Chapter III - Cytoskeleton And Cell Motility
Items 183-186
Examine the electron micrograph of a longitudinal section of skeletal muscle in Tissue-culture microscope 3.1 below and then match the lettered structure on the micrograph with the most appropriate description of this structure in the items below.
183. This structure contains an enzyme whose ability to release ADP and Pi is the rate limiting step in muscle contaction.
184. The protein actin is the principal component of this structure.
185. When Ca 2+ binds to troponin here, it facilitates cyclic interactions between mysosin and actin.
186. Glucose-6-phosphate is produced from material stored here.
Item 187-191
Choose the lettered protein component of skeletal muscle that is the most appropriate much with the numbered item below the list of answers.
(A) ?-actin
(B) Tropomyosin
(C) Troponin-T
(D) Actin
(E) Myosin
(F) Calsequestrin
(G) Troponin-C
(H) C-protein
(I) Dystrophin
(J) Troponin-I
187. This protein serves to anchor the ends of thin filaments at the Z-line.
188. This protein is a rod-like molecule, consisting of two nearly identical ?-helical subunits. It binds to 7 actin monomers within a thin filament.
189. Thick filaments are held together at the M-line via this protein.
190. This protein shows extensive sequence homology to calmodulin.
191. This protein has as subunit MW = 42,000 and binding sites for ATP, mysosin, tropomyosin and troponin.
Items 192-194
Choose the best response.
192. The coupling of verve cell excitation to muscle fiber contraction involves all of the following events EXCEPT:
(A) release of acetylcholine from vesicles that fuse with the presynaptic membrane
(B) binding of acetylcholine to receptors
(C) hydeolysis of acetylcholine by tropomyosin
(D) opening of ion channels within the sacrolemma
(E) conduction of an anction potential alon- t-tubule membranes
193. When skeletal muscle is to stimulated to contract, which event occurs fist?
(A) troponin C dinds Ca2+
(B) myosin cross bridge attach to actin
(C) actin hydrolyses ADP
(D) tropomyosin dissociates from myosin
(E) Ca2+ diffuses out of mitochondria
194. When skeletal muscle relaxes following contraction, all of the following occur EXCEPT:
(A) release of myosin molecules from think filaments
(B) reduction of free Ca2+ concentration in the sarcoplasm
(C) uptake of Ca2+ by the sarcoplasmic criatine and ATP
(D) release of lactate into the bloodstream
ANSWER AND TUTORIAL OF ITEMS 183-194
The answers are: 193-B; 184-A; 186-E; 187-A; 188-B; 189-H; 190-G; 191-D; 192-C; 193-A; 194-A Tissue-culture microscope 1.3 show that the contractile apparatus of skeletal muscle is organized in cylindrical structures called myofibrils. These consist of interdigitating arrays of thick (B) and (A) filaments. The principal component of thick filaments is myosin, a protein consisting of two heavy chains (MW ? 200,000) and four smallest light chains. Myosin molecules have a rod-like that associates with other myosin tails to form the shaft of the thick filaments. The “heads” of myosin molecules protrude from the shaft of thick filaments forming cross bridge that transiently associate with the action of the thin filaments. The letters C. D and E label Z-lines, M-line, and glycogen granules, respectively. Toolmakers Microscopes USB Computer Connected Microscopes Video Eyepiece Cameras. Actin (D) has a subunit MW = 42,000. The globular (G-) actin subunits bind ATP. When G-action polymerizes to filamentous (F-) actin, the bound ATP is hydrolysed to ADP. While this hydrolysis is important in the functioning of non-muscle actins, it appears to play no role in muscle contraction. In addition to the nucleotide binding site, actin has biding sites for myosin and number of other proteins. The myosin heads bind ATP and rapidly hydrolyse it is ADP and inorganic phosphate (P). They energy liberated by splitting the terminals phosphate bond in the ATP is stored in an altered configuration of myson (E) and is only released when myosin heads can interact with actin in the adjacent thin filaments. When myosn heads bind actin, they release the ADP and Pi and the stored energy is used to drag thin filament past thick filaments, thus doing work as the myofibrils shorten. The sliding movement of thick and thin filaments over on another is what is referred to as the sliding filament model of skeletal muscle contraction. The normal functioning of myofibrils is dependent on several proteins in addition to actin and myosin. Thin filaments are anchored to Z-discs via interaction with the proteins ?-actinin (A), while C-protein (H) forms cross-lick between thick filaments, maintaining them in register.
In resting muscle, interaction of actin and myosin is inhibited by the proteins tropomyosin (B) troponin. Tropomyosin is a rod-like molecule made up of two nearly identical ? helical polypeptides (MW = 35,000) that wrap around each other in a coiled coil. Tropomyosin binds in the groove of the F-actin core of the thin filaments; each tropomyosin makes contact with seven actin monomers. Troponin (Tn) consist of three distinct subunits. Tn-T (C) binds the complex to tropomysin and thus to the filament. Tn-I (J) interacts with actin. Tn (G), a Ca2+ binding protein with 70% sequence homology to calodulin, is a Ca2+ activated wsitch. At the low levels characteristic of resting muscle, Tn-C has no bound Ca2+ and the Tn complex exits in a conformation that cause tropomyosin to be positioned so that it blocks the access of myosin to the active site on actin.
Skeletal muscle contraction is stimulated when as action potential, propagating along a nerve fiber, arrives at the neuromuscular junction and triggers fusion of synaptic vesicles with the presynaptic nerve cell membrane. Acetylcholine (ACh) released from vesicles diffuses across the synaptic cleft and binds to ACh to the sarcolemma triggers the opening of ion channels in the membrane and propagation of an action potential over the surface of the muscle fiber as well as inward, along extensions of the sarcolemma (muscle cell plasma membrane) called t-tubes. The electrical signals moving along the t-tubes cause opining of Ca2+ channels in adjacent patches of the sarcoplasmic reticulum (CR). The Ca2+ that is released is then bound to TN-C, causing a conformational change in the Tn complex that moves tropomyosin to a position no longer blocking actin-myosin interaction. During this interaction, the ADP and Pi are released and the energy stored in the myosin is used to case sliding of the actin filament past the myosin filament. Myosin heads repeatedly binds molecules of ATP which cause dissociation of myosin from actin, hydrolyse them to ADP and Pi and, CA2+ levels remain high enough, bind to thin filaments, again releasing ADP and Pi and causing another step of movement of thin filaments past thick filaments.
Microscopes. Inverted phase contrast microscopes are used for visualizing the cells. Microscopes should be kept covered and the lights turned down when not in use. Before using the microscope or whenever an objective is changed, check that the phase rings are aligned.
Cells are preserved and stored in liquid nitrogen. Vessels. Anchorage dependent cells require a nontoxic, biologically inert, and optically transparent surface that will allow cells to attach and allow movement for growth. The most convenient vessels are specially-treated polystyrene plastic that are supplied sterile and are disposable. These include petri dishes, multi-well plates, microtiter plates, roller bottles, and screwcap flasks - T-25, T-75, T-150 (cm2 of surface area). Suspension cells are either shaken, stirred, or grown in vessels identical to those used for anchorage-dependent cells.
When nerve stimulation ceases, release of ACh stops, residual ACh in the neuromuscular cleft is destroyed by acetylcholinesterase, the sarcolemma repolarizes, and elements of the SR regain their relatively high impermeability to CA2+ ions. These ions are pumped back into the lumen of the SR where they once again become bound to the protein calsequestrin (F). With cytosolic free CA2+ levels low, actin-myosin interaction is again blocked by the troponin-troomyosin complex. During brief contractile activity, ATP (broken down by actomyosin) is transiently maintained at needed levels by the action of creatine kinase, an enzyme which catalyzes the transfer of high energy phosphate from phosphoceatine to ADP. Contractile events lasting more than a few second deplete phosphocratine and ATP levels are maintained by breakdown of glycogen stores. Glucose-6-phosphate derived from glycogen is degraded anaerobically to lactate, yielding ATP. Lactate accumulates in the sarcoplasm during prolonged muscular activity. Recovery after prolonged contraction involves resynthesis of ATP and phosphocratine. Lactate released from the muscle is carried by the bloodstream to the liver, where it is reconverted to glucose by gloconeogenesis.
Dystrophin (I) is a protein associated with the inner leaflet of the sarcolemma. It is thought to serve an important function in reinforcing the cell membrane to strengthen it during contractions. Dystrophin structure is abnormal in Duchenne’s muscular dystrop
