Lec 17 Kinesins + Dyenins

Overview of motor proteins

  • Motor proteins convert chemical energy (ATP) into mechanical energy (force/movement).
  • Molecular motors move unidirectionally along cytoskeletal tracks in a stepwise manner.
  • Three broad superfamilies of motor proteins: Kinesins, Dyneins, and Myosins. (No motor uses intermediate filaments as tracks.)
  • Major roles include:
    • Fast axonal transport in neurons
    • Sliding of microtubules in cilia and flagella
    • Transport of membrane organelles and mitosis-related movements
  • Cytoskeletal tracks:
    • Microtubules (MTs) and microfilaments (actin filaments) support motor movement
    • Intermediate filaments are not used as tracks
  • Core terms:
    • Anterograde transport: movement toward MT plus ends
    • Retrograde transport: movement toward MT minus ends
    • Processivity: ability of a motor to take many steps along a track without detaching

Kinesins

  • Kinesins are a family of plus-end–directed motors that primarily move organelles and vesicles along MTs toward the MT plus ends.
  • Structure of a typical kinesin-1 motor (tetramer):
    • Head region: globular motor domain that binds MTs and hydrolyzes ATP (ATPase engine)
    • Neck region: connects head to stalk
    • Coiled helical stalk: provides flexibility during movement
    • Light-chain region: attaches kinesin to cargo (proteins, organelles, or other cargo)
  • Kinesin-1 is a tetramer composed of 2 heavy chains and 2 light chains.
  • Evolution and diversity:
    • Composed of ~14 kinesin-related proteins (KRPs) families
    • Classified into families based on structural features; can form homodimers, heterodimers, or tetramers
    • Kinesin-14 is an exception: minus-end directed motor
    • KIF5A, KIF5B, KIF5C are genes encoding Kinesin-1 in mammals
  • Directionality:
    • Most kinesins are plus end–directed motors
    • Kinesin-14 is minus end–directed
  • Key properties of kinesin-1:
    • Step size: 8 nm per step along a MT (one tubulin dimer to the next)
    • Mechanism: “hand-over-hand” movement with two head domains taking turns as the leading head, powered by ATP hydrolysis
    • High processivity: can move long distances along MTs before detaching; steps can repeat without full detachment
    • Movement is coupled to ATP hydrolysis, ensuring directional stepping
  • Functional roles:
    • Organellar transport and maintaining correct organelle localization in many cell types
    • Identification historically from squid giant axon studies; primary motor for anterograde transport in neurons
  • Motor-domain architecture:
    • N-terminal motor domain (head) binds MTs and catalyzes ATP hydrolysis
    • Neck linker and stalk contribute to stepping mechanics
    • C-terminal tail (varies by cargo) interacts with cargo adaptors and receptors
  • Kinesin-1: classic motor often cited as KIF5 family (KIF5A/B/C)
  • Kinesin-1 vs other kinesins: tail diversity allows cargo specificity; motor domain conservation supports similar ATPase mechanism
  • Related kinesin families (examples and roles):
    • Kinesin-3: monomer; moves synaptic vesicles in neurons
    • Kinesin-5: bipolar tetramer; drives bipolar MT sliding during mitosis (spindle dynamics; spindle elongation)
    • Kinesin-6: involved in completion of cytokinesis
    • Kinesin-13: catastrophins; destabilize MT plus ends
    • Kinesin-14: minus-end directed motors; contribute to spindle dynamics in meiosis/mitosis
  • Structural notes:
    • Kinesin-1 has 500 amino acids in the motor domain region mentioned in some references; overall protein length varies by family
    • KRP tail regions are highly divergent, reflecting cargo diversity and specialization
  • Movement directions along MTs:
    • Plus-end–directed kinesins move toward MT plus ends; minus-end–directed kinesins (e.g., kinesin-14) move toward MT minus ends

Dyneins

  • Dyneins are large motor proteins that move toward MT minus ends (minus-end directed).
  • Major class: Cytoplasmic dynein
  • Structure:
    • Two identical heavy chains containing the force-generating motor domains and MT-binding sites; heavy chains form the core motor units
    • A stalk that projects toward the MT- binding region
    • Tail domain important for interactions with adaptor proteins
    • Multiple light and intermediate chains that assist in regulation and cargo binding
    • AAA+ ATPase domains (AAA1–AAA6) form the catalytic engine (ATP hydrolysis drives conformational changes)
  • Cargo interaction:
    • Dynein does not bind cargo directly; it requires adaptor molecules such as dynactin and spectrin to link cargo to the motor
  • Regulation and mechanism:
    • ATP hydrolysis induces conformational changes in the motor domain, particularly in the linker region that connects the motor domain to the MT-binding site
    • The motor domain’s activity enables processive movement toward MT minus ends
  • Roles in cells:
    • Central to retrograde transport in cells and critical for positioning the spindle and moving chromosomes during mitosis
  • Notable structural features (from references):
    • The dynein motor is large and complex, with multiple subunits and regulatory components
    • The dynactin complex acts as a critical adaptor for cargo linkage

Cargo movement and motor cooperation

  • Motors on the same MT can engage in tug-of-war for shared cargo:
    • Kinesin (plus-end directed) vs. dynein (minus-end directed) interactions can bias cargo movement
    • Some organelles can bind both kinesin and dynein, enabling regulated bidirectional transport or stalling depending on motor engagement
  • In neurons and other cells, distinct motor assignments coordinate directional transport:
    • Anterograde transport (cell body toward axon terminal) is typically kinesin-1–driven
    • Retrograde transport (axon terminal toward cell body) is dynein-driven
  • Conceptual takeaway: the same microtubule network supports opposing directional flows mediated by different motor proteins with cargo adaptors guiding specificity

