Motor Protein and Cilia/Flagella

Categories of Motor Proteins

1. Interaction between microtubules and motor proteins: Kinesins and dyneins

   1. Used in fast axonal transport in neurons or the sliding of MTs in cilia and flagella, transporting membrane organelles mitosis

2. Interactions between actin microfilaments and members of the myosin motor protein

   1. Muscle contraction, organelle transport, cytokinesis, cell motility, maintaining cell shape

Overview of motor proteins

• Converts chemical energy (ATP) in to mechanical energy (force/movement)

• It moves unidirectionally in a stepwise manner, It travels to the plus end of microtubules (anterograde direction)

• No motor proteins uses intermediate filaments as tracks

Kinesin structure and directional movements

• A tetramer, 2 heavy chains and 2 light chains

• Has a globular head regions that attaches to microtubules, acts as a ATP-hydrolyzing engine

  • Uses head to walk on MT

  •  KRP heads are evolutionarily conserved

• A neck regions that connects head to the stalk

• A coiled helical stalk region that provides flexibility during movement

• A light-chain regions that attaches the kinesis to proteins, organelles or other cargo

  • KRP Tails are highly divergent that reflects the cargo diversity

• Kinesins are plus end directed motors

Kinesin Family

• Made of 14 kinesin-related protein with different structures and functions

  • Classified based on their structure

  • Some are homodimers; heterodimers or tetramers

• Majority of Kinesins have the motor domain on the N-terminus

  • Kinesin 1 is the classic protein most studied

• Kinesin 13 have its motor domain in the middle of the protein

• Kinesin 14 has a motor domain on the C-terminus causing it to be a minus end directed motor, travels in the retrograde direction

• Kinesins are involved in many different cellular processes

Kinesin 1

Dimer, moves cargo to plus end of MT

Kinesin 3

Monomer movement of synaptic vesicles in neurons

Kinesin 5

Bipolar, tetrameric, bidirectional sliding of MTs during anaphase of mitosis

Kinesin 6

completion of cytokinesis

Kinesin 13

• Dimer; destabilization of plus ends of MTs

• Catastrophins

Kinesin 14

• Spindle dynamics in meiosis and mitosis

• Moves towards minus ends of MTs

About Movement of Kinesin 1

• Kinesin motor step is 8nm from 1 B-tubulin to the next B-tubulin

  • Globular binding domains, head, from the heavy chains bind to the microtubules

• Uses a hand-over-hand model of movement with 2 globular head domains taking turns as the lead hand

  • 1 head remained on the microtubule while 1 head swings over to take a step

  • It is coupled with ATP hydrolysis

• Each kinesin molecule exhibits high processivity

  • It can move long distances along he MT before detaching

Steps of Kinesin movement

1. Leading heavy chain binds to ATP

2. ATP binding causes a conformational change allowing the trialing heavy chain to swing forward

3. Trailing heavy chains finds a new MT binding site

4. New leading chain releases ADP and the new trailing head hydrolyzes ATP to ADP and Phosphate

Kinesin-1 role in the cell

Responsible for organelle transport and maintaining the correct organelles’ localization occurs in most cells

• Comparing the MTs within the cell and the location of the organelles close to the MT tracks, you can discern the function of the kinesin for that cell

Types of Dynein

Cytoplasmic and Axonemal

• Axonemal is in cilia and flagella

Cytoplasmic Dyneins

• High processivity toward the minus head of microtubules

  • Able to catalyze consecutive reaction son a single substrate molecules without releasing it

• Structure

  • 2 identical heavy chains that has a force generating head

  • Protruding stalk

  • A Binding site for MTs and Tail

  • Number of light and intermediate chains

• ATP hydrolysis causes conformational changes in the motor domain in the linker region that connects the motor domain to the MT-binding domain

• Dynein requires an adaptor molecule to interact with the cargo (dynactin and spectrin)

• Moves towards the minus end of MT

Roles of Cytoplasmic Dynein

1. Position the centrosome and Golgi complex and moving organelles, vesicles and particles through the cytoplasm

2. Position the spindle and move chromosomes during mitosis

Kinesin vs Dyneins

• Both move similar materials but in opposite directions on the same railway network (both move on MT)

• organelles may bind to both kinesin and dynein simultaneously engaging in a tug a way battle for the cargo

Structure of Cilia

• Cilia is 2-10 micrometers long, Most cells have one to many cilia

• Occurs in both unicellular and multicellular eukaryotes

• Generates a force perpendicular to the cilium like an oar-like pattern

  • Motile cilia move fluid through tracts by mucus propellers

  • Non-motile ciliar have sensory function

Flagella

• Moves cell through a fluid environment

• Is the same diameter as cilia but is much longer (up to 200 micrometers)

• Limited to one or few per cell and move with a propagated bending motion

  • (like a wave)

• Force generated is parallel to the flagellum

Axoneme in Cilia/Flagella

Cilia and flagella is made of an axoneme connected to the basal body

• In all regions the microtubular plus end points towards the tip and the minus ends point to the base

• Cilium/flagellum emerger from a basal body like a centrosome

• Its surrounded by an extension of the cell membrane

• There is a transition zone between axoneme and basal body

  • Its where the microtubule takes on the characteristic pattern of axoneme

Basal Bodies

It is like the MTOC

• consists of 9 triplets MTs

• act as a nucleation site for MTs

Cross section of axoneme

• Axonemes have a "9+2" pattern with 9 outer doublets (A and B tubules)

  • 1 doublet is complete with 13 subunits

  • 1 incomplete microtubule, 10 or 11 subunits

• 2 MTs in the centre, central pair

• Doublet are connected to one another by a bridge composed of an elastic nexin link

• Axonemal dyneins project from the complete MT as pair of arms

Intraflagellar Transport

• Assembly and disassembly of the cilium and flagellum require the transport of material to and from the distal

• Movement of structural components is intraflagellar transport which occurs between the peripheral doublets and the cell membrane

• IFT proteins assemble as linear trains to carry cargo

  • Kinesin-2 a plus-end-directed motor pulls the IFT trains towards the cilium/flagellum

  • Cytoplasmic dynein (minus-end directed motor)  returns IFT trains to the cell body

Bending of Axoneme

Axonemal dynein is involved in the sliding of MTs against each other

• The stem of each axonemal dynein molecule is anchored to outer surface of A tubule (complete one)

• Globular heads/stalk point towards B tubule of the neighboring doublet

• The minus-end directed motor axonemal dynein exert force of the neighboring microtubule (B), it pulls the A tubule to the minus end

  • Like if you are on a boat in the pool (boat is Tubule A) you hold onto the wall (tubule B) to pull yourself along the wall (towards the minus end)

• dynein produces microtubule sliding

  • Linkages holding neighbouring microtubules together are broken. The addition of ATP allows the motor action of dynein heads to slide one pair of doublet MT against the other pair

• Dynein causes MT bending

  • An intact axoneme flexible protein linkage prevent the sliding of the doublet

  • The actual motion action is a bending motion

At any given time the axonemal dynein on one side of teh axoneme are active while the other side is inactive

Primary Cilia an Non-motile cilia

• Primary cilia is used in sensory structures

• Important in development; defects can cause disorders like deafness and left-right asymmetry reversals

• Non-motile cilia does not have the central pair of MT that the dynein links to is it are unable to move

Motile Cilia Functions in the body

Respiratory system and fertility in reproductive system

Non-motile Function in the body

• Eyes

• Smell (nose)

• Hearing  (ears)

• Skeletal system

• Reproductive

• Brain

• Kidney

• liver