Motor Proteins - Introduction Part 1 Biochemistry 4000 Dr. Ute Kothe.

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 FunctionMyosin (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

Voet Fig. 35-89

Tubulin+ GTP

Tubulin+ GDP

• building block of microtubules

• heterodimer of closely related Tubulin &

• G proteins: N-terminal residues fold into G domain-like structure

• -tubulin’s GTP buried at subunit interface, nonexchangable, not hydrolyzed

• -tubulin’s GTP is solvent exposed until tubulin dimers polymerize

• upon polymerization, -tubulin from adjacent dimer provides catalytic residue to hydrolyze b-tubulin’s GTP; resulting GDP is nonexchangeable unless tubulin dissociates from microtubule

Microtubules

Voet Fig. 35-92

1. Tubulins interact head to tail to form a long protofilament2. Protofilaments align side by side in curved sheet3. Sheet of 13 (9-16) protomers closes on itself to form microtubule4. Microtubule lengthens by addition of tubulins to both ends

(preferentially to + end, i.e. the end terminating in -tubulins)

Structure of the Axoneme

Voet Fig. 35-102

• Bundle of microtubules called axoneme• coated by plasma membrane• Forms eukaroytic flagella & cilia

Dynein

Valle, Cell 2003; Voet Fig. 35-107

• 1 or more heavy chains (motor domain)• several intermediate and light chains• motor domain is 7-membered ring, ATP-

hydrolyzing unit• coiled-coil extension forms stalk that

interacts via globular domain with microtubules

• long stem (with intermediate and light chains) binds cargo

Conventional Kinesin

Voet Fig. 35-94

• 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)

Kinesin Structure

Voet Fig. 35-95 & 96 & 97

• Globular head: tubulin-binding site & nucleotide binding site

• flexible neck linker• -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. 35-98

Kinesin Cycle

Hand-over-Hand Mechanism

ATP-bound state: strong microtuble bindingADP-bound state: weak microtubule binding

1. ATP binds to leading head - globular kinesin head which is already bound to the microtubule and oriented towards (+) end

2. Neck linker of leading head “zips up” agains catalytic core 3. trailing head is thrown forward (trailing head has bound ADP and

reduced affinity to microtubule): 4. Trailing head swings by ~ 160 Å, net movement of dimeric kinesin

is ~ 80 Å = length of one microtubule dimer5. ATP in new trailing head is hydrolyzed & phosphate released:

affinity for microtuble decreases6. ADP in new leading head dissociates

Two heads work in a coordinated fashion ATP binding to leading head induces power stroke

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

Valle, Cell 2003; Voet Fig. 35-107

• 1 or more heavy chains (motor domain)• several intermediate and light chains• motor domain is 7-membered ring, ATP-

hydrolyzing unit• coiled-coil extension forms stalk that

interacts via globular domain with microtubules

• long stem (with intermediate and light chains) binds cargo