10. MODULE 10.docx

**Motor Proteins** Move on cytoskeletal filaments by using ATP energy Carry membrane-enclosed organelles and assist the sliding of cytoskeletal filaments against each other Associate with filaments through a "head" region (motor domain) that binds/hydrolyzes ATP Move through a cycle: fil...

**Motor Proteins** Move on cytoskeletal filaments by using ATP energy Carry membrane-enclosed organelles and assist the sliding of cytoskeletal filaments against each other Associate with filaments through a "head" region (motor domain) that binds/hydrolyzes ATP Move through a cycle: filament binding → conformational change → filament release → conformational relaxation → filament re-binding Track and direction of movement are determined by the motor domain; type of cargo is determined by the tail Three groups **Actin-based Motor Proteins: Myosins** Myosin II in skeletal muscle has two heavy chains (green) and four light chains (blue) Light chains are of two types: one copy of each type is present at each myosin head Two heavy chains wrap around to form a coiled-coil tail Myosin II molecules aggregate by their tails to form bipolar filaments, with heads projecting to the outside of the filament and the bare zone in the center consisting of tails Opposing head orientation in a myosin filament allows for sliding of oppositely oriented actin filaments **Actin-based Motor Proteins: Myosins (Additional)** Many types: heavy chains have a motor domain (N-terminus), and there is a variety of C-terminal tails All myosins (except myosin VI) move toward the plus end of an actin filament Myosin II: contraction in muscle and non-muscle cells; cytokinesis, forward cell migration **Microtubule Motor Proteins: Kinesins** Kinesin moves toward the plus end of microtubules Functions: (1) Anterograde transport of organelles, protein complexes, and vesicles along microtubules in an ATP-dependent manner; (2) Organization of chromosomal and mitotic spindle movements Two heavy chains and two light chains (two motor domains, and a coiled-coil) Many kinesin-related proteins (KRPs) bind microtubules via the motor head domains and cargo via the light chains (tails) **Microtubule Motor Proteins:** Dyneins: minus end-directed (retrograde), the largest and fastest molecular motors 2-3 heavy chains with motor domains + variable number of light chains Cytoplasmic dyneins: heavy-chain homodimers with two large motor domains as heads; required for vesicle trafficking, localization of the Golgi apparatus Axonemal dyneins: heterodimers and heterotrimers, with two or three motor-domain heads; used for sliding movements of microtubules in cilia and flagella **ATP Hydrolysis and Conformational Changes: Myosins** Myosin II is tightly bound to the actin filament with no associated nucleotide (the \'rigor\' state) ATP binds and releases the head from the filament ATP hydrolysis occurs while the myosin head is detached from the filament, causing the head to assume a cocked conformation, with ADP and inorganic phosphate still bound to the head The head rebinds to the filament; the release of phosphate and ADP triggers the power stroke that moves the filament ATP binding releases the head to allow the cycle to begin again (head bound to actin for \~5% of the time) **ATP Hydrolysis and Conformational Changes: Kinesins** The front head (dark green) is bound to the microtubule, with the rear head (light green) detached. Binding of ATP to the front head causes the rear head to be thrown forward, past the binding site of the attached head, to another binding site further toward the plus end of the microtubule. Release of ADP from the second head (now at the front) and ATP hydrolysis on the first head (now in the rear) brings the dimer to the original state, but the two heads have switched their positions, and the motor protein has moved ahead; in this cycle, each head spends about 50% of its time attached to the microtubule. **Functions of Motor Proteins** Kinesins and dyneins move in a processive fashion: kinesin travels for hundreds of ATPase cycles along a microtubule without dissociating. Myosin does not have processivity, but it has speed; many motorheads interact with the same actin filament. Microtubule arrays have minus ends at the centrosome, plus ends in the cell's periphery; therefore, organelle movement to the cell center requires minus end-directed motor proteins (dyneins); movements to the periphery require plus end-directed motors (e.g., kinesins). **Regulation of Motor Proteins** Signals regulate the attachment and activity of motor proteins Example: Phosphorylation controls the two myosin II conformations **Muscles** Convert chemical (ATP) into mechanical energy; can contract Types: skeletal, cardiac, and smooth muscle **Muscle Contraction** ATP-driven sliding of actin (thin) filaments against myosin II (thick) filaments. Muscle fibers in skeletal muscle are large, single cells formed by the fusion of multiple cells, with the cytoplasm containing myofibrils. A myofibril is a cylindrical structure consisting of repeated contractile units known as sarcomeres. At each end of the sarcomere is a Z disc, which serves as an attachment site for the plus ends of actin filaments. The M line is where proteins link adjacent myosin II filaments to one another. Sarcomeres shorten as myosin filaments slide past actin filaments, a movement driven by interactions between myosin heads and actin filaments. **Ca2+ Initiates Muscle Contraction** A nerve signal triggers an action potential in the muscle cell's plasma membrane, which spreads to the transverse tubules (T-tubules). Voltage-sensitive Ca2+ channels in the T-tubule membrane open, triggering the opening of Ca2+-release channels in the sarcoplasmic reticulum (SR), leading to a wave of Ca2+ entering the cytosol. The increase in Ca2+ is transient, as Ca2+ is rapidly pumped back into the SR by Ca2+-ATPase. In the resting state, the troponin I-T complex pulls tropomyosin out of its normal binding groove on actin, preventing myosin heads from binding. When cytosolic Ca2+ binds to troponin C, it prompts troponin I to release its hold on actin, allowing tropomyosin to move and enabling myosin heads to bind to actin filaments. **Putting It Together** Motor proteins use ATP hydrolysis to move along microtubules or actin filaments. Motor proteins mediate the sliding of filaments relative to one another and the transport of organelles, complexes along filaments. Myosins move on actin filaments; kinesins and dyneins move on microtubules. Myosins and kinesins have a motor head domain; the heads can be attached to a variety of tails. The tails (light chains) attach to cargo and enable various functions. Dynein complexes are larger and more complex than kinesins and myosins. Dyneins are composed of two or three heavy chains and a variable number of light chains. Myosin and actin in sarcomeres power muscle contractions.