You are working out the component order of a signaling pathway. You are studying

Question

You are working out the component order of a signaling pathway. You are studying two protein kinases, STK1 and STK2. These two kinases act sequentially in the signaling pathway you study. When either kinase is inactivated (inactivating mutation), cells do not respond to the normal extracellular signal. When cells contain a mutant form of STK1 in which the kinase is constitutively active (activating mutation), they respond even in the absence of an extracellular signal. Cells that contain the constitutively active mutant STK1 along with an inactivate mutant of STK2 also respond in the absence of a signal. Given this information, answer the following questions.

A. In the normal signaling pathway, does STK1 activate STK2, or does STK2 activate STK1? Explain your reasoning using the information given.

B. What would the outcome be if a cell contained an activating mutation in STK2 and an inactivating mutation in STK1? Explain your reasoning.

Explanation / Answer

a. In the normal signaling pathway, STK2 activates STK1. This can be concluded from the fact that even when STK2 is inactive, the mutants respond. So STK1 is the second kinase in the cascade and is enough to trigger the cascade of events.

b. If the cell contained an active STK2 and an inactive STK1, the mutants cannot respond even in the presence of a signal.

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