Exam 16: The Cytoskeleton

In the following graph, the elongation rate of pure actin filaments as a function of actin subunit concentration is shown as a dashed line. Which one of the other five lines (A to E) would you think better shows what happens when a plus-end capping protein such as CapZ is present?

Indicate true (T) and false (F) statements below regarding the cell cytoskeleton. Your answer would be a four-letter string composed of letters T and F only; e.g. TTFF. ( ) The three major building blocks of cytoskeletal filaments can bind to and hydrolyze nucleotides. ( ) The building blocks of microfilaments and microtubules are globular proteins, whereas those of intermediate filaments are themselves filamentous proteins. ( ) Intermediate filaments are typically thicker than actin filaments but thinner than microtubules. ( ) Plant cells lack microtubules.

From left to right, indicate if each of the following schematic drawings represents the typical overall organization of actin filaments (A), microtubules (M), or intermediate filaments (I) in an animal cell. Your answer would be a three-letter string composed of letters A, M, and I only; e.g. AMI.

The γ-tubulin ring complex is to microtubules what … is to actin filaments.

The actin-nucleating protein formin has flexible "whiskers" containing binding sites that help recruit actin subunits in order to enhance polymerization by this protein. What protein would you expect to bind to these sites?

Indicate whether each of the following structures has actin organized mostly in a one- (1), two- (2), or three- (3) dimensional arrangement. Your answer would be a four-digit number composed of digits 1 to 3 only, e.g. 1333. ( ) Stress fibers ( ) Invadopodia ( ) Lamellipodia ( ) Filopodia

If the concentration of free subunits is C, under which condition does the growth of a cytoskeletal filament proceed spontaneously?

In the following schematic drawing, in which direction (1 or 2) is the keratocyte migrating? What is the approximate length of the cell indicated by the bar on the left?

What is the major microtubule-organizing center in animal cells?

Which of the following is NOT an example of a crawling cell?

Indicate whether each of the following descriptions matches actin filaments (A), microtubules (M), or intermediate filaments (I). Your answer would be a three-letter string composed of letters A, M, and I only, e.g. MMA. ( ) They have the smallest persistence length. ( ) They lack polarity. ( ) They have the highest tensile strength.

In the following schematic drawing of a polarized neutrophil engaged in chemotaxis, in what region (a or b) does Rac activity dominate? What other member of the Rho family dominates at the other region?

Unlike a myosin head, a kinesin head is tightly bound to its cytoskeletal track when bound to ...

In the structure of which of the following proteins are the actin-filament binding sites furthest apart in space?

Sort the following events to reflect the sequence during smooth muscle contraction upon delivery of muscle stimulation. Your answer would be a five-digit number composed of digits 1 to 5 only, e.g. 13452. (A) Activation of the myosin light-chain kinase (B) Interaction of myosin head with actin (C) Phosphorylation of myosin (D) Calcium release into the cytoplasm (E) Activation of calmodulin

Using time-lapse fluorescence microscopy, you have recorded the transport of fluorescently labeled mitochondria in a region of an axon in a fruit-fly larva; you have plotted the results in the following graph, in which the vertical axis is time (from the top to the bottom), the horizontal axis is the position along the axon, and each trace represents one mitochondrion. Is the retrograde transport or the anterograde transport of mitochondria faster in these axons? Does the trace indicated by an asterisk correspond to a mitochondrion that was transported by a dynein or a kinesin?

Keratins are intermediate filaments that ...

Dynamitin is a subunit of the dynactin complex. Its overexpression leads to the disassembly of the complex, and it is therefore considered a dynactin inhibitor. Which of the following processes would happen if dynamitin is overexpressed in a cell?

Indicate if each of the following descriptions applies to (1) EB1, (2) kinesin-1, (3) kinesin-13, or (4) katanin. Your answer would be a four-digit number composed of digits 1 to 4 only, e.g. 1432. ( ) This is a microtubule-severing protein that can release microtubules from the microtubule-organizing centers. ( ) It increases the frequency of catastrophe by deforming microtubule protofilaments. ( ) This is a conventional motor protein that moves toward the plus end of a microtubule. ( ) It recognizes the structure of a growing microtubule end and binds to it, helping other proteins to also bind to the plus end.

Consider the ATP-bound and ADP-bound forms of actin and the polarized nature of the actin filaments. In the following diagram that shows the various actin polymerization rate constants (k?n and k?ff values), which monomer corresponds to an ADP-bound actin incorporated at the plus end?