Cilia and flagella: structure and function

  • Cilia and flagella are motile cellular appendages built on a shared structural basis
  • Cilia:
    • Typically 2–10 µm long; many cells have multiple cilia
    • Beat in an oar-like pattern, generating force perpendicular to the cilium; motile cilia move fluids (e.g., mucus transport in airways)
    • Non-motile (primary) cilia act mainly as sensory antennas and are important in development
  • Flagella:
    • Similar diameter to cilia but much longer (up to ~200 µm)
    • Usually one or a few per cell and move with a propagated bending motion; force generated is parallel to the flagellum, propelling the cell through fluid
  • Structure common to both:
    • Axoneme: central MT-based core
    • Basal body: basal MTOC that nucleates MTs and anchors the axoneme to the cell
    • Transition zone between axoneme and basal body marks MT pattern changes toward the axoneme
  • Basal bodies:
    • Microtubule organizing center (MTOC) composed of nine triplets of MTs that nucleate MTs for the cilium or flagellum
  • Axoneme architecture:
    • Classic "9+2" pattern: 9 outer doublets (A and B tubules) and 2 central MTs (central pair)
    • Each doublet comprises a complete A tubule (13 protofilaments) and an incomplete B tubule (10–11 protofilaments)
    • Doublets are linked by nexin links (elastic bridges)
    • Outer arm dyneins project from the A tubule toward the neighboring B tubule, enabling sliding between doublets
  • Axonemal dynein function:
    • Dynein arms generate sliding between adjacent MT doublets; when sliding is constrained by nexin links, bending occurs, producing ciliary/flagellar beating
  • IFT (intraflagellar transport):
    • Essential for assembly/disassembly by transporting building blocks to and from the distal tip
    • IFT trains carry cargo such as tubulin and nexin; movement occurs along MTs inside the cilium/flagellum
    • Anterograde transport to the tip is driven by kinesin-2 (plus-end directed)
    • Retrograde transport back to the cell body is driven by cytoplasmic dynein (minus-end directed)
  • Growth and maintenance:
    • Proteins synthesized in the cell body must be transported to the distal tip by IFT for assembly

Intraflagellar transport (IFT)

  • IFT is the bidirectional transport system within cilia/flagella that sustains their structure and function
  • Anterograde IFT:
    • Motor: kinesin-2 (plus-end directed)
    • Carries IFT particle complexes and cargo toward the distal tip
  • Retrograde IFT:
    • Motor: cytoplasmic dynein (minus-end directed)
    • Returns IFT trains to the cell body for recycling
  • IFT trains assemble as linear cargo-carrying units and require coordinated motor activity for proper cilium/flagellum growth and maintenance

Cilia in health and disease; non-motile vs motile cilia

  • Primary (non-motile) cilia have sensory roles and are critical in development
  • Defects in cilia can lead to a spectrum of ciliopathies, including deafness and left-right asymmetry defects
  • Humans have diverse ciliopathy-related disorders, including Bardet-Biedl syndrome and polycystic kidney disease (PKD) among others
  • Proper dynein function and IFT are essential for correct ciliary beat patterns and developmental signaling

Key structural and functional concepts to remember

  • Directionality and tracks:
    • MTs serve as tracks for kinesins and dyneins with defined plus-end and minus-end directions
  • Motor domain and cargo-binding domains:
    • Motor domain (ATPase) drives movement; tail/cargo-binding domains determine cargo specificity
  • Movement mechanisms:
    • Kinesin-1 uses a hand-over-hand stepping mechanism with a fixed 8 nm step size per ATP hydrolysis cycle
    • Dyneins use ATP-driven conformational changes across AAA+ domains to generate motile force toward MT minus ends
  • Axoneme structure and beating:
    • The 9+2 axoneme and dynein arms generate controlled sliding between MT doublets, producing bending motion
  • IFT as a growth mechanism:
    • Anterograde transport delivers building blocks to the distal tip; retrograde transport recycles components and returns used material

Notable numerical references for quick recall

  • Kinesin-1 step size: s=8 nms = 8~\text{nm} per step
  • Fast axonal transport rate (observed): about 2 μm/s2~\mu\text{m/s}
  • Axoneme pattern: 9+29+2 MT arrangement
  • Basal body organization: nine triplet MTs
  • Cilium length range: 2−10 μm2-10~\mu\text{m}; Flagellum length can reach up to 200 μm200~\mu\text{m}

Final notes for exam preparation

  • Be able to contrast kinesin and dynein motor directionality and basic mechanism
  • Understand cargo transport concepts: plus-end vs minus-end transport, processivity, and the tug-of-war concept
  • Explain the structural organization of kinesins (head, neck, stalk, tail) and why tail diversity is important
  • Describe the key kinesin families and their cellular roles (KIF5A/B/C as kinesin-1; Kinesin-5, Kinesin-13, Kinesin-14, etc.)
  • Describe dynein’s large multi-subunit architecture, the role of dynactin/spectrin adaptors, and why cargo association is indirect
  • Understand cilia and flagella structure (axoneme, basal body, transition zone) and the 9+2 MT arrangement
  • Explain IFT: anterograde transport by kinesin-2 and retrograde transport by cytoplasmic dynein; its role in cilium growth and maintenance
  • Recognize physiological relevance and disease associations linked to cilia and flagellar dysfunction