Transcript L 9 Myosin
Cytoskeltal Motors Network of long protein strands located in the cytosol not surrounded by membranes Consist of microtubules and microfilaments Microfilaments protein threads of actin cell movement and muscle contraction Microtubules : help in movement of organelle around made up of tubulin longest strands of cytoskeleton make up spindle fibers (role in mitosis and meiosis) Cytoskeletal motors Motor proteins utilizing the cytoskeleton for movement fall into two categories based on their substrates: •Actin motors such as myosin move long microfilaments through interaction with actin. •Microtubule motors such as dynein and kinesin move along microtubules through interaction with tubulin. Cytoskeletal motors – Myosin is responsible for muscle contraction – Kinesin moves cargo inside cells away from the nucleus along microtubules track. – Dynein transports cargo along microtubules towards the cell nucleus. MYOSIN MOTORS Structure of a Muscle Cell • Vertebrate muscle that is under voluntary control has a banded (striated) appearance when examined under a microscope. • It consists of multinucleated cells that are bounded by plasma membrane. • A muscle cell contains many parallel myofibrils, each about 1 mm in diameter. • The functional unit, called a sarcomere, typically repeats every 2.3 mm (23,000 Å) along the fibril axis in relaxed muscle Structure of Muscle Cell Structure of a Muscle Cell Skeletal muscle myofibril showing a single sarcomere • A dark A band and a light I band alternate regularly. The central region of the A band, termed the H zone, is less dense that the rest of the band. The I band is bisected by a very dense, narrow Z line. Sarcomere • A sarcomere of a myofibril consists of two kinds of interacting protein filaments. • The thick filaments have diameters of about 15 nm (150 Å) and consist primarily of myosin. • The thin filaments have diameters of approximately 9 nm (90 Å) and consist of actin as well as tropomyosin and the troponin complex. Skeletal muscle myofibril showing a single sarcomere Sliding-Filament Model • Muscle contraction is achieved through the sliding of the thin filaments along the length of the thick filaments, driven by the hydrolysis of ATP . Sliding-Filament Model • Tropomyosin and the troponin complex regulate this sliding in response to nerve impulses. • Under resting conditions, tropomyosin blocks the intimate interaction between mysosin and actin. • A nerve impulse leads to an increase in calcium ion concentration within the muscle cell. • A component of the troponin complex senses the increase in calcium and, in response, relieves the inhibition of myosin - actin interactions by tropomyosin. Thick Filament: Myosin head domains at each end and a relatively narrow central region Interaction of thick and thin filaments in skeletalmuscle contraction Actin Is a Polar, Dynamic Polymer • Actin monomers (often called G-actin for globular) come together to form actin filaments (F-actin filament). • The structure is polar, with discernibly different ends. One end is called the barbed (plus) end, and the other is called the pointed (minus) end. Myosin Power Stroke • Individual myosin heads bind the actin filament and undergo a conformational change (the power stroke) that pulls the actin filament. • After a period of time, the myosin head releases the actin, which then snaps back into place. • The complete cycle of ATP-binding, hydrolysis, and phosphate release is called the "power stroke" cycle • All myosins are composed of a globular catalytic head domain, a converter and a lever arm. • Conformational changes in the catalytic head are amplified by the lever arm during the ATPase cycle through an ~ 70° rotation of the lever arm. Myosin motors are plus end motors The 'powerstroke' cycle of (+)-end-directed myosins. (Myosin II, Myosin V motor) Step 1: At the end of the previous round of movement and the start of the next cycle, the myosin head lacks a bound ATP and it is attached to the actin filament in a very short-lived conformation known as the 'rigor conformation'. Step 2: ATP-binding to the myosin head domain induces a small conformational shift in the actin-binding site that reduces its affinity for actin and causes the myosin head to release the actin filament. Step 3: ATP-binding also causes a large conformational shift in the 'lever arm' of myosin that 'cocks' the head into a position further along the filament. ATP is then hydrolysed, but the inorganic phosphate and ADP remain bound to myosin. Step 4: The myosin head makes weak contact with the actin filament and a slight conformational change occurs on myosin that promotes the release of inorganic phosphate. Step 5: The release of inorganic phosphate reinforces the binding interaction between myosin and actin and subsequently triggers the 'power stroke'. Step 6: As myosin regains its original conformation, the ADP is released, but the myosin head remains tightly bound to the filament at a new position from where it started, thereby bringing the cycle back to the beginning. The complete cycle of ATP-binding, hydrolysis, and phosphate release is called the "power stroke" cycle Refer to following link for animation: http://highered.mcgrawhill.com/sites/0072495855/student_ view0/chapter10/animation__breakd own_of_atp_and_crossbridge_movement_during_muscle_c ontraction.html