Exam 15: Cell Signaling

Consider a signaling protein that can be phosphorylated by a dual-specificity protein kinase at two independent sites (one tyrosine and one serine residue), such that each site is phosphorylated in about 1% of the protein molecules under normal conditions. Phosphorylation at the first site has a short half-life, since a fast tyrosine phosphatase removes the phosphate soon after the residue is phosphorylated, whereas dephosphorylation at the second site-carried out by a serine/threonine phosphatase-is relatively slow. Upon activation of a signaling pathway, the concentration of the dual-specificity kinase increases several-fold rapidly. The phosphorylation state of the tyrosine and serine residues following stimulation is compared in the following graph. Which curve (1 or 2) do you think corresponds to the serine residue? Write down 1 or 2 as your answer.

Consider visual transduction in rod photoreceptors in the vertebrate retina. Which of the following steps does NOT normally amplify the signal in this pathway?

Indicate true (T) and false (F) statements below regarding the mTOR complexes. Your answer would be a four-letter string composed of letters T and F only, e.g. TTTF. ( ) mTOR in complex 1 contains the protein raptor, is sensitive to rapamycin, and stimulates cell growth. ( ) Akt activation is stimulated by mTOR in complex 2, which contains the protein rictor. ( ) Akt activates mTOR in complex 2 by activating a Rheb-GAP called Tsc2. ( ) mTOR in complex 1 is activated in the presence of growth factors.

How does the expression of Delta on the surface of a cell activate the expression of certain genes in the nucleus of its neighboring cell?

In which of the following schematic drawings of signaling pathways does the activation of the receptor lead to gene expression? Activating and inhibitory steps are indicated with (+) and (-), respectively.

In the following simplified diagram of the NF?B-dependent signaling pathway, indicate which boxes (A to C) correspond to the NF-?B, I?B, and IKK proteins, respectively. Your answer would be a three-letter string composed of letters A to C only, e.g. BCA.

Cells can adapt to extracellular signal molecules in a variety of ways. Interestingly, exposure to a signal molecule can sometimes not only desensitize the cell to this molecule, but also to one or more other signal molecules that are recognized by different receptors, even if those receptors can still bind to their ligand at the cell surface and initiate signaling. Which subset of the following adaptation mechanisms (for the original signal) can in principle account for this type of desensitization? Choose all that apply. Your answer would be a string composed of letters A to E only, in alphabetical order; e.g. BDE. (A) Receptor sequestration (B) Receptor down-regulation (C) Receptor inactivation (D) Inactivity of signaling protein (E) Production of inhibitory protein

Which of the following is NOT part of a negative feedback mechanism in adaptation to light in retinal rod cells?

Fill in the blank in the following paragraph regarding plant hormones. DO NOT use abbreviations. "As simple a signaling molecule as it is, ... is also a key regulator of plant physiology. It can promote processes such as fruit ripening, flower opening, and leaf abscission; it can also be produced in response to various types of stress including drought, flooding, and bacterial and viral infections."

A certain effector protein can be activated by phosphorylation at a key tyrosine residue. An upstream kinase rapidly phosphorylates this tyrosine in the presence of a signal. However, the kinase also phosphorylates and activates a slow-acting phosphatase that can dephosphorylate the phosphotyrosine. Which curve in the following graph would you expect to represent the activity of the effector molecule over time? The input signal is present during the period indicated in gray. The dashed line represents the response in the absence of the phosphatase.

Sort the following events to reflect the normal order in which they occur in G-protein-coupled receptor signaling leading to transcription of genes with cAMP response elements. Your answer would be a four-letter string composed of letters A to E only, e.g. ACBD. (A) Binding of CREB to PKA (B) Binding of cAMP to PKA (C) Dissociation of PKA into catalytic and regulatory subunits (D) Activation of adenylyl cyclase

Which of the following molecular mechanisms is NOT used in the cell in order to produce abrupt responses to a signal?

Which of the following represents the active form of β-catenin in cells stimulated with Wnt?

PI 3-kinase ...

Activated STATs can induce the expression of SOCS proteins, which in turn inhibit the JAK-dependent phosphorylation of the STATs. Under sustained serum stimulation of mouse fibroblast cells, a number of oscillations can be observed as a consequence of this delayed negative feedback loop, as shown in the following simplified graph. According to this graph, approximately how long is the delay in the feedback response, and what process is mainly responsible for it?

In the following graphs, the cellular response, as measured in real time by the concentration of a certain active effector protein, is plotted over time for five cell types (A to E) that are treated with three different concentrations of a signal molecule. During the time period indicated by the horizontal gray bar, the signal molecule is present in the culture media at a concentration of 1 nM (dotted curve), 5 nM (gray curve), or 25 nM (solid curve). Answer the following question(s) based on these graphs. -Which of the cell types A to E shows the lowest sensitivity to the signal? A) B) C) D) E)

In the following graphs, the cellular response, as measured in real time by the concentration of a certain active effector protein, is plotted over time for five cell types (A to E) that are treated with three different concentrations of a signal molecule. During the time period indicated by the horizontal gray bar, the signal molecule is present in the culture media at a concentration of 1 nM (dotted curve), 5 nM (gray curve), or 25 nM (solid curve). Answer the following question(s) based on these graphs. -Which of the cell types A to E shows a response with the highest persistence? A) B) C) D) E)

Which of the following mechanisms is NOT employed by I-Smads to negatively regulate Smad signaling?

Ethylene receptors in plant cells ...

Indicate whether each of the following descriptions better applies to phytochromes (P), cryptochromes (C), or phototropins (T). Your answer would be a four-letter string composed of letters P, C, and T only, e.g. TPCC. ( ) They respond to red light. ( ) They are found in animals as well as in plants. ( ) They are flavoproteins. ( ) They are dimeric, cytosolic serine/threonine protein kinases that are normally activated by autophosphorylation.