Cytoskeleton: Microtubules Lecture 14

Cytoskeleton: Microtubules 2 BIOL2020 Microtubules are made of alpha and beta tubulin heterodimers ● ● Microtubules have polarity They are hollow tubes Classic kinesins are motors that move towards the plus ends of microtubule Kinesin is a tetramer protein • • • • Two heavy chains and two l...

Cytoskeleton: Microtubules 2 BIOL2020 Microtubules are made of alpha and beta tubulin heterodimers ● ● Microtubules have polarity They are hollow tubes Classic kinesins are motors that move towards the plus ends of microtubule Kinesin is a tetramer protein • • • • Two heavy chains and two light chains Heavy chain has many domains Head splits ATP and converts the energy into motion Tail is the cargo-binding The movement of kinesin molecules is for long-range • Step size (4 tubulin monomers) • at least one of the heads is always attached to the microtubule Progressive steps (all the same size) See kinesin dynamics by a kinesin in-vitro motility assay • • • Kinesins are deposited onto a coverslip, microtubules labeled with a fluorescent marker Motor activity of the kinesins causes the microtubules to glide kinesins are a large protein superfamily in which the motor domain of the heavy chain is the common element Kinesin-13 proteins (microtubule depolymerases) induce depolymerization uniquely from both ends of the microtubule. • • are incapable of movement. regulate microtubule dynamics to control spindle assembly kinesin-14 (Ncd) is unusual as it moves from microtubule plus-ends towards the minus-ends in motility assays The tail of kinesin-14 can bind microtubules and allows it to organize microtubule bundles Cytoplasmic Dynein is another microtubule motor protein • • • • • Dynein is ~4 times bigger and more complex than kinesin Head is force generating motor(AAA =ATPase domain) stalk contains the microtubule‐binding site at its tip Tail binds cargo ATP changes the conformational structure to dissociate microtubule binding Dynein steps are big but irregular and it moves toward the minus ends Cytoplasmic Dynein moves toward the minus ends Ron Vale Discovered Kinesin https://www.youtube.com/watch?v= lVwKiWSu8XE&t=328s Comparison Dynein and Kinesin classical Kinesin cytoplasmic Dynein Small Big Towards plus end Towards minus end ATP driven ATP driven Regular steps Irregular movements Microtubule motors move vesicles in the secretory pathway kinesin‐mediated and dynein‐mediated transport of vesicles, vesicular‐tubular clusters, and organelles Evidence of kinesin transport of organelles Normal microtubules Kinesin mutant microtubules • Normal mitochondria Kinesin mutant mitochondria Kinesin transport mitochondria and other organelles to the periphery The rapid movement of melanosomes is mediated by dynamin and microtubules • • The melanosome organelle synthesizes and stores melanin Coordinated movement because microtubules are uniformly polarized Axonal transport of organelles Cell body axon • • synapse Neurons transport vesicles with neurotransmitters to the synapse a vesicle contains both types of motor proteins Motor proteins can be inactivated to get them back These are moving mitochondria Kinesin must be inhibited for minus-end transport • Kinesin-binding protein prevents Kinesin-microtubule binding Organelle transport goes a long way ~3 mm motoneurons Brain axons muscles • Synaptic vesicle speed = 5 μm/second or 0.4 m/day Both cilia and flagella are hairlike cell appendages that have a bundle of microtubules at their core. ● ● Flagella are found on sperm and many protozoa and have an undulating motion. Cilia beat with a whiplike motion The core of the cilium and flagellum is called the axoneme, which is composed of microtubules and their associated proteins, Axonemal dynein bends the axoneme which move the cilium and flagellum Summary Kinesin • Kin1 Progressive plus end movement • Kin1 tetramer • Large diversity of functions • Dynein Organelle transport • Axons • Melanosomes • Link to secretory pathway Axoneme • Axonemal Dynein • Flagellum • Cilia Dynein • Large many subunits • Irregular steps • Towards the minus ends