Question

Two protein kinases, K1 and K2, function sequentially in an intracellular signaling pathway. If either kinase contains a mutation that permanently inactivates its function, no response is seen in the cells when an extracellular signal is received. A different mutation in K1 makes it constitutively active, so that in cells containing that mutation, a response is observed even in the absence of an extracellular signal. You characterize a double-mutant cell that contains K2 with the inactivating mutation and K1 with the activating mutation. You observe that the response is seen even in the absence of an extracellular signal. In the normal signaling pathway, does K1 activate K2 or does K2 activate K1? Explain your answer.

Answer 1

K1 mutant: no response to extracellular signal

K2 mutant: no response to extracellular signal

constitutive active K1 mutant: response in absence or presence of extracellular signal

double mutatnt: mutant K2 / constitutive active K1 mutant

In normal cells K2 is upstream of K1 or K2 activates K1. If K1 activates K2 then in double mutant there will be no response because K2 is inactive. However, if K2 activates K1, then absence of K2 should not matter because K1 is already active to carry intracellular signalling.