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, PK-B and PK-X, act sequentially in an intracellular signaling pathway: - When either kinase is completely inactivated, cells do not respond to the normal extracellular signal.
- Cells containing a mutant form of PK-B that is permanently active respond even in the absence of an extracellular signal.
- Doubly mutant cells that contain inactivated PK-X and permanently active PK-B respond in the absence of a signal.

Based on this information, answer the following questions.

a. What is the normal sequence of the kinases in the signaling pathway?
b. What outcome would you predict for a doubly mutant cell line with a permanently active mutation of PK2 and an inactivating mutation of PK1?