Suppose you are studying a kinase signaling cascade using a

Suppose you are studying a kinase signaling cascade using a yeast model system. It has been determined that the key output signal of the cascade is production of a rare phosphorylated plasma membrane lipid, phosatidylinositol 3-phosphate (PI-3-P), which is simply PI that has been phosphorylated at the 3-position on the inositol head-group. You have identified three genes that specifically affect production of this signal, called pik1, pik2, and pik3. All three genes are predicted to be kinases. The problem is that kinases in this family can either be protein kinases or lipid kinases, this cannot be determined from the sequence, and you need to know which one is the lipid kinase that produces PI-3-P. Your screen produced knockout alleles of all three kinases. The phenotype of the pik1 mutant and pik3 single mutants was decreased production of PI-3-P. The phenotype of the pik2 mutant was over-production of PI-3. You also made and tested the following double mutant strains: pikl-pik2. increased PI-3-P production: pik2+pik3, decreased PI-3-P production. Which gene encoded the PI-3 kinase and how do you know? Speculate on the roles of the other two kinases, what docs each of their phosphorylations do to the activity of their targets?

Solution

pik3 is the lipid kinase.

When pik1 was mutated, there was decreased production of PI-3-P because pik2 was controlling pik3.

When pik2 was mutated, there was increased production of PI-3-P because pik3 was not controlled by pik2

When pik3 was mutated, there was decreased production of PI-3-P because pik3 is the lipid kinase and that was mutated

When both pik1 and pik2 were mutated, production increased because pik2 was not there to control pik3

When both pik2 and pik3 were mutated, production decreased because pik3 is the lipid kinase and that was mutated