CELLULAR MOVEMENT

  1. Microtubule-based movements

    • Kinesins and Dyneins in intracellular cargo transport

    • Cilia and Flagella in cellular movements

  2. Microfilament-based movements

    • Myosin motor proteins

    • Muscle cell contractility

    • Non-muscle cell motility

—> Contractility is also a type of motility. Contractility is the shortening of the muscle cells.

—> Motility occurs at the tissue, cellular, and sub-cellular levels.

—> Movement occur due to the motor proteins that attached themselves to the scaffolding provided by microtubules and microfilament in cells.

Motor proteins

  • convert chemical energy (ATP) into mechanical energy by changing their shape and attachment to scaffolds.

  • They also undergo cycling of ATP-binding, hyrdolysis, and ADP release and this process allows the motors to attach, move and detach over and over again as they move along a filament.

—> “Outbound”- Transport towards the plus end.

—> “Inbound”- Transport towards the minus end.

Microtubule-Associated Motor Proteins- Facilitate traffic along microtubules.

—>Dyneins are inbound, they transport towards the minus end.

—>Two types of Dyneins: Cytoplasmic and Axonemal dyneins.

  • Cytoplasmic Dyneins moves cargo toward minus ends. It also catalyzes inbound(minus end) ATP-dependent transport, which is involved in Retrograde axonal transport.

  • Axonemal Dyneins activates the microtubule sliding in flagella and cilia.

—>Kinesins are outbound, they transport towards plus end.

  • Kinesin-1 catalyzes outbound(plus end) ATP-dependent transport, which is involved in Anterograde axonal transport.

  • Kinesin-3 is a monomer that moves synaptic vesicles in neurons. (outbound movement)

  • Kinesin-5 is responsible for the bidirectional sliding of the microtubules during anaphase of mitosis. (bipolar movement).

  • Kinesin-6 completes the cytokinesis.

  • Kinesin-13 is a catastrophin that destabilize the plus ends of microtubule.

  • Kinesin-14 helps with the formation of the spindle in meiosis and mitotic and moves towards minus ends of microtubule.

KINESIN WALKING

1. The leading heavy chain binds to ATP.

2. ATP binding causes conformational change and trailing heavy chain swings forward.

3. Trailing heavy chain finds new MT binding site.

4. New leading heavy chain releases ADP. New trailing head hydrolyzes ATP to ADP + pī. and pí is released.

  • Kinesin-1:

    • Forms as a heterotetramer of 2 light chain and 2 heavy chain proteins.

    • Binds only to beta-tubulin subunits in 8nm increments.

    • Have high processivity.

    • Converts energy from ATP into work.

  • The heavy chain globular domains will bind and hyrolyze ATP for the step-wise walking movement to occur.

-AAA+: ATPase Associated with diverse cellular Activities.

-The rotation of the AAA+ drives the lever arm to pull the cargo in the direction of the minus-end of MT.

ENDOMEMBRANE TRANSPORT

-Kinesins play a role in anterograde [forward] transport which flows through ER→Golgi→Plasma Membrane.

-Dyneins play a role in the retrograde [backward] transport which flows through Plasma Membrane→ Golgi→ ER.

AXONEME STRUCTURE

-Axoneme is a bundled doublet microtubule shared by cilia and flagella.

-The axoneme is anchored at the base by a basal body.

-Axonemes are constructed in a ring of doublet microtubules tethered by radial spokes.

  • 9+2 is a ring of 9 doublets around a central pair.

  • 9+0 is a ring of doublets that lacks a central pair, found in sensory cilia.

  • A tubule= complete ring.

  • B tubule= incomplete ring.

→Adjacent mt doublets are linked by axonemal dynein complexes.

→Mutations in axonemal dyneins can lead to the dysfunction of the cilia which is a condition called situs inversus totalis-it is the mirroring of symmetry of organs.

→The 3-bead experiment measure the force exerted by motor proteins against a microfilament tethered between two ‘trapped’ beads.

