Motor proteins, cellular motility Regulation of actin treadmilling

04/10/2013
Motor proteins, cellular motility
Common characteristics and types of motor proteins.
The ATPase cycle.
Properties, function and partner proteins of the different motor
proteins.
25.09.2013.
Regulation of actin treadmilling
Le Clainche C, Carlier MF. Regulation of actin assembly associated with protrusion and adhesion in cell migration. Physiol Rev. 2008 Apr;88(2):489-513.
1

04/10/2013
Motor proteins
• Proteins that can bind to specific filaments
• chemo-mechanical enzymes (mechanochemical coupling)
− they hydrolyze ATP (chemical energy)
− they can produce kinetic energy
• Their thermodynamic efficiency is between 30 and 60%.
• They convert chemical energy to mechanical work directly.
• The cytoskeletal filaments:
− Serve as tracks to carry organelles or vesicles
− Can be moved by the motor proteins
Families of motor proteins
Cytoskeletal motors
• Actin-based motors (myosins)
• Microtubule based motors (kinesins and dyneins)
Non-cytoskeletal motors
• Nucleic acid based (DNA and RNA polymerases,
helicases)
• Rotary motors: bacterial flagellum, FI-F0-ATPase
2

04/10/2013
Basic components of the
cytoskeletal transport system
• Motor
− Myosin, kinesin, dynein
• Track
− Microfilaments, microtubules
• Cargo
− organelles, chromosomes, vesicles
• Fuel
− ATP, GTP
Common properties
Structure
• N-terminal globular head:
motor domain
− nucleotide binding and hydrolysis
− specific binding sites for the corresponding filaments
• C-terminal: structural and functional role with specificity
N
C
Mechanical properties, function
In principle: cyclic function and work
Motor → binding to a filament → force → dissociation → relaxation
1 cycle requires 1 ATP hydrolysis
They can either move or produce force
3

04/10/2013
Directionality
• Track polarity: + and – ends of the actin filaments and the
microtubules
• Motors move unidirectionally
−
−
−
myosin: toward the + end of microfilaments
kinesin: toward the + end of microtubules
dynein: toward the – end of microtubules
Myosins
• Actin-based motors
• conventional myosins (myosin II – 1864: Wilhelm Kühne)
• nonconventional myosins
• Myosin families: myosin I-XVIII
• Functions:
− muscle contraction
− cytokinesis
4

04/10/2013
Myosin II
Head: actin binding and
nucleotide binding region
Neck
Tail
S1 head tilting model
• S1 head attaches to the actin filament
• The angle of attachment change from 90° to 45° → sliding of the filament
• detachment
Huxley AF, Simmons RM. Proposed mechanism of force generation in striated muscle. Nature. 1971 Oct 22;233(5321):533-8
5

04/10/2013
S1 head tilting model
Huxley AF, Simmons RM. Proposed mechanism of force generation in striated muscle. Nature. 1971 Oct 22;233(5321):533-8
The swinging lever arm model
• Rigor-dissociation step: ATP binding → dissociation form the actin filament
• Priming (back swing) of the lever arm
• ATP hydrolysis: reattachment
• Power stroke: force generating swing of the lever arm
MECHANOCHEMICAL COUPLING!
Kühner S, Fischer S. Structural mechanism of the ATP-induced dissociation of rigor myosin from actin. Proc Natl Acad Sci U S A. 2011 May 10;108(19):7793-8.
6

04/10/2013
• 1985: Ron Vale
• XIV kinesin families
• microtubule based motor proteins
• They move towards the plus end of MT
• Functions:
− organelle movement
− Assembly of cilia and flagella
−
−
Kinesins
Signaling pathways
Mitotic spindle formation and transportation of the
chromosomes
• The end!
7