Question

You have discovered a protein important for regulating cell size. You name it PLUS+1. In the laboratory you are investigating the signal transduction pathway that leads to increased PLUS+1 expression. You discover that increased expression begins with activation of the GRG3 receptor which then leads to activation of the Ras-like GTP-binding protein, Fds. Fds then activates a kinase that activates the transcription factor, JJ7D. JJ7D is only imported into the nucleus when it is phosphorylated. In the nucleus, JJ7D activates the transcription of the PLUS+1 gene.

When cells are not exposed to the ligand, they are an average of 15.4 micrometres in diameter. When the cells are exposed to a GRG3 ligand, they have a diameter of around 8.5 micrometres. Which situation will more likely produce smaller cells?
- A GRG3-binding toxin that stimulates the GTPase activity of Fds
- A GRG3-binding drug that stimulates the degradation of PLUS+1
- A GRG3-binding poison that inhibits JJ7D binding to DNA
- A GRG3-binding chemical that inhibits the activity of the phosphatase that acts on JJ7D

Answer 1

A GRG3-binding toxin that stimulates the GTPase activity of Fds is more likely to produce smaller cells.

Step-by-step explanation:

The situation that is more likely to produce smaller cells is a GRG3-binding toxin that stimulates the GTPase activity of Fds.

When cells are exposed to a GRG3 ligand, their diameter decreases from 15.4 micrometers to around 8.5 micrometers. This indicates that the GRG3 ligand leads to a decrease in cell size. In the given signal transduction pathway, Fds is activated by GRG3 and plays a role in regulating the expression of PLUS+1, a protein important for cell size regulation.

Therefore, a GRG3-binding toxin that stimulates the GTPase activity of Fds would lead to increased GTP hydrolysis by Fds, resulting in reduced activation of the downstream kinase and transcription factor. As a result, the transcription of the PLUS+1 gene and the regulