Biology · Cell Signaling

Two protein kinases, K1 and K2, regulate an intracellular pathway in response to an extracellular signal. The following observations were made: i. Response is observed even in the absence of an extracellular signal when a mutation permanently activates K1. ii. Response is observed even in the absence of an extracellular signal when K1 contains an activating mutation and K2 has an inactivating mutation. iii. No response is observed even in the presence of an extracellular signal when K2 is inactivated by mutations. iv. Response is observed even in the absence of an extracellular signal when both kinases are activated by mutations. Which one of the following cascade relationships is correct?