bs101 lecture 08 ytoskeleton 2012-13

1

MOLECULAR CELL BIOLOGY BS101

12-13

Lecture 8

The cytoskeleton and cytosol, microfilaments and microtubules

LEARNING OBJECTIVES Lecture 8

• List and describe the molecules that compose the cellular cytoskeleton.

• Describe the main functions of the cytoskeleton.

Original views on cell structure

• In the early days of light microscopy, scientists thought that organelles floated freely in the cytoplasm of a cell

• Eventually, improvements in both light and electron microscopy revealed that the cell contained a CYTOSKELETON

5

Cytoskeleton

Figure 6.20 Reece et al (2011)

Green= microtubules

Red= actin filaments

Blue= DNA in nucleus

•This shows that the cytoskeleton extends throughout the cell

•It acts like a dome tent, stabilised by opposing forces

•But it also very dynamic, and can be dismantled and reassembled quickly, to change the shape of the cell

6

The cytoskeleton is a network of fibres that organizes structures and activities in

the cell• The cytoskeleton is a network of fibres extending

throughout the cytoplasm, providing mechanical support and to maintain the cell shape

• It organizes the cell’s structures and activities, anchoring many organelles

• It is composed of three types of molecular structures:– Microtubules– Microfilaments– Intermediate filaments

7

Roles of the Cytoskeleton: Support, Motility, and Regulation

• The cytoskeleton helps to support the cell and maintain its shape

• It interacts with motor proteins to produce motility

• Inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton

VesicleATP

Receptor for motor protein

Microtubuleof cytoskeleton

Motor protein (ATP powered)

(a)

Microtubule Vesicles

(b)

0.25 µm

Reece et al (2011) Fig. 6-21

•This shows how vesicles can be carried around the cell on ‘monorails’ of microtubules

•Here, motor proteins can attach themselves to a vesicle, then also to a microtubule, and using ATP, this motor protein can ‘walk’ the vesicle along the rail, to its destination with the cell

•In this figure of a squid giant axon, two vesicles containing neurotransmitters migrate toward the tip of the axon, by the same mechanism as above

Kinesin walking along a microtubule

http://www.youtube.com/watch?v=YAva4g3Pk6k

Movement of vesicles within the cell

• Vesicles of proteins that leave the ER and move to the Golgi (endomembrane system lecture 6) are transported along cytoskeletal tracks

• The cytoskeleton also manipulates the plasma membrane, making it bend inward to form food vacuoles or phagocytic vesicles

11

Components of the Cytoskeleton

• Three main types of fibres make up the cytoskeleton:– Microtubules are the thickest of the three

components of the cytoskeleton– Microfilaments, also called actin filaments, are the

thinnest components– Intermediate filaments are fibres with diameters in

a middle range

12

Campbell and Reece (2011)

Table 6.1

Structure and function of the cytoskeleton

•Microfilaments

•Intermediate filaments

•Microtubules

10 µm 10 µm 10 µm

Column of tubulin dimers

Tubulin dimer

Actin subunit

25 nm

7 nm

Keratin proteins Fibrous subunit (keratins coiled together)

8–12 nm

13

10 µm

Column of tubulin dimers

Tubulin dimer

25 nm

Campbell and Reece (2011) Table 6-1a

Dimer of and tubulin

e.g. movement of secretory vesicles from Golgi to membrane surface

e.g.Flagella/cilia

e.g. mitotic spindle

14Figure 17-8 Essential Cell Biology (© Garland Science 2010)

Microtubule functions•25nm diameter

•Organising role

•Grow out from a centrosome

•Provide a system of tracks, along which other organelles and vesicles are moved

•Also responsible for anchoring organelles within the cell

•Form the mitotic spindle during cell division

•Also present in cilia and flagella for movement (e.g. sperm)

Centrosomes and Centrioles• In many cells, microtubules grow out from a

centrosome near the nucleus• The centrosome is a “microtubule-organizing

centre”• In animal cells, the centrosome has a pair of

centrioles, each with nine triplets of microtubules arranged in a ring


Centrosome

Microtubule

Centrioles0.25 µm

Longitudinal section of one centriole

Microtubules Cross sectionof the other centriole

•Most animal cells have a centrosome- a region near the nucleus where the cells’ microtubules are initiated

•Within the centrosome is a pair of centrioles: each 0.25um in diameter

•The two centrioles are at right angles to each other, and each is made up of 9 sets of three microtubules

17

Campbell and Reece (2011)

Figure 12.8

The mitotic spindle at metaphase

-condensed chromatin

-mitotic spindle is made of tubulin (microtubules)

-mitotic spindle radiates out from the centrosome

-chromosomes bind to spindle via kinetochores

- the kinetochore is bound to centromeric DNA

• Microtubules control the beating of cilia and flagella, locomotor appendages of some cells

• Cilia and flagella differ in their beating patterns• Many unicellular organisms are propelled

through water by cilia or flagella

Cilia and Flagella

06_23aChlamydomonas_SV.mpgChlamydomonas (green unicellular algae) showing moving flagella

Cilia and flagella• Cilia can be found on the surface of a cell, and

can move liquids along• E.g. ciliated lining of trachea can move mucus

and dust particles out of lungs• Cilia in oviducts of women’s reproductive tract

can move egg towards uterus• Cells can have many cilia, but only 1 or 2

flagella• The beating patterns of cilia and flagella is

different…


5 µm

Direction of swimming

(a) Motion of flagella

Direction of organism’s movement

Power stroke Recovery stroke

(b) Motion of cilia15 µm

The motion of flagella is undulating, like a snake, driving the cell forward in the direction it is facing- for example, this sperm cell