→Atomic Force Microscopy uses tiny probes to scan across the surface of a sample.

→Optical Laser Trapping uses lasers to trap small particles.

MICROFILAMENT-BASED MOTILITY

  • Myosin II- slides microfilaments in muscle. Its involved in muscle contractility.

  • Myosin V- Vesicle positioning and trafficking.

  • Myosin VI- Involves in endocytosis; moves toward minus ends of microfilament

→ Both kinesin and myosin are highly efficient motors converting chemical energy (ATP) to work

→Myosins operate in large quantities and typically exhibit low processivity, while kinesins operate alone or smaller groups and shows a higher processivity.

→ A contractile unit in muscle cells called sarcomere has parallel fibres of thin filaments of actin and thick filaments of myosin II slide against each other due to the ATPase activity of myosin II.

SKELETAL MUSCLE STRUCTURE

→Muscle fibres are bundles of elongated multinuclear muscle cells.

  • Each muscle cell contains bundles of myofibrils→Myofibrils contains thick myosin filaments and thin actin filaments in a repeating contractile units of sarcomeres.

    →muscle→muscle fibres→single muscle fibres→ myofibrils→ single myofibril

→The globular domains of myosin II heavy chain interact with the adjacent actin thin filaments.

Sarcomere Structure

-Striated muscle is the repeating units of sarcomeres that generates the striated banding pattern to skeletal and cardiac muscle.

→ “I bands” are isotropic

→ “A bands” are anisotropic- Represents the contractile structure and contains the M (myomesin)

Thin Filaments

→Troponins (Tn) bind along the filament and regulate myosin interactions.

  • Tnl- (Inhibitory) binds actin

  • Tnc- Ca2+ binding

  • Tnt- tropomyosin binding

→ Troponin complex blocks the myosin binding site until Ca2+ signal is received.

Sarcomere Structural Proteins

→ all actin filaments are oriented with the plus end anchored at the Z-line.

→CapZ stabilize the plus end of the filament and bind to the alpha actinin to anchor the filament to the Z-line.

→Myomesin anchors the thick filaments at the M-line.

→Nebulin stabilizes the thin filament assembly and links thin filaments to Z-line.

→G-actin is the major component of thin filaments.

→Titin links the thick filaments to Z-line.

→Neurons release acetylcholine at the neuromuscular junction which triggers the depolarization of the muscle cell membrane.

→Ca2+ is pumped back into the sarcoplasmic reticulum by the Ca2+ ATPase to reduce the contraction.

→Voltage-gated Ca2+ channels triggers the ryanodine receptors to release Ca2+ from the Sarcoplasmic reticulum.

→ Cardiac cells are directly electrically coupled through intercalated discs that contains GAP junctions.

SMOOTH MUSCLE

→Non-striated muscles

→Responsible for slow and prolonged contractions.

→smooth muscle contractions is Ca2+ regulated.

→Single cell

→Contractile bundles formed actin and myosin.

→Anchored at densed bodies to intermediate filaments.

→Involuntary contractions

→Inactivated by dephosphorylation by myosin light chain phosphate.

SKELETAL MUSCLE

→Multinuclear cells

→Sarcomeres formed from actin and myosin

→Anchored to adjacent sarcomeres at Z-lines to form a continous myofibril

→Activated by Ca2+ binding to troponins to unblock myosin binding sites on thin filaments.

→ Inactivated by Ca2+=ATPase removing Ca2+ to the Sarcoplasmic reticulum.

→Voluntary contractions

CYTOPLASMIC STREAMING

→Actin cytoskeleton is involved in the streaming of organelles through the cytoplasm.

→Cytoplasmic streaming depends on actin-myosin dynamics.

→Actin-myosin drives retrograde flow of actin fibres from the leading edge for disassembly

→Actin-myosin provides tension and pull the trailing cell towards the new point of contact while breaking the trailing contacts with the substrates.