PROBLEM SET 5 MCB2210 Spring 2015 1 Which of the following is true As a cell increases in size a Cytoplasmic volume increases faster than membrane surface area b Membrane surface area increases faster than cytoplasmic volume c Cytoplasmic volume and membrane surface area increase at the same rate d Cytoplasmic volume remains the same while surface area increases e Membrane surface area increases but surface area remains the same 2 Which does not describe diffusion correctly a Diffusion in a membrane is two dimensional b Diffusion does not have directionality for individual molecules c Diffusion is due to thermal agitation of molecules d Large molecules cannot diffuse only molecular motors can move them e The rate of movement of molecules depends on factors such as temperature 3 Which of the following are types of cytoskeleton polymers a Actin filaments b Intermediate filaments c Microtubules d All of the above e Two of the above 4 Which of the following activities are identified as functions of the cytoskeleton a dynamic scaffold that provides structural support that helps to determine cell shape b internal framework that positions organelles in cell interior c a network of tracks that direct the movement of materials and organelles within the cell d a force generating apparatus e all of the above 5 WASP family proteins are known to activate which protein or proteins a G actin b myosin c Arp2 3 complex d formins e profilins 6 What limits F actin assembly in cells a The number of nucleation sites This never occurs in cells b c b The inability of actin to assemble on its own cells have constant cytosolic c The sequestration of G actin ionic concentration and d The G actin concentration is below the critical concentration polymerization is controlled by e a and c ABPs Only in vitro 7 New F actin assembly in cells can be increased by a Creation of nucleation sites Cofilin b Severing of existing filaments to generate new plus ends 1 c Addition of sequestering proteins to G actin Sequestering to remove or isolate d Removal of phosphate from G actin by phosphatases Removing phosphate from actin NO more ATP e a and b 8 The solid line below shows the amount of F actin formed from G actin over time a pyrene actin assembly assay When this experiment is repeated with G actin and an unknown actin binding protein a different plot is obtained dashed line How has the addition of this protein changed the assembly dynamics of the actin filaments a The critical concentration has substantially increased b The critical concentration has substantially decreased c The rate of polymerization has decreased d The protein has stabilized the filaments Add nucleators to e The number of nucleation sites has increased change RATE of 9 Actin polymerization in a test tube requires polymerization a ATP b GTP c both A B d profilin e Arp2 3 10 A new actin nucleator has been identified that generates long unbranched filaments This nucleator is least likely to function like the a Arp2 3 complex b Formins c Tandem actin monomer binding nucleators d b and c e a b and c 11 F actin a is composed of four linear strands b is made from G actin dimers c is composed of polymerized G actin monomers wound around each other in a helix d polymerizes at the same rate in both directions from both ends of the filament e is the abbreviation for Fat actin 2 12 If you were to assemble actin filaments from G actin in a test tube containing ATP S a non hydrolyzable form of ATP instead of ATP what would you expect to happen a Filaments will polymerize and depolymerize normally b Filaments will polymerize and be more dynamic than usual c Filaments will not polymerize d Filaments will polymerize but will not treadmill e Filaments will polymerize with a lower critical concentration 13 The solid line below shows the amount of F actin y axis formed at steady state starting with various concentrations of total actin monomer x axis When this experiment is repeated with actin and an unknown actin binding protein a different plot is obtained dashed line How has the addition of this protein changed the assembly dynamics of the actin filaments a The rate of polymerization has decreased b The rate of cross linking has decreased c The number of nucleation sites has increased d The critical concentration has decreased e The critical concentration has increased Change Critical Concentration to begin polymerization at diff TIME 14 What happens at the minus ends of actin filaments when the concentration of G actin is above the critical concentration at the minus end a Monomers are lost from it b Monomers add on to it c Monomers are not lost from it or added on to it d Monomers bind GTP e Monomers undergo dynamic instability 15 All of the following are important features of the Arp2 3 complex except a It remains on the barbed end as it nucleates new actin filaments NOT TRUE Arp2 3 complex b Arp2 and Arp3 are distant relatives of actin remains on pointed minus c It binds to the sides of existing actin filaments end as it nucleates new actin d It can nucleate actin filament polymerization e It leads to dendritic branching of actin filaments 16 Decoration of actin filaments with myosin S1 the head is commonly used to 3 a attach actin filaments to cell membranes b disassemble actin filaments Myosin S1 pointed barbed ends of actin c reveal the polarity of actin filaments d reveal the polarity of myosin filaments e nucleate actin filaments 17 All of the following statements about actin assembly are correct except a ATP actin can assemble into filaments b Actin subunits can treadmill through an actin filament c Actin assembly can produce force for movement d Actin ends assemble more rapidly than actin ends e Actin assembly is accelerated by nucleators 18 During treadmilling actin subunits add a predominantly to filament ends b predominantly to filament ends c equally to both filament ends d along the sides of filaments e to the Arp2 3 complex 19 Choose a statement that describes the dynamic behavior of actin filaments when the concentration of actin monomers is between the critical concentration for plus and minus ends a Treadmilling b Dynamic instability c Net disassembly d Crosslinking e Nucleation 20 In an in vitro experiment with purified actin the kinetics of actin filament polymerization was examined in the presence or absence of activated Arp2 3 How do you think the results would differ between the two experiments a Not much difference would be observed