1 A cell expresses a transmembrane protein that is cleaved at the plasma membrane to
release an extracellular fragment. The fragment binds to receptor proteins on nearby cells and
activates signaling pathways resulting in altered gene expression patterns in the cells. What form
of intercellular signaling does this represent?
A. Contact-dependent signaling
B. Paracrine signaling
C. Synaptic signaling
D. Endocrine signaling
E. Autocrine signaling
2 Which of the following is NOT a common second messenger in cell signaling?
A. Ca2+
B. Cyclic adenosine monophosphate
C. Diacylglycerol
D. Tyrosine
E. Inositol trisphosphate
3 Which of the following events normally activates a GTP-binding protein?
A. GTP hydrolysis by the protein
B. Activation of an upstream GTPase-activating protein
C. Activation of an upstream guanine nucleotide exchange factor
D. Phosphorylation of a bound GDP molecule by an upstream phosphorylase
E. Pi release after GTP hydrolysis
4 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.
Reference: Examples of Cellular Response Curves Questions 5-8
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.
5 Which of the cell types A to E shows the lowest sensitivity to the signal?
6 Which of the cell types A to E shows a response with the highest persistence?
7 Which of the cell types A to E shows the fastest signal adaptation?
8 Which of the cell types A to E shows the widest dynamic range of signal concentration?
, 9 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.
Percentage of molecules
phosphorylated
100
1
2
0
0 20
Time after stimulation (min)
10 The Src kinase can be regulated by at least two mechanisms. Kinases of the CSK family
inactivate Src by phosphorylating a C-terminal tyrosine residue. On the other hand, binding of
activating ligands (including some activated receptor tyrosine kinases, or RTKs) results in
autophosphorylation at a tyrosine residue near the active site, stimulating Src activity. Full
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