Chapter Questions
During signal transduction (a) the cell converts an extracel. lular signal into an intracellular signal that leads to a change in some cell process (b) a signaling molecule directly activates or represses several genes (c) each enzyme catalyzes production of one molecule of product (d) enzymes in the signal cascade remain active until the last component of the pathway alters a cellular process (e) the signal is terminated by cyclic AMP
When a signaling molecule binds with a receptor, (a) G proteins are inactivated (b) a third messenger is activated (c) cell signal. ing is terminated (d) cAMP is produced by the receptor (e) the receptor becomes activated
G protein-linked receptors (a) inactivate C proteins (b) activate first messengers (c) consist of 18 transmembrane alpha helices (d) have a tail that extends into the cytosol with a binding site for a C protein (e) are located in the cytoplasm or nucleus
An enzyme-linked receptor (a) is a cytoplasmic protein (b) would not be found on plant cell surfaces (c) forms a dimer with another enzyme-linked receptor when a ligand binds to it (d) is typically an adenylyl cyclase molecule (e) typically activates ion channels
G proteins (a) relay a message from an activated receptor to an enzyme that activates a second messenger (b) are CTP molecules (c) terminate cell signaling (d) directly activate protein kinases (e) function as first messengers
Calcium ions (a) can act as second messengers (b) split calmodulin (c) are kept at higher concentration in the cytosol than in the extracellular fluid $(\mathrm{d})$ are produced in the ER by protein kinases and protein phosphatases (e) typically terminate signaling cascades
When growth hormone binds to an enzyme-linked receptor, (a) C proteins are amplified into a cascade of molecules (b) the enzyme portion of the receptor becomes dephosphorylated (c) the receptor becomes activated and phosphorylates signaling proteins in the cell (d) an ion channel is opened (e) an immediate signal is sent into the nucleus, and specific genes are activated or inhibited
Scaffold proteins (a) release kinases and phosphatases into the extracellular fluid (b) bind C proteins to cell membranes (c) increase accuracy but slow signaling cascades (d) organize groups of intracellular signaling molecules into signaling complexes (e) are transcription factors found mainly in plant cells
Each adenylyl cyclase molecule produces many cAMP molecules in an example of (a) receptor up-regulation (b) receptor downregulation (c) signal amplification (d) scaffolding (e) similarities produced by evolution
In response to a hormone secreted by a cell of the opposite mating type, a yeast cell undergoes a complex series of physiological changes involving the activity of about 200 genes and cytoplasmic proteins. They include blocking DNA synthesis, growing toward the mating partner, fusion of the plasma membranes of the two cells, and fusion of their nuclear mem. branes. Explain how all these events can be controlled through a complex signaling cascade that is triggered by the binding of the hormone to a G protein-linked receptor.
More than five hundred genes have been identified in the human genome that code for protein kinases. What does such identification imply regarding the role of protein kinases in cellular functions? Explain your answer.
Which is the correct sequence? $1 .$ protein kinase activated 2 . adenylyl cyclase activated 3. cAMP produced 4. proteins phosphorylated 5. G protein activated(a) 1,2,3,5,4(b) 5,3,2,1,4(c) 5,2,3,4,1(d) 5,2,3,1,4 (e) 2,3,1,4,5
Which is the correct sequence? 1 . phospholipase activated2. C protein activated 3. PIP $_{2}$ split 4. proteins phosphorylated5. DAC produced(a) 1,2,5,3,4(b) 2,1,3,5,4(c) 4,2,3,1,5(d) 5,2,3,1,4(e) 2,3,5,4,1
Cell signaling in plant and animal cells is similar in some ways and different in others. Offer one or more hypotheses for these similarities and differences, and cite specific examples.
Some of the same G protein-linked receptors and signal transduction pathways found in plants and animals have been identified in fungi and algae. What does that suggest about the evolution of these molecules? What does it suggest about these molecules and pathways?
Mutant tyrosine kinase signaling proteins are implicated in many types of human cancer. Hundreds of millions of dollars are required for the basic research and development of each new drug; consequently, many of these drugs are very expensive when used for cancer treatments. In 2012, eight tyrosine kinase inhibitors were approved for cancer treatments by the U.S. Food and Drug Administration, and more than one hundred potential inhibitors were undergoing clinical trials for approval. What do you think would be some of the difficulties of finding these drugs given that similar kinases are active in normal cells? Do you think new medications of this type should be developed through government-sponsored research? Why or why not? If not, what alternatives do you propose?