Question

Working out the order in which the individual components in a signaling pathway act is an essential step in defining the pathway. Imagine that two protein kinases, PK1 and PK2, act sequentially in a kinase cascade that activates a set of target proteins bringing about a cellular response. When either kinase is completely inactivated, cells do not respond to the normal extracellular signal. By contrast, cells containing a mutant form of PK1 that is permanently active respond even in the absence of an extracellular signal. Cells that contain both an inactivated PK2 and a permanently active PK1 respond in the absence of a signal.

Given these data, what is the order of action of PK1 and PK2? What outcome would you predict for a doubly mutant cell line with an activating mutation in PK2 and an inactivating mutation in PK1?
1) PK1 activates PK2. The doubly mutant cells would not respond in the presence of signal.
2) PK2 activates PK1. The doubly mutant cells would not respond in the presence of signal.
3) PK2 activates PK1. The doubly mutant cells would respond in the absence of signal.
4) PK1 activates PK2. The doubly mutant cells would respond in the absence of signal.

Answer 1

PK1 activates PK2 in the signaling pathway, and cells with an inactivating mutation in PK1 and activating mutation in PK2 do not respond since the sequence begins with PK1.

Step-by-step explanation:

The correct answer to the question is Option 1: PK1 activates PK2. The doubly mutant cells would not respond in the presence of the signal. This is because the permanently active mutant form of PK1 can initiate the cellular response even without the extracellular signal, while the inactivated PK2 cannot act downstream to continue the signaling cascade.

The explanation relies on the data provided: when either kinase is completely inactive, the normal response to an extracellular signal is abolished. This indicates that both kinases are essential. Since cells with permanently active PK1 can respond without the extracellular signal, PK1 is upstream of PK2 and activates it. Finally, if both PK1 is inactive and PK2 is permanently active, there would be no response since the upstream activator, PK1, is required to activate PK2.