The motion of cilia is different however- they have a back and forth motion, which moves the cell sideways- its like one side on a boat oarThe image shows a Colpidium- a freshwater protozoa

– A core of microtubules sheathed by the plasma membrane

– A basal body that anchors the cilium or flagellum– A motor protein called dynein, which drives the

bending movements of a cilium or flagellum– Arranged in a ‘9+2’ pattern

Cilia and flagella share a common ultrastructure:

Animation: Cilia and flagellaAnimation: Cilia and flagella

In Mastering Biology Study Area, Chapter 6, Activity: Cilia and Flagella

0.1 µmTriplet

(c) Cross section of basal body:

(a) Longitudinal section of cilium: showing microtubules running the length of the cilium

0.5 µm

Plasma membrane

Basal body

Microtubules (b)Cross section of cilium: Shows a cross section through

the middle of a cilium, showing the 9+2 arrangement of microtubules- the outer ring of doublets are held together by motor proteins called DYENINS (red)

The central doublet is held to the outer ring by other cross linking proteins (blue)

Plasma membraneOuter microtubule

doubletDynein proteinsCentral microtubuleRadial spoke

Protein cross-linking outer doublets

0.1 µm


The bottom part of the cilia- each doublet joins up to another microtubule to form a triplet. The central doublet has gone and the structure is held together by the cross linking proteins (blue)

23

Actin subunit

10 µm

7 nmCampbell and Reece (2011) Table 6-1b

e.g. interaction with myosin in sarcomere

Microfilaments (Actin Filaments)

• Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits

• The structural role of microfilaments is to bear tension, resisting pulling forces within the cell

• They form a 3-D network called the cortex just inside the plasma membrane to help support the cell’s shape

• Bundles of microfilaments make up the core of microvilli of intestinal cells


Microfilament/Actin filament functions

•7nm diameter•Actin filaments are thin and flexible, provide shape to the cell•A shows microvilli•B shows contractile bundles in cytoplasm•C shows fingerlike protrusions from the leading edge of a moving cell•D shows contractile ring/cleavage furrow during cytokinesis


Microvillus

Plasma membrane

Microfilaments (actin filaments)

Intermediate filaments

0.25 µm

This image shows the structural role of microfilaments

This shows microvilli in the intestine, and inside the microvilli are microfilaments (actin filaments)

• Microfilaments that function in cellular motility contain the protein myosin in addition to actin

• In muscle cells, thousands of actin filaments are arranged parallel to one another

• Thicker filaments composed of myosin interdigitate with the thinner actin fibres

Microfilament/Actin filament functions

Reece et al (2011) Fig, 6-27a

Muscle cell

Actin filament (yellow)

Myosin filament (purple)

Myosin arm

(a) Myosin motors in muscle cell contraction

•In muscle cells during contraction, myosin arms ‘walk’ over the actin filaments- this causes shortening of the gaps between the actin filaments•A muscle contraction involves shortening of many muscle cells at the same time

• Is a circular flow of cytoplasm within cells• This streaming speeds distribution of materials within the

cell• Cytoplasmic streaming is a way of stirring up the

contents of the cytoplasm• In plant cells,

actin-myosin interactionsdrive cytoplasmic streaming

Video: Cytoplasmic StreamingVideo: Cytoplasmic Streaming

Cytoplasmic streaming

30

5 µm

Keratin proteins

Fibrous subunit (keratinscoiled together)

8–12 nm

Campbell and Reece (2011) Table 6-1c

:structural filaments that help to keep the shape of the nucleus, and are depolymerised during mitosis, when the nuclear envelope disintegrates

e.g. skin, hair, nails

Intermediate Filaments• Intermediate filaments range in diameter from 8–12

nanometres, larger than microfilaments but smaller than microtubules

• They support cell shape and fix organelles in place• They also bear tension in the cell- and nucleus sits in a

cage of intermediate filaments that hold it in centre of cell

• Intermediate filaments are more permanent cytoskeleton fixtures than the other two classes

• Eg keratin framework still remains in place, even after skin cells have died


Intermediate filament functions

•10nm diameter•Intermediate filaments have great tensile strength•Allow cells to withstand mechanical stress when stretched•They span the entire cell, strengthening it•They are the toughest and most durable of the 3 filament types

33Table 17-1 Essential Cell Biology (© Garland Science 2010)

(Crocuses)(Yew tree)

(Madagascar periwinkle)

Microtubules can be targets for anti-cancer drugs

•Cancer cells divide rapidly and with less control•Inhibition of the mitotic spindle can prevent cell division, and cause death of these cells•These drugs block microtubule polymerisation- anti-mitotic agents

34

LEARNING OBJECTIVES Lecture 8

• List and describe the molecules that compose the cellular cytoskeleton.

• Describe the main functions of the cytoskeleton.

35

Reading: Campbell Biology 9th Ed. Reece et al (2011). Ch 6 pp.158-166

Homework: watch http://www.bbc.co.uk/iplayer/episode/b01nln7d/Secret_Universe_The_Hidden_Life_of_the_Cell/Only available until 8pm, Sunday 28th Oct

Homework:

Go to Mastering Biology BS101 account online

Complete assignment 4 ‘Cytoskeleton and cytosol’

Opens: Tues 23rd Oct, 5pm

Closes: Tues 6th Nov, 5pm

bs101 lecture 08 ytoskeleton 2012-13

Documents