Question

PK1 and PK2 are protein kinases involved in a phosphorylation cascade. If either protein is inactive due to a mutation the response is not observed. A different mutation causes PK1 to be permanently active. This activity induces a response even if PK2 is inactive. Which of the following conclusions is supported by the above information?

A. PK1 activates PK2

B. PK2 activates PK1

C. PK2 is a transcription factor

D. PK1 inhibits PK2

Ca²⁺ can act as a second messenger because it is _________.

A. produced by the enzyme calcium synthase

B. normally at a high level in the cytoplasm

C. normally at a low level in the cytoplasm

D. stored in the cytoplasm

(For each answer of the above questions explain reasoning and explanation for being the correct answer).

Answer 1

B. PK2 activates PK1

It is given that if either PK1 or PK2 is inactive due to a mutation the response is not observed. It means that PK1 and PK2 function in a sequence in the signalling cascade with either PK1 activating PK2 or PK2 activating PK1. Also it is given that a mutation in PK1 causes it to be permanently active even if PK2 is inactive and induces a signalling cascade. This infers that PK2 must be upstream of PK1 i.e, PK2 activates PK1. So PK1 will be always active even if PK2 is inactive. We can rule out option A as PK1 cannot alone induce response if present upstream of PK2. In this case PK2 should also be active to induce the signalling cascade. However, it is given that PK1 induces a signalling cascade even if PK2 is inactive . So PK2 must activate PK1.

Ca2+ can act as a second messenger because it is _________.

C. normally at a low level in the cytoplasm.

Calcium is present in very low concentration in the cytoplasm which is maintained by calcium pumps in the plasma membrane or intracellular compartments like endoplasmic reticulum. When the level of Ca²⁺ rises in the cytoplasm, it acts as a second messenger to activate downstream signalling cascade like activation of myosin light chain kinase , CaM kinase, etc.