AP Biology 4.2 Signal Transduction

Practise AP Biology 4.2 questions by tracing ligand recognition, receptor activation, intracellular amplification and responses such as gene expression.

[Maximum number: 1]

Questions 43-48

Figure 1. Model of synapse

Researchers investigating the regulation of neurotransmitter release from presynaptic neurons proposed a model
(Figure 1) in which CDK5, a protein expressed in axon terminals, inhibits the movement of synaptic vesicles to the
presynaptic membrane.
To test their model, the researchers used a modified version of green fluorescent protein (GFP*). In slightly
alkaline conditions, GFP* exhibits a bright green fluorescence. In acidic conditions, GFP* exhibits no fluorescence.
Using standard techniques, the gene encoding GFP* is easily introduced into living cells. By engineering the
expression of GFP* in laboratory-cultured nerve cells, the researchers found that a bright green fluorescence was
exhibited only when a presynaptic neuron was given a certain stimulus.

Based on the model, which of the following
describes the most likely mechanism by which
CDK5 regulates neurotransmitter release?

A

CDK5 adds methyl groups to DNA, altering
expression of genes required for synthesis
of neurotransmitters.

B

CDK5 promotes the rearrangement of the
lipid molecules of two bilayers into a single
membrane.

C

CDK5 alters the activity of other proteins
involved in the movement of synaptic
vesicles to the plasma membrane.

D

CDK5 binds to gated ion channels in the
postsynaptic membrane, resulting in
diffusion of calcium ions.

[Maximum number: 5]

In eukaryotic microorganisms, the PHO signaling pathway regulates the expression of certain genes. These genes, Pho target genes, encode proteins involved in regulating phosphate homeostasis. When the level of extracellular inorganic phosphate (Pi) is high, a transcriptional activator Pho4 is phosphorylated by a complex of two proteins, Pho80-Pho85. As a result, the Pho target genes are not expressed. When the level of extracellular Pi is low, the activity of the Pho80-Pho85 complex is inhibited by another protein, Pho81, enabling Pho4 to induce the expression of these target genes. A simplified model of this pathway is shown in Figure 1.

Figure 1. A simplified model of the regulation of expression of Pho target genes in (A) a high-phosphate (high-Pi) environment and (B) a low-phosphate (low-Pi) environment

To study the role of the different proteins in the PHO pathway, researchers used a wild-type strain of yeast to create a strain with a mutant form of Pho81 (pho81mt) and a strain with a mutant form of Pho4 (pho4mt). In each of these mutant strains, researchers measured the activity of a particular enzyme, APase, which removes phosphates from its substrates and is encoded by PHO1, a Pho target gene (Table 1). They then determined the level of PHO1 mRNA relative to that of the wild-type yeast strain, which was set to 10.

TABLE 1. APase ACTIVITY AND RELATIVE AMOUNTS OF PHO1 mRNA IN WILD-TYPE AND MUTANT STRAINS OF YEAST IN HIGH- AND LOW-PHOSPHATE ENVIRONMENTS

(a)

Describe the effect that the addition of a charged phosphate group can have on a protein that would cause the protein to become inactive. Explain how a signal can be amplified during signal transduction in a pathway such as the PHO signaling pathway.

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(b)

Based on Table 1, identify a dependent variable in the researchers' experiment. Justify the researchers' using the wild-type strain for the creation of the mutant strains. Justify the researchers' using mutant strains in which only a single component of the pathway was mutated in each strain.

[ 3 ]

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AP Biology 4.2 Signal Transduction

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4.2 Introduction to Signal Transduction question 1

4.2 Introduction to Signal Transduction question 2

Question (a)

Question (b)