12. Cell signalling pathways I and II were studied independently, and researchers
arrived at the following schematics:
𝐼: 𝐴 → 𝐵 → 𝐶 → 𝐷
𝐼𝐼: 𝐸 → 𝐹 → 𝐺 → 𝐻
Later these pathways were found to cross-talk. When only pathway I is activated, C
and H are phosphorylated. An inhibitor of B inhibits this phosphorylation. When
pathway II is activated, G and C are phosphorylated while H is not phosphorylated.
An inhibitor of F inhibits phosphorylation of G and C while restoring phosphorylation
of H. Identify the kinases and phosphatases.
a. B,F,G are kinases
b. B,C,G are phosphatases
c. B,G are kinases and F is a phosphatase
d. B,F are kinases and G is a phosphatase

The correct answer is option c: B and G are kinases, F is a phosphatase. This fits all experimental observations in the cell signalling pathways crosstalk scenario.

Pathway I Activation

Activation of pathway I (A → B → C → D) alone phosphorylates both C and H. An inhibitor of B blocks this phosphorylation of C and H, indicating B acts as a kinase that phosphorylates C directly and influences H through crosstalk. B’s inhibition prevents signal propagation to both targets.

Pathway II Activation

Activation of pathway II (E → F → G → H) alone phosphorylates G and C but not H. This shows G (kinase) auto-phosphorylates and cross-phosphorylates C, while F (phosphatase) dephosphorylates H to prevent its activation. H remains unphosphorylated due to active F.

Inhibitor Effects

Inhibiting F blocks phosphorylation of G and C but restores H phosphorylation. F normally dephosphorylates H (keeping it off); its inhibition allows H to become phosphorylated via crosstalk from pathway I’s basal activity or G’s influence. This confirms F as phosphatase and G as kinase. B’s role as kinase is consistent across activations.

Option Analysis

a. B, F, G are kinases fails because F inhibition restores H phosphorylation, indicating phosphatase activity (dephosphorylation block), not kinase loss.

b. B, C, G are phosphatases contradicts observations; phosphatases remove phosphates, but B inhibition blocks addition to C/H, and G/C gain phosphates in pathway II.

d. B, F kinases, G phosphatase mismatches pathway II where G phosphorylates itself and C (kinase behavior), not dephosphorylates; F inhibition enabling H fits phosphatase only.