Lecture 8 - Microtubule motor proteins Flashcards
What are the function of microtubule motor proteins?
Kinesin and dynein motor proteins transport membrane bound vesicles, proteins and organelles along microtubules at the expense of ATP.
Most kinesins move cargo from (-) to (+) - anterograde
Most dynein’s transport cargo from the (+) end to the (-) end of microtubules - retrograde
Describe the structure and function of kinesin.
Motor/head- The kinesin family motor (or head) domain contains the ATP binding site and the microtubule-binding site of the motor. The kinesin motor domain is required to generate movement but does not appear to determine the direction a kinesin motor will move on a microtubule.
Neck - the neck is a flexible region connecting the motor domain to the central stalk domain. The neck determines the direction of movement.
Kinesin is a large family of different structures and functions.
Kinesin 1 has a globular, bulbous region at one end of the molecule and this region is enlarged when antibody that binds to the motor domain is present. Accordingly, these observations suggest that the globular region contains the kinesin motor domain. The two heavy chains appear to interact in parallel, as the motor domain is observed only at one end of the molecule.
In contrast, kinesin 5 has two bulbous regions, each of which is enlarged when antibody to the motor domain is present. These data suggest that kinesin 5 has motor domains at each end of the molecule, i.e., it is a bipolar kinesin. Because it is a tetramer, rather than a dimer, it may be formed by two dimers interacting tail to tail in an antiparallel fashion. This allows it to slid two microtubules past each other.
Kinesin 2 is heterotrimeric which means it has two different motor proteins.
Kinesin 13 (Kinl) doesn’t have a stalk and is important in end disassembly.
Describe kinesin movement along microtubules
Kinesin Movement along Microtubules
1. Forward motor binds β-tubulin, releasing ADP
2. Forward head binds ATP
3. Conformational change in neck linker causes rear head to swing forward
4. New forward head releases ADP, trailing head hydrolyses ATP and releases Pi
Describe dynein movement.
Cytosolic dynein’s transport cargo towards the (-) end of microtubules (retrograde). Very different structure to kinesins, but do have ATPase domains and microtubule binding domains. Cytosolic dynein’s are linked to their cargos (which include vesicles and chromosomes) by large complexes of microtubule binding proteins (dynactin).
Dynein movement
Hydrolysis of ATP causes the rotation of the ATPase domain which applies force to the neck. The force of rotation moves the structure relative to the binding site.
How is cargo movemetn direction determined.
Although microtubule orientation is fixed by the MTOC (and any given motor moves only in one direction), some cargoes are able to move in both directions along a microtubule because they are able to interact with both (+) -end- and (−)end-directed motor proteins.
The direction that a given cargo moves along a microtubule appears to be controlled by swapping one motor protein for the other e.g. kinesin for dynein (it may also be possible to activate one motor and inactivate the other). Kinesin travels at about 600nm per second – no problem within a normal cell - but this does mean it takes several days to transport components along the lengthier nerves like the sciatic nerve.
Microtubule associated proteins
MAP2 - if not present disrupts spacing
Tau - Tau tangles occur in brain cells in Alzheimer’s disease
What is the function of microtubules in cillia and flagella
Cilia and flagella contain a highly organized core of microtubules and associated proteins. This core, termed the axoneme, is typically made of nine outer doublet microtubules and two central pair microtubules (known as the 9 + 2 arrangement). Each outer doublet consists of a 13-protofilament microtubule and a10- protofilament microtubule, while the central pair contains 13
protofilaments each.
Cell movement depends on axoneme bending, which, in turn, depends on force generated by axonemal dynein’s. These motor proteins act to slide outer doublet microtubules past each other, but this sliding motion is converted into bending because of restrictions imposed by cross-linking proteins (nexin) in the axoneme.
What are the three types of microtubule that make up the mitotic spindle?
The three types of microtubules that make up the spindle are kinetochore microtubules, polar microtubules and astral microtubules.
The (−) ends of all three types associate with spindle poles.
* Kinetochore microtubules connect chromosomes, via the kinetochore attachment site, to the spindle poles.
* Polar microtubules from each pole overlap and are involved in holding the poles together and regulating pole-to-pole distance.
* Astral microtubules radiate from each spindle pole toward the cortex of the cell, where they help position the spindle and determine the plane of cytokinesis.
What are intermediate filaments?
Intermediate filaments are formed from a family of related proteins such as keratin or Lamin. The subunits assemble to create a
strong, ropelike polymer that, depending on the specific protein, may provide support for the nuclear membrane or for cell adhesion.
Intermediate filaments provide mechanical support to the plasma membrane and the nuclear membrane and are much more stable than actin filaments or microtubules. This stability is derived from the α helical rod structure of the subunits, which overlap along the filament long axis to generate rope-like filaments.
All intermediate filament proteins assemble to form 10 nm diameter filaments and all share a common domain structure: globular N-terminal and C-terminal domains connected by a central, -helical core (or rod) domain. The core domain is conserved among all intermediate filament proteins and mediates dimer and ultimately tetramer formation. The N-terminal and C-terminal domains vary among the different types of intermediate filament proteins and therefore distinguish the different types.
There is evidence that the N-terminal domain may play some role in filament assembly, and that both the N-terminal and C-terminal domains are important for filament organization and interaction with other cellular components.
IF’s don’t have any motor proteins
Medical importance
* Because they are tissue/cell specific Intermediate filament proteins can serve as markers of for specific cell types e.g. stem cells.
* Mutations to IF genes can also result in a number of diseases for example Epidermolysis bullosa simplex due to a mutation in a keratin gene.
