Kinesins and Dyneins

Kinesins and dyneins• Cytoskelton includes two components in addition to

actin: intermediate filaments and microtubules.• Microtubules act as tracks for two classes of motor

proteins: kinesins and dyneins.• Importance

– Kinesins: moving along microtubules usually carry cargo such as organelles and vesicles from the center of a cell to its periphery.

– Dyneins: important in sliding microtubules relative to one other during the beating of cilia and flagella on the surfaces of some eukaryotic cells. They carry cargo from periphery to centre of cell.

Microtubules are a component of the cytoskeleton.

These cylindrical polymers of tubulin can grow as long as 25 micrometers and are highly dynamic.

The outer diameter of microtubule is about 25 nm.

Microtubules

• Microtubules are long, hollow cylinders made up of polymerised α- and β-tubulin dimers.

• They are highly dynamic structures that grow through the addition of α- and β-tubulin dimers to the ends of existing structures.

• Like actin, tubulins also bind and hydrolyze nucleoside triphosphates, although for tubulin the nucleotide is GTP rather than ATP.

• Thus, a newly formed microtubule consists primarily of GTP-tubulins. • Through time, the GTP is hydrolyzed to GDP.• The GDP-tubulin subunits in the interior length of a microtubule remain stably

polymerized, whereas GDP subunits exposed at an end have a strong tendency to dissociate.

• Thirteen protofilaments associate laterally to form a single microtubule and this structure can then extend by addition of more protofilament

Assembly of microtubules

+ end- end

The heavy chain of kinesin-1 comprises a globular head (the motor domain) at the amino terminal end connected via a short, flexible neck linker to the stalk – a long, central alpha-helical domain – that ends in a carboxy terminal tail domain which associates with the light-chains

Kinesin

The head regions bind to microtubules and also bind ATP. The head domains are thus ATPase motors.

The tail domain binds to the organelle to be moved. ATP is needed for both binding and movement. Hydrolysis is absolutely essential for movement.

Kinesin Motion is highly processive• Kinesins are motor proteins that move along microtubules.• When a kinesin molecule moves along a microtubule, the

two head groups of the kinesin molecule operate in tandem: one binds, and then the next one does.

• A kinesin molecule may take many steps before both heads groups are dissociated at the same time.

• A single kinesin molecule will typically take 100 or more steps toward the plus end of a microtubule in a period of seconds before the molecule becomes detached from the microtubule.

• The average step size is approximately 80 Å, a value that corresponds to the distance between consecutive α- or β-tubulin subunits along each protofilament.

Kinesin movement along microtubule

• The addition of ATP strongly increases the affinity of kinesin for microtubules.

• This is in contrast with the behavior of myosin; ATP binding to myosin promotes its dissociation from actin.

• In a two-headed kinesin molecule in its ADP form, dissociated from a microtubule, the neck linker binds the head domain when ATP is bound and is released when ADP is bound.

• The initial interaction of one of the head domains with a tubulin dimer on a microtubule stimulates the release of ADP from this head domain and the subsequent binding of ATP.

• The binding of ATP triggers a conformational change in the head domain that leads to two important events. • First, the affinity of the head domain for the microtubule

increases, essentially locking this head domain in place.• Second, the neck linker binds to the head domain.

• This change repositions the other head domain acting through the domain that connects the two kinesin monomers.

• In its new position, the second head domain is close to a second tubulin dimer, 80 Å along the microtubule in the direction of the plus end.

• Meanwhile, the intrinsic ATPase activity of the first head domain hydrolyzes the ATP to ADP and Pi.

• When the second head domain binds to the microtubule, the first head releases ADP and binds ATP.

• Again, ATP binding favors a conformational change that pulls the first domain forward.

• This process can continue for many cycles until, by chance, both head domains are in the ADP form simultaneously and kinesin dissociates from the microtubule.

• Because of the relative rates of the component reactions, a simultaneous dissociation occurs approximately every 100 cycles.

• Kinesin hydrolyzes ATP at a rate of approximately 80 molecules per second.

• Thus, given the step size of 80 Å per molecule of ATP, kinesin moves along a microtubule at a speed of 6400 Å per second. This rate is considerably slower than the maximum rate for myosin, which moves relative to actin at 80,000 Å per second

Kinesin ‘walks’ along the microtubule while carrying its cargo

Kinesin movement along microtubule

Dyneins• First microtubule motors to be identified.• Very large in size.• Dyneins are called "minus-end directed

motors, because directed towards minus end

Dynein is a motor protein in cells which converts the chemical energy contained in ATP into the mechanical energy of movement.

Dynein transports various cellular cargo by "walking" along cytoskeletal microtubules towards the minus-end of the microtubule.

They transport cargo from the periphery of the cell towards the centre.

Two types: cytoplasmic dyneins and axonemal dyneins.

Cytoplasmic dynein

• Cytoplasmic dynein probably helps to position the Golgi complex and other organelles in the cell.

• It also helps transport cargo needed for cell function such as vesicles made by the endoplasmic reticulum, endosomes, and lysosomes.

• Dynein is involved in the movement of chromosomes and positioning the mitotic spindles for cell division.

Axonemal Dyneins

• Present in flagella or cilia of eukaryotes.• Help in their beating for effective movement.

• Flagella are usually 1 or 2 per cell. Generally longer. Have rotary motion.

• Cilia are usually many per cell. They tend to have a whip-like movement.

Cilia & flagella Bounded by plasma

membrane. Basal body: a single centriole

cylinder at the base of each cilium or flagellum.

Core axoneme: a complex of microtubules & associated proteins.

Some distinctions:

plasma membrane axoneme basal body (centriole)

Cilium

cytosol

Axonemal dynein• Each dynein molecule forms a cross-bridge

between two adjacent microtubules of the ciliary axoneme.

• During the "power stroke", which causes movement, the ATPase motor domain undergoes a conformational change that causes the microtubule-binding stalk to turn relative to the cargo-binding tail with the result that one microtubule slides relative to the other .

• This sliding produces the bending movement needed for cilia to beat and propel the cell or other particles.

• Groups of dynein molecules responsible for movement in opposite directions are probably activated and inactivated in a coordinated fashion so that the cilia or flagella can move back and forth.

Protein Dynein:–Is responsible for the bending movement

of cilia and flagella

Microtubuledoublets ATP

Dynein arm

Powered by ATP, the dynein arms of one microtubule doublet grip the adjacent doublet, push it up, release, and then grip again. If the two microtubule doublets were not attached, they would slide relative to each other.

(a)

Outer doubletscross-linkingproteins

Anchoragein cell

ATP

In a cilium or flagellum, two adjacent doublets cannot slide far because they are physically restrained by proteins, so they bend. (Only two ofthe nine outer doublets in Figure 6.24b are shown here.)

(b)

Figure 6.25 B

Localized, synchronized activation of many dynein arms probably causes a bend to begin at the base of the Cilium or flagellum and move outward toward the tip. Many successive bends, such as the ones shown here to the left and right, result in a wavelike motion. In this diagram, the two central microtubules and the cross-linking proteins are not shown.

(c)

1 3

2

Figure 6.25 C

Bidirectional dyneins

• Several reports suggest bidirectional movement of specific dyneins but no conclusive evidence of unidirectional plus-end motility.

• Motility can be switched to unidirectional minus end transport by phosphorylation.

• Probability that phosphorylation of motor regulates its directionality.

• Positive End Directed motors: move from –ve to + end.

Eg: Myosin and Kinesin Motors

• Negative End Directed motors: move from +ve end to –ve end

Eg:Dynein Motors