Two protein kinases, K1 and K2, regulate an intracellular pathway in response to the extracellular signal. The following observations were made regarding the pathway.

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 statements is correct?

Correct option is D

Observations Breakdown:

1. Response is observed even in the absence of an extracellular signal when a mutation permanently activates K1.

   • This means that when K1 is permanently activated (likely through a mutation), the pathway can be activated even without an external signal. This suggests K1 is a key player in initiating the pathway.

2. Response is observed even in the absence of an extracellular signal when K1 contains an activating mutation and K2 has an inactivating mutation.

   • If K1 is mutated to be active and K2 is inactivated, the response still occurs. This indicates K2 normally plays an inhibitory role, and K1 can bypass K2 when it is activated. This suggests that K2 negatively regulates K1 in the normal state.

3. No response is observed even in the presence of an extracellular signal when K2 is inactivated by mutations.

   • If K2 is inactivated, even an extracellular signal cannot activate the pathway. This strongly suggests that K2 plays a positive role in the pathway's activation. Thus, K2 is essential for the signaling response, likely activating K1.

4. Response is observed even in the absence of an extracellular signal when both kinases are activated by mutations.

   • If both K1 and K2 are mutated to be always active, the pathway is activated even without an extracellular signal. This observation implies that both K1 and K2 are critical and can work together to maintain activation of the pathway, indicating that K2activates K1 in the signaling cascade.

Analyzing the Statements:

1. K1 inhibits K2:

   • Based on the data, K1 does not inhibit K2. The evidence suggests K2 is important for the pathway's activation, especially when K2 is inactivated (Observation 3), making this statement incorrect.

2. K2 inhibits K1:

   • This is partially correct as K2 can inhibit K1 in the absence of a mutation. However, this does not fully explain the observed behavior when both K1 and K2 are mutated (Observation 4), where both kinases appear to activate the pathway together.

3. K1 activates K2:

   • There is no direct evidence from the observations that K1 activates K2. The data suggests that K2 plays a role in activating K1, particularly in the presence of the signal and in the absence of mutations.

4. K2 activates K1:

   • Correct: The key observation that supports this statement is Observation 4, where both K1 and K2 are activated by mutations and lead to the activation of the pathway. This suggests that K2 plays a crucial role in activating K1, as the pathway is turned on when both are mutated to be constantly active.

Conclusion:

The correct statement based on the observations is that K2 activates K1, making Option 4 the correct answer.

Correct Answer: Option 4 (K2 activates K1)