Molecular Motors. Neil Thomas, University of Birmingham, UK.

Transcription

1 Molecular Motors Neil Thomas, University of Birmingham, UK.

2 Acknowledgements My thanks to: The British Council (Tsuji-san & Shimura-san) Monbukagakusho, JSPS & JST Dr Yasuhiro Imafuku (Kyushu University) Dr Shinji Kamimura (Tokyo University) 2

3 Birmingham University The University was established in It now has over students. 3

4 Birmingham and the UK Birmingham is England s second city. It is in the Midlands, about 200 km north-west of London. 4

5 Canals and Railways Birmingham is at the centre of England's canal system. Canals were once very important for carrying materials for industry. Nowadays, 'narrow boats' can be hired for leisure. Railways were built from about The trains were pulled by steam locomotives. Photo: Brian Townsley The 'Age of Steam' made Britain very wealthy. 5

6 James Watt James Watt ( ), pioneer of the steam engine, worked with Matthew Boulton in Birmingham. He invented the condenser and the governor. Watt s Beam Engine Animation: Matt Keveney Steam engines led to the science of Thermodynamics. 6

7 Molecular Motors & Movement I m a physics lecturer. My main research interest is the molecular motors that produce movement in living systems, from single cells to whole animals. This seminar will show you why molecular motors are an exciting and important topic. A single-celled amoeba A galloping horse 7

8 Outline of Seminar How Muscles Work How Cells Make DNA How Cells Move How Cells Generate Energy Videos from Essential Cell Biology CD-ROM 8

9 Muscles & Movement Cheetah Humming-bird Shark Bumble-bee Videos from How Animals Move CD-ROM 9

10 Muscles Human arm muscles Structure of muscle Diagrams from The Ultimate Human Body CD-ROM 10

11 Bundle of Muscle Fibres (viewed with an optical microscope) Fibres show a periodic striation pattern Black blobs are cell nuclei Photo from How Animals Move CD-ROM 11

12 Single fibre of striated muscle Muscle fibres are single biological cells Fibre diameter is typically about 100 microns (0.1 mm) Fibres consist of bundles of smaller myofibrils Photo from How Animals Move CD-ROM 12

13 Single myofibril (viewed with an electron microscope) Each myofibril is about 1 micron in diameter Myofibrils also show the periodic striation pattern The repeating unit is called a sarcomere Photo from How Animals Move CD-ROM 13

14 Muscle Contraction (Sliding-filament model) A nervous impulse (guided by T-tubules) causes the release of calcium inside a muscle cell. Calcium switches on the molecular motors that cause sliding of the actin and myosin filaments. 14 Video from Essential Cell Biology CD-ROM

15 Actomyosin: the molecular motor that powers our muscles Video from Essential Cell Biology CD-ROM The actomyosin motor works in a cycle. Myosin binds to actin, its lever arm tilts over, and then it detaches. Each cycle uses up one molecule of ATP. ATP is the fuel that allows the motor to perform work. The chemical reaction is ATP ADP + P i 15

16 Heat Engines Work in a Thermodynamic Cycle Steam Engine (Rankine Cycle) Animations: Matt Keveney Car Engine (Otto Cycle) Actomyosin also works in a cycle. Its power stroke is only 5 10 nm. (1 million nanometres = 1 millimetre) We apply Thermodynamics to molecular motors. 16

17 Cilia and Flagella Cilia (Stentor) Flagellum (Sperm) Beating cilia (Simulation) Beating flagellum (Simulation) Videos from How Animals Move CD-ROM 17

18 Molecular Motors in Cilia Dynein motors cause sliding of microtubules. Sliding produces bending: Cross-section of a cilium showing microtubules and dynein arms. (Diameter = 250 nm) Cilia and flagella beat spontaneously! 18

19 How Do Cells Move? An amoeba moves rapidly by extending pseudopodia. A keratocyte (from a fish scale) crawls rapidly. Neurites grow slowly from nerve cells by means of growth cones. 19

20 Cells move by using their cytoskeleton. 20

21 The Cytoskeleton Actin filaments (red) allow the cell to crawl. Microtubules (blue) transport chemical signals that control the movement. Intermediate filaments (green) add strength to the cell. Photo from Video Tour of Cell Motility by Vic Small 21

22 Crawling Cancer Cell Formation of actin filaments inside the lamellipodium pushes the front of the cell forwards. The rear of the cell is pulled along by actomyosin. 22 From Video Tour of Cell Motility by Vic Small

23 Crawling of white blood cells A neutrophil chases a bacterium. Lymphocytes migrate to a wound. In both cases, the movement is in response to a chemical signal ( chemotaxis ). 23 From Video Tour of Cell Motility by Vic Small

24 Propulsion by Actin An actin filament moves by treadmilling. The motion consumes ATP. Listeria bacteria move by hijacking a cell s actin. From Video Tour of Cell Motility by Vic Small Video from Essential Cell Biology CD-ROM 24

25 Movement inside Cells 25

26 Organelle Movement Molecular motors transport material along the cytoskeleton. The video shows organelle transport along a microtubule. Kinesin motors transport material to the plus end (away from the nucleus). Dynein motors transport material in the opposite direction. 26 Video from Essential Cell Biology CD-ROM

27 The Kinesin Molecular Motor Video from Essential Cell Biology CD-ROM The video shows how two-headed kinesin may step along a microtubule. The model is based on many different experiments. We have used a similar picture to develop a mathematical model of the kinesin motor for analysing the results of laser-tweezer experiments. 27

28 Laser Tweezers Laser tweezers are used to trap tiny beads in a focussed laser beam. They exert a force comparable to that due to a single molecular motor (typically a few piconewtons). 1 gram weighs about 10 thousand million piconewtons Video from Essential Cell Biology CD-ROM 28

29 Laser Tweezers Visscher et al. (1999) Laser tweezers are used to study kinesin stepping along a microtubule. The kinesin molecule is attached to a bead that is trapped in the focussed laser beam. The tweezers exert a force that slows down the kinesin motor. 29

30 Molecular Motors and DNA 30

31 DNA carries the Genetic Code A DNA molecule is a double helix (like a spiral staircase) The two strands of the helix are joined by base pairs Base C always pairs with base G Base A always pairs with base T The sequence of bases CAA stores the genetic information 31 Videos from Essential Cell Biology CD-ROM

32 DNA Motors 1:Transcription (Reading a single gene) The RNA polymerase motor copies (transcribes) a gene from DNA to RNA inside the cell nucleus. Video from Essential Cell Biology CD-ROM 32

33 DNA Motors 2: Translation (Making a protein from a gene) A ribosome translates the genetic code in the RNA to make a protein from the correct sequence of amino acids. Our bone marrow makes 100 million million molecules of haemoglobin per second. Video from Essential Cell Biology CD-ROM 33

34 DNA motors 3: Replication (Making an exact copy of DNA) A DNA helicase motor separates the DNA double helix into two strands. DNA polymerase motors copy each strand. 34 Videos from Essential Cell Biology CD-ROM

35 DNA Motors 4: Mitosis (Cell division) Microtubules form a spindle (green). Pairs of chromosomes containing DNA (blue) attach to the spindle by kinetochores (pink) along the equator. Cells in a frog embryo divide rapidly. Molecular motors pull chromosomes to opposite poles of the spindle. Each cell gets the same DNA. Videos from Essential Cell Biology CD-ROM 35

36 How Do Cells Generate Energy? 36

37 Mitochondria: the Cell s Power-Stations A heart muscle cell contains myofibrils (orange). It requires many mitochondria (red), which supply ATP for the actomyosin molecular motors. Glucose oxidation produces protons in the inter-membrane space. Proton flow back across the inner membrane generates ATP. 37

38 ATP Synthase: A Molecular Turbine ATP synthase is a molecular turbine that converts energy from a proton gradient into the chemical energy in the form of ATP that powers the cell. The molecular structure illustrates how ATP may be synthesized. 38

39 Rotation of ATP Synthase Kinoshita et al Kinoshita and colleagues observed rotation of ATP synthase by attaching a fluorescent actin filament to the shaft. 39

40 Electricity Generation A steam turbine rotates a generator to produce electricity. The Chiba #1 turbine generator produced 125 MW (125 million watts). #1 Turbine from Chiba Thermal Power Station (TEPCO Museum) A marine turbine from a ship 40

41 Turbines Compared Chiba #1 Turbine TEPCO Museum (Kawasaki). The Chiba steam turbine is over 10 m long. It can generate about 100 million watts (10 8 W = 100 MW). ATP synthase is about 20 nm long. It generates 1/(million million million) watts (10-18 W =1 aw). Both machines are governed by the laws of Thermodynamics. 41

42 Summary Molecular motors are the fundamental agents of movement in living systems They are cyclic machines like steam engines They generally consume one molecule of ATP per cycle They typically produce force 1 pn and power ~ 1 aw 42

43 But molecular motors working together in muscle can produce large forces 43

44 44