Figure 16-90 Molecular Biology of the Cell (© Garland Science 2008)
______Lamellipodia ______
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______Assembly of the contractile ring followed by severing is required in cytokinesis ______
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______Pseudopod‐Driven Phagocytosis in Neutrophils ______
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______Figure 13-46 Molecular Biology of the Cell (© Garland Science 2008)
______Actin polymerization is required to extend pseudopods ______
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Actin depolymerization is required to seal off the ______phagosome ______Figure 13-47a Molecular Biology of the Cell (© Garland Science 2008)
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Regulation of the active actin structure is associated ______with known Rho‐family signaling pathways ______
stress fibers ______
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microspikes lamellipodia or ______filopodia
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______Figure 16-98b Molecular Biology of the Cell (© Garland Science 2008)
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______Figure 16-98a Molecular Biology of the Cell (© Garland Science 2008)
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______b. Microtubules are also assembled and disassembled rapidly and are used to ______regulate movement and positioning of materials, including the cell ______
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______γ‐Tubulin ring complex (γ‐TuRC) nucleates assembly from the ______MTOC and remains associated with the minus end ______Figure 16-30a Molecular Biology of the Cell (© Garland Science 2008)
______A high concentration of free tubulin subunits is required ______for assembly and elongation ______
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______Figure 16-30b Molecular Biology of the Cell (© Garland Science 2008) ______
______MAP2, Tau and Plus‐end tracking proteins (TIPs) bind and stabilize microtubules ______
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______Figure 16-41 Molecular Biology of the Cell (© Garland Science 2008) ______
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Green = MT ______
Red = +Tip protein ______
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Long‐term stable ______structures ______Figure 16-45a Molecular Biology of the Cell (© Garland Science 2008) ______Expression of stathmin protein will bind free tubulin subunits and inhibit assembly and elongation ______
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Each stathmin protein binds two dimers ______
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Phosphorylation of stathmin will inactivate it ______
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______Microtubule Severing ______
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______Figure 16-44 Molecular Biology of the Cell (© Garland Science 2008) ______Disassembly and Reassembly in Mitosis ______
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c. Intermediate filaments are the most stable element in the ______cytoskeleton but their structure can be regulated to facilitate specific cell activities ______
Remember.... ______
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______Figure 16-19 Molecular Biology of the Cell (© Garland Science 2008) ______
______The aminoterminal head and carboxyterminal tail appear to be key points of regulatory control ______
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______Glycosylation and phosphorylation of intermediate filaments.
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Hyder C L et al. J Cell Sci 2011;124:1363-1372 ______©2011 by The Company of Biologists Ltd ______
______Disassembly of nucleus in mitosis ______
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______Nuclear Envelope Fragmentation ______
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______CELLULAR ______ADHESION ______
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Hemidesmosomes bind ______intermediate filaments to anchor proteins to integrins to ECM fibrous ______proteins Very stable ______Figure 19-46 Molecular Biology of the Cell (© Garland Science 2008) ______
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______Focal adhesions bind microfilaments to anchor proteins ______to integrins to ECM fibrous proteins ______
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Can be stable or used for migration ______Figure 19‐45 Molecular Biology of the Cell (© Garland Science 2008) ______Internal binding and external binding are linked ______
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______Figure 19-48b Molecular Biology of the Cell (© Garland Science 2008) ______
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Desmosomes bind ______intermediate filaments to anchor proteins to their ______own cadherins which bind to the adjacent cell’s ______cadherins Very stable ______Figure 19‐17a Molecular Biology of the Cell (© Garland Science 2008) ______
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______Adherens junctions bind microfilaments ______to anchor proteins to their own cadherins which bind to the ______adjacent cell’s cadherins ______
______A fairly large protein family, all have homophillic ______binding and are Ca++‐dependent ______
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______Figure 19‐7 Molecular Biology of the Cell (© Garland Science 2008) ______
______Tight Junctions can block molecules as small as ions from passing between cells ______
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______Figure 19-24 Molecular Biology of the Cell (© Garland Science 2008) ______
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______Figure 19-26a Molecular Biology of the Cell (© Garland Science 2008) ______
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______When there is an infection or inflammation, endothelial cells will express selectins on their surface that specifically bind WBC. ______
WBC also express integrins that bind strongly, stop their rolling and aid in escape ______
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______Ig superfamily cell adhesion molecules
______NCAM is found on a variety of cell types and ______mediates homo‐ philic binding. ICAM is found on endothelial ______cells and binds heterophilically to integrins on ______white blood cells. ______Figure 19‐20 Molecular Biology of the Cell (© Garland Science 2008) ______
______This is a large gene family ______
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