During the Mating Process, Yeast Cells Respond to Pheromones Secreted

Multiple Choice

During the mating process, yeast cells respond to pheromones secreted by other yeast cells.These pheromones bind GPCRs on the surface of the responding cell and lead to the activation of G proteins inside the cell.When a wild-type yeast cell senses the pheromone, its physiology changes in preparation for mating: the cell stops growing until it finds a mating partner.If yeast cells do not undergo the appropriate response after sensing a pheromone, they are considered sterile.Yeast cells that are defective in one or more components of the G protein have characteristic phenotypes in the absence and presence of the pheromone, which are listed in Table 16-14. $$\begin{array} { l l l } & {\underline{ \quad\quad\quad\quad\quad\quad\text { phenotype\quad\quad\quad\quad }} } \\ \text { mutation } & \text { minus pheromone } & \text { plus pheromone } \\\hline \text { none (wild type) } & \text { normal growth } & \text { arrested growth, mating response } \\\alpha \text { subunit deleted } & \text { arrested growth } & \text { arrested growth, sterile } \\\beta \text { subunit deleted } & \text { normal growth } & \text { normal growth, sterile } \\\gamma \text { subunit deleted } & \text { normal growth } & \text { normal growth, sterile } \\\alpha \text { and } \beta \text { deleted } & \text { normal growth } & \text { normal growth, sterile } \\\alpha \text { and } \gamma \text { deleted } & \text { normal growth } & \text { normal growth, sterile } \\\beta \text { and } \gamma \text { deleted } & \text { normal growth } & \text { normal growth, sterile }\end{array}$$ Table 16-14
Which of the following models is consistent with the data from the analysis of these mutants? Explain your answer.

A) The ? subunit activates the mating response but is inhibited when bound to ??.
B) The ?? subunit activates the mating response but is inhibited when bound to ?.
C) The G protein is inactive; either the free ? or free ?? complex is capable of activating the mating response.
D) The G protein is active; both free ? and free ?? complex are required to inhibit the mating response.