Transcript Introduction Part1

Motor Proteins
- Introduction Part 1
Biochemistry 4000
Dr. Ute Kothe
Motor Proteins
Motor Proteins convert chemical energy into motion.
• chemical energy is derived from ATP hydrolysis
• motion is generated by conformational changes depending on the
bound nucleotide
Myosin
Kinesin
Dynein
Motor Protein Function
Myosin (18 known classes, 40 different myosins in humans)
 Movement along actin fibres
• Muscle movement
• Cytokinesis (cytoplasmic division, tightening of contractile ring)
• Transport of cargo along microfilaments (vescicles etc.)
Kinesin (16 classes)
 Movement along microtubule tracks, usually to (+) end
• Transport of cargos: vesicles, organelles, cytosolic components
such as mRNAs & proteins, chromosomes
Dynein (12 mammalian dyneins)
 Movement along microtubule tracks, to (-) end, i.e. cell center
• Cytoplasmic dyneins: transport of cargos such as vesicles
• Axonemal dyneins: Movement of cilia and flagella
Tubulin
• building block of microtubules
• heterodimer of closely related Tubulin a
&b
Tubulin b
+ GDP
• G proteins: N-terminal residues fold into
G domain-like structure
• a-tubulin’s GTP buried at subunit
interface, nonexchangable, not hydrolyzed
• b-tubulin’s GTP is solvent exposed until
tubulin dimers polymerize
Tubulin a
+ GTP
• upon polymerization, a-tubulin from
adjacent dimer provides catalytic residue
to hydrolyze b-tubulin’s GTP; resulting
GDP is nonexchangeable unless tubulin
dissociates from microtubule
Voet Fig. 35-89
Microtubules
1.
2.
3.
4.
Tubulins interact head to tail to form a long protofilament
Protofilaments align side by side in curved sheet
Sheet of 13 (9-16) protomers closes on itself to form microtubule
Microtubule lengthens by addition of tubulins to both ends
(preferentially to + end, i.e. the end terminating in b-tubulins)
Voet Fig. 35-92
Structure of the Axoneme
• Bundle of microtubules called axoneme
• coated by plasma membrane
• Forms eukaroytic flagella & cilia
Voet Fig. 35-102
Dynein
• 1 or more heavy chains (motor domain)
• several intermediate and light chains
• motor domain is 7-membered ring, ATPhydrolyzing unit
• coiled-coil extension forms stalk that
interacts via globular domain with
microtubules
• long stem (with intermediate and light
chains) binds cargo
Valle, Cell 2003; Voet Fig. 35-107
Conventional Kinesin
• two identical heavy chains forming two large globular heads
attaching to microtubules and a coiled-coil
• two identical light chains interacting wit cargo
• transports vesicles and organelles in (-) to (+) direction (towards cell
periphery)
Voet Fig. 35-94
Kinesin Structure
• Globular head: tubulin-binding site &
nucleotide binding site
• flexible neck linker
• a-helical stalk leading into coiled-coil
• ATP hydrolysis triggers conformational
change in neck linker via 2 switch regions:
• When ATP is bound, neck linker docks
with catalytic core
• Upon ATP hydrolysis, the neck linker
“unzips”
Voet Fig. 35-95 & 96 & 97
Kinesin Cycle
Voet Fig. 35-98
Hand-over-Hand Mechanism
ATP-bound state: strong microtuble binding
ADP-bound state: weak microtubule binding
1.
6.
ATP binds to leading head - globular kinesin head which is
already bound to the microtubule and oriented towards (+) end
Neck linker of leading head “zips up” agains catalytic core
trailing head is thrown forward (trailing head has bound ADP and
reduced affinity to microtubule):
Trailing head swings by ~ 160 Å, net movement of dimeric kinesin
is ~ 80 Å = length of one microtubule dimer
ATP in new trailing head is hydrolyzed & phosphate released:
affinity for microtuble decreases
ADP in new leading head dissociates


Two heads work in a coordinated fashion
ATP binding to leading head induces power stroke
2.
3.
4.
5.
Processivity
Kinesin is highly processive: it takes several 100 steps on a
microtubule without detaching or sliding backwards!
How?
• coordinated, but out of phase ATP cycle in both heads
• one head is always firmly attached to microtubule
Movie demonstrating kinesins processivity:
http://www.proweb.org/kinesin/axonemeMTs.html
Dynein
• 1 or more heavy chains (motor domain)
• several intermediate and light chains
• motor domain is 7-membered ring, ATPhydrolyzing unit
• coiled-coil extension forms stalk that
interacts via globular domain with
microtubules
• long stem (with intermediate and light
chains) binds cargo
Valle, Cell 2003; Voet Fig. 35-107