Exam 16: The Cytoskeleton

Heart conditions known as cardiomyopathies include two main subtypes, namely the hypertrophic and dilated cardiomyopathies, in which some portions of the cardiac muscle become stiff or dilated, respectively. Drugs such as the calcium channel blocker verapamil are used to treat the former, while the Na?-K? ATPase inhibitor digoxin (which elevates intracellular calcium concentration in cardiac muscle cells) is sometimes used for the latter. Mutations in the subunits of the troponin complex are among the genetic causes of these diseases, and can be grouped into mutations that result in either decreased (D) or increased (I) calcium-ion sensitivity of actin-myosin contraction. Which of these mutations would you expect to be associated with hypertrophic cardiomyopathy? Write down D or I as your answer.

If a certain isoform of myosin II is depleted from a cell, stress fibers are lost and focal adhesions disappear. If these cells are placed on a surface coated with an array of flexible pillars to measure traction forces, would you expect the traction to increase (I) or decrease (D) in these cells compared to wild-type cells? Write down I or D as your answer.

SUN and KASH proteins embedded in the nuclear envelope provide connections between the organization of the nucleus and the cytoplasm. Which of the following pairs of proteins DO NOT bind directly to each other in these connections?

Indicate true (T) and false (F) statements below regarding the microtubule cytoskeleton in neurons. Your answer would be a five-letter string composed of letters T and F only, e.g. TTFFF. ( ) All microtubules in an axon are normally oriented in the same direction. ( ) All microtubules in a dendrite are normally oriented in the same direction. ( ) Each microtubule in an axon has one end at the cell body and its other end at the axon terminal. ( ) Anterograde transport in axons is exclusively carried out by kinesins. ( ) Anterograde transport (toward the tip) in dendrites is exclusively carried out by kinesins.

You have prepared actin filament seeds in a microfluidic chamber through which you then pass either unlabeled or fluorescently labeled actin subunits in alternation several times, keeping the total monomer concentration constant. You then observe the resulting filaments under a fluorescence microscope. If the filaments appear as drawn schematically in the example below (in which fluorescence is indicated by black color), which end do you think is the plus end-(a) or (b)? Was the concentration of free actin subunits below or above the critical concentration (Cc) for polymerization at the barbed end?

In contrast to growing microtubules, shrinking microtubules …

In which of the following structures are actin-myosin II bundles regulated by the troponin complex?

Considering the diagram below, which summarizes two signaling pathways initiated by activated GTPases of the Rho family, what letter (A to D) corresponds to each of the following? Your answer would be a four-letter string composed of letters A to D only, e.g. ABCD. ( ) Rac-GTP ( ) Rho-GTP ( ) Lamellipodia formation ( ) Stress-fiber formation

Which of the actin-binding proteins (1 to 5) in the following schematic drawing represents tropomyosin, ?-actinin, Arp 2/3 complex, CapZ, and myosin, respectively? Your answer would be a five-digit number composed of digits 1 to 5 only; e.g. 15324.

In the polymerization in vitro of actin filaments and microtubules from their subunits, what does the "lag phase" correspond to?

Fill in the blank: Each microtubule is typically made of thirteen parallel …. that associate laterally to form a hollow tube.

The proteins of the ERM family such as moesin …

Which of the following is an actin homolog?

The time courses of seeded actin polymerization under two different conditions are compared in the following graph. If the overall kₒn for polymerization is known to be the same under both of these conditions, which curve-(1) or (2)-corresponds to the condition with a higher kₒff rate constant? Which one corresponds to a higher Cc for polymerization?

In a cross section of a vertebrate axon, longitudinally organized cytoskeletal proteins can be seen as dots, as shown in the schematic drawing below. What type of filaments do these dots represent?

Consider the structure of a sarcomere. Which of its features does NOT shorten during skeletal muscle contraction?

Sort the following events to show the sequence in the mechanochemical cycle of myosin II, following ATP binding by the head domain. Your answer would be a four-digit number composed of digits 1 to 4 only, e.g. 1342. (A) Myosin head binds tightly to actin. (B) Power stroke is triggered. (C) Cocking occurs and the head is displaced along the actin filament. (D) The binding affinity of myosin for actin is reduced.

Which of the following proteins do you expect to be enriched near the plus end of microtubules?

Indicate if each of the following structures is based on actin filaments (A), microtubules (M), or intermediate filaments (I). Your answer would be a five-letter string composed of letters A, M, and I only; e.g. AAAMM. ( ) The cell cortex ( ) The mitotic spindle ( ) The nuclear lamina ( ) Cilia ( ) Filopodia

What are the typical consequences of mutations in keratins and their associated proteins?