NROS 310 EXAM 3 /175 questions /Solved 100%

What are the three main components of the cytoskeleton and what are their main functions? - ANSWER Microfilaments (actin): sense the environment and direct movement Intermediate Filaments: maintain cell shape/structure, anchor organelles Microtubules: for transport What are the main similarities and differences between the three components of the cytoskeleton? - ANSWER Similarities: -important for cytoskeletal structure -charged ends; form a highway for motor protein Differences: -size and function Describe how the assembly of actin filaments is regulated. Include both the role of associated proteins and monomer concentration. - ANSWER Rac/Rho/CDC42 trigger Arp 2/3 to nucleate: capping of (-) end -- grow filament via actin-ATP monomers -- stable filaments -- cap (+) end -- ADF breaks filament to actin-ADP filaments -- profilin phosphorylates What would be the effect of incorporating a nonhydrolyzable analog of ATP in the assembly process? - ANSWER Actin filaments would grow uncontrollably, there would be no dynamic instability since ATP cannot be converted to ADP What is the rate-limiting step in actin filament assembly? - ANSWER Nucleation; clumping together of actin Define critical concentration. - ANSWER rate of addition = rate of subtraction How do the ends of the actin filament differ? - ANSWER Positive end: sterically easier to add actin, lower critical concentration because easier to add monomers so don't need a large concentration Negative end: higher critical concentration What is dynamic instability? How does the regulation of ATP hydrolysis figure into the process? - ANSWER 1.) The process of growing and shrinking of the actin-based off ATP/ADP concentration 2.) ATP's attached to the individual monomers eventually hydrolyze to ADP at random 3.) When hydrolysis catches up to addition, it is difficult to add to the (+) end, so addition must be ahead of the hydrolysis for addition to continue How do the properties of actin subserve both stable and dynamic functions in the cell? - ANSWER -Actin-ATP cap allows temporary capping to get more stability: add capping proteins that mimic the function of ATP -Arp 2/3 can cap and still allow growth What are the main functions of the different classes of actin associated proteins? - ANSWER -providing stability -reducing stability -hydrolyzing ATP -- ADP and vice versa -regulating nucleation Why are there motor proteins associated with both actin filaments and microtubules but not intermediate filaments? - ANSWER Intermediate filaments don't have directionality/polarity How does the hydrolysis of ATP cause myosin to move along the actin filament? - ANSWER Each myosin head binds/hydrolyzes ATP, using energy of ATP hydrolysis to walk toward (+) end Why is it important that each individual myosin head group is only transiently attached to the actin filament? - ANSWER They are attached only transiently to actin filament so as not to interfere with one another-- you don't want muscles to contract all the time, need to be released for muscle relaxation Diagram the components of a sarcomere in skeletal muscle. Identify the proteins involved including actin, myosin, titin, CapZ, and tropomodulin. B.) If you looked at an electron micrograph of skeletal muscle, could you tell if the muscle was contracted or relaxed? What would you see if there was a difference? C.) Predict the functional effects of a defect in titan. - ANSWER A.) ON DIAGRAM B.) Yes, you would see a shorter light band and a much longer dark band C.) Titan works like a spring that keeps the muscles from being pulled apart too far apart --If defective, muscle contraction would be negatively affected; muscles could be too relaxed unable to contract again with no actin-myosin interaction Describe how an increase in calcium causes the contraction of skeletal muscle and smooth muscle. What are the key differences? - ANSWER Skeletal Muscle: Ca++ binds troponin complex, push off tropomyosin (expose myosin binding sites) so actin and myosin can interact. Smooth Muscles: Ca++ activate myosin light chain kinase (MLCK), MLCK phosphorylates ADP - ATP, which activates myosin tail to extend and activates the head group to interact w/actin. You have created a new transgenic mouse in which the troponin complex is now independent of calcium regulation. What would happen in the skeletal muscles of these mice if troponin was A.) never activated or B.) always activated. - ANSWER A.) the troponin will not pull the tropomyosin from its binding groove, causing it to not have any contractions B.) Troponin will constantly be relieving the hold it has on actin where tropomyosin will return on the A.F. causing a constant contraction to occur continuously Rigor mortis is the stiffening of the muscles that occurs after death. It results from an increase in the level of Ca++ and a lack of ATP in the dead muscle cells. (Assume that all of the protein components of the muscle remain intact.) A.) Why do the increased Ca++ levels and low ATP levels cause muscle stiffness? - ANSWER Increase Ca++ = myosin and actin interaction Decrease ATP = stuck with myosin bound to actin Not contracting but can't relax, just stuck and stiff Rigor mortis is the stiffening of the muscles that occurs after death. It results from an increase in the level of Ca++ and a lack of ATP in the dead muscle cells. (Assume that all of the protein components of the muscle remain intact.) B.) What would happen if you tried to reanimate the tissue by decreasing calcium and increasing ATP levels in the muscle cells? - ANSWER Decrease Ca++ = tropomyosin binds, release myosin Increase ATP = can contract To reanimate, need ATP and Ca++ regulation What are the differences in structure and function between myosin V and myosin II? - ANSWER Myosin V: -2 headed myosin w/a large step size -involved in organelle transport along actin filament -moves continuously along actin filament w/o letting go -carry a wide range of cargo: mRNA, ER, & secretory vesicles -mediates correct sorting of organelles Myosin II: -work in ensembles & are attached only transiently to A.F. so as not to interfere w/one another -elongated protein formed from 2 heavy chains & 2 copies of each of 2 light chains -bipolar thick filament in muscle Outline the components necessary for the actin-polymerization based movement of listeria. - ANSWER 1. ) act A hijacks cell's actin jet machinery 2.) activates Arp 2/3 3.) capping of (+) end and nucleation 4.) filament grows and becomes stable 5.) capping of (-) end