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Exam 19: Cell Junctions and the Extracellular Matrix

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Question 1

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Consider two cells attached to each other via adherens junctions. The forces generated by the actin cytoskeleton in one cell …

A) are dependent on myosin activity.
B) are normally balanced by similar forces in the neighboring cell.
C) can strengthen the adherens junction by unfolding catenin proteins.
D) All of the above.

E) None of the above
F) All of the above

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Question 2

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Indicate true (T) and false (F) statements below regarding fibronectin in the extracellular matrix. Your answer would be a four-letter string composed of letters T and F only, e.g. TFTT. ( ) Fibronectin carries asparagine-linked oligosaccharides. ( ) Fibronectin often forms homodimers by cross-linking through covalent attachment of lysine residues. ( ) Fibronectin binds to integrins, collagens, and glycosaminoglycans through its type III repeats. ( ) Fibronectin molecules can assemble into fibrils only in the vicinity of cells.

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Fibronectin is a matrix multidomain gl...

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Question 3

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Neuromuscular junctions of vertebrates are special types of chemical synapses formed from close association between motor neuron axon terminals and muscle fibers. A thin layer of basal lamina separates the two cell membranes at the neuromuscular junction and is important for its organization and maintenance. Which of the following would you NOT expect to find in the basal lamina in these structures?

Consider a sheet of epithelial tissue that invaginates to form a tube along the anterior-posterior axis of a developing vertebrate embryo. The tube is then closed concomitant with a "convergence and extension" process similar to that observed in germ-band extension in Drosophila melanogaster. During this process, the tube elongates and narrows before pinching off from the sheet. Which axis (1 or 2) in the following schematic drawing of the tube cells do you think corresponds to the anterior-posterior axis? Write down 1 or 2 as your answer.

Fill in the blank in the following paragraph. Do not use abbreviations. "Particularly abundant in cells of heart muscle and the epidermis, … are structurally similar to adherens junctions and contain a number of homologous components. However, their cadherin molecules are linked to intermediate filaments instead of the actin cytoskeleton."

A cell's response is not the same when placed on a rigid compared to a soft matrix. Indicate whether each of the following is expected to occur in a cell that is placed on a rigid matrix (R) or a soft matrix (S). Your answer would be a three-letter string composed of letters R and S only, e.g. RSR. ( ) More tension is generated in cell-matrix contacts. ( ) More vinculin proteins would bind to the C-terminal tail of talin. ( ) More actin filaments are recruited to the site of cell-matrix adhesion.

The size distribution of exons coding for collagen helical domains in various vertebrates reveals a unique pattern. This pattern provides clues to the evolutionary history of the genes encoding fibrillar collagen α chains. What is this pattern?

Indicate true (T) and false (F) statements below regarding the interaction of cells with the extracellular matrix using matrix receptors. Your answer would be a four-letter string composed of letters T and F only, e.g. TFTT. ( ) Integrins are the only known matrix receptors in animal cells. ( ) Integrins can transmit signals in both directions across the membrane; that is, both outside-in and inside-out. ( ) Tension can increase the binding affinity of an integrin for its intracellular and extracellular ligands. ( ) Integrins can convert molecular signals into mechanical ones.

A giant aggrecan aggregate can have a molecular mass of 10⁸ daltons or more, and can be as large as a bacterium. Its structure resembles a brush. What is responsible for attaching the bristles of the brush to its core?

Indicate whether each of the following descriptions better applies to adherens junctions (A), desmosomes (D), gap junctions (G), or tight junctions (T). Your answer would be a four-letter string composed of letters A, D, G, and T only, e.g. AAAD. ( ) They play a direct role in structural polarization of epithelial cells. ( ) They couple cells both electrically and metabolically. ( ) They are formed from hexameric hemichannels that can assemble into heterotypic complexes. ( ) They create semipermeable paracellular pores.

Indicate true (T) and false (F) statements below regarding glycosaminoglycan chains in the extracellular matrix. Your answer would be a four-letter string composed of letters T and F only, e.g. TFTT. ( ) They are highly branched polysaccharides. ( ) They are strongly hydrophilic and absorb a large amount of water. ( ) They all contain N-acetylglucosamine or N-acetylgalactosamine. ( ) They are highly positively charged.

Indicate true (T) and false (F) statements below regarding cadherins. Your answer would be a four-letter string composed of letters T and F only, e.g. TFTT. ( ) Cadherins are present in all multicellular organisms. ( ) Adhesion by cadherins is dependent on Ca²? ions. ( ) In contrast to nonclassical cadherins, classical cadherins are more closely related in sequence. ( ) E, N, and P cadherins are mainly found in desmosomes.

Cells expressing either N-cadherin (A), high levels of E-cadherin (B), or low levels of E-cadherin (C) have sorted themselves out on a substratum in a cadherin-dependent manner as shown in the schematic drawing below. Which cells (A to C) would you expect to correspond to white, gray, and black in the drawing, respectively? Your answer would be a three-letter string composed of letters A to C only, e.g. CAB.

In insects such as Drosophila melanogaster, electrical synapses are abundant in the ventral nerve cord. Unlike a chemical synapse, the cytosols of pre- and postsynaptic cells in an electrical synapse are connected, allowing action potentials to spread rapidly and without delay. What junctional proteins in the flies are chiefly responsible for this unique feature of the electrical synapses?

In the active state of the integrin dimer, …

Indicate whether each of the following descriptions better applies to the intracellular (I) or extracellular (E) domain of integrins. Your answer would be a four-letter string composed of letters I and E only, e.g. EEEE. ( ) It binds to talin. ( ) It binds to the RGD tripeptide motif. ( ) It is bulkier. ( ) It contains divalent cation-binding domains.

A researcher has coated the wells in a multiwell plate with human plasma fibronectin. She introduces integrin-expressing cells into these wells in the presence of different concentrations of a synthetic peptide. She then washes the wells to remove unbound cells and counts the number of cells that remain attached to each well. The results of her experiment are presented in the following schematic graph. The synthetic peptide sequences used in these experiments were GRGDSPC (1) or GGDRSLN (2) . Which one of these peptides (1 or 2) would you expect to correspond to curve a in the graph? Which curve (a or b) would you expect if a synthetic peptide with the sequence GDGRSPC was used? C is cysteine, D is aspartic acid, G is glycine, L is leucine, N is asparagine, P is proline, R is arginine, and S is serine.

In the following schematic diagram of a cubic plant cell that has just left the meristem zone, the cellulose microfibrils are shown as black lines. In which direction (1 or 2) do you think the cell will elongate when the cell grows? If the cell wall is momentarily dissolved, in what direction (3 or 4) would water tend to flow?

Cadherin molecules at the cell surface are often clustered side-to-side to create a molecular Velcro that attaches the cell to another cell or to the extracellular matrix. In the following simplified drawings, two cells are initially attached via such a Velcro. When one of the cells moves in either of three different ways (A to C), it is faced with a mechanical resistance from the cadherin interactions. The resistance against which type of movement do you think is the weakest? Write down A, B, or C as your answer.

Indicate whether each of the following descriptions better applies to the plant cell wall (W) or the extracellular matrix of animal cells (M). Your answer would be a three-letter string composed of letters W and M only, e.g. WMM. ( ) It is generally thicker and stronger. ( ) It is more rigid. ( ) It has higher nitrogen content.