fibrous components and motors - sacramento state · fig. 6‐26 microvillus plasmamembrane...

B. The Cytoskeletal System and Adhesion

• Red = Actin Microfilaments

• Green = Tubulin Microtubules

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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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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Figure 16-12 Molecular Biology of the Cell (© Garland Science 2008)

ATP Binding

ADP Binding

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Figure 16-74c,d Molecular Biology of the Cell (© Garland Science 2008)

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Fig. 6‐26

Microvillus

Plasma membrane

Microfilaments (actin filaments)

Intermediate filaments

0.25 µm

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Figure 16-69a Molecular Biology of the Cell (© Garland Science 2008)

Actin tracks allow specific movement between two points

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Remember.....

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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

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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

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Figure 16-30c Molecular Biology of the Cell (© Garland Science 2008)

Large‐scale, long distance movement of cellular components, including organelles

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Fig. 6‐22Centrosome

Microtubule

Centrioles

0.25 µm

Longitudinal section of one centriole

Microtubules Cross sectionof 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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Directed transport along the neuronal axon

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Cilia andflagella

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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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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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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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Keratinsin the cytosol

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Lamins in the Nucleus

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Sarcomere Support

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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

Helical proteins give cell its shape by distributing themselvesthroughout the cell and directing sites of cell wall synthesis.

Mbl+ Mbl‐

Analogous to plant tubulin!

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Actin homolog: ParM

Analogousto tubulin!

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Tubulin homolog: FtsZ

As a bacterium divides: after the two chromosome copieshave been segregated, the Z‐ring forms and then contractsto split the daughter cells. Two rings reform in the daughters.

This is analogous to actin!

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Intermediate filament homolog: Crescentin

Creates the curved shape by forming a cortical band

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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)

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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

Molecular Motors are ATPases that Couple Hydrolysis with Movement

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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 dyneinsmove ciliaand flagella

2. Generally similar structure to the others – can have trimer heads

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The motorcargo complexgrabs onto

meshwork and transmembrane

proteins.

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“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

Nucleation is the rate‐limiting step in assembly and the first control

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To make a dense actin network, or gel, the cell must express and activate ARP complexes at that site

Activating factor can followinternal or external signals

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Formin facilitates nucleation and causes rapid extension

Expressed with ARP complexes, leads to very rapid assemblage and branching

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Thymosin and Profilin Regulate Speed

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Caps mean slow growth and stabilized fibers

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Lateral association of double helical fibers by bundling proteins

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Stabilization of gel structures

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Microvilliassemblage

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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

Severing Depolymerization

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Combine disassembly with localized assembly

Profilin promotes ATP exchange and ARP nucleation

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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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Actin polymerizationis required to extendpseudopods

Actin depolymerization isrequired to seal off thephagosome

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Regulation of the active actin structure is associated with known Rho‐family signaling pathways

stressfibers

microspikesor

filopodialamellipodia

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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

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A high concentration of free tubulinsubunits is required for assembly and elongation

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MAP2, Tau and Plus‐end tracking proteins (TIPs) bind and stabilize microtubules

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Green = MT

Red = +Tipprotein

Long‐termstable

structures

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Expression of stathmin protein will bind free tubulinsubunits and inhibit assembly and elongation

Each stathmin protein binds two dimers

Phosphorylation of stathmin will inactivate it

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Microtubule Severing

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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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The aminoterminal head and carboxyterminaltail appear to be key points of regulatory control

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Glycosylation and phosphorylation of intermediate filaments.

Hyder C L et al. J Cell Sci 2011;124:1363-1372

©2011 by The Company of Biologists Ltd

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Disassembly of nucleus in mitosis

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Nuclear Envelope Fragmentation

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CELLULARADHESION

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Hemidesmosomes bind intermediate filaments to anchor proteins to integrins to ECM fibrous proteins

Very stable

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Figure 19‐45 Molecular Biology of the Cell (© Garland Science 2008)

Focal adhesions bind microfilaments to anchor proteins to integrins to ECM fibrous proteins

Can be stable or used for migration

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Internal binding and external binding are linked

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Figure 19‐17a Molecular Biology of the Cell (© Garland Science 2008)

Very stable

Desmosomes bind intermediate filaments to anchor proteins to their own cadherinswhich bind to the adjacent cell’s cadherins

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Adherens junctions bind microfilaments to anchor proteins to their own cadherinswhich bind to the adjacent cell’s cadherins

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A fairly large protein family, all have homophillicbinding and are Ca++‐dependent

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Tight Junctions can block molecules assmall as ions from passing between cells

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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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NCAM is foundon a variety ofcell types and mediates homo‐philic binding.

Ig superfamily cell adhesion molecules

ICAM is foundon endothelialcells and bindsheterophilicallyto integrins onwhite blood cells.

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This is a large gene family

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fibrous components and motors - sacramento state · fig. 6‐26 microvillus plasmamembrane...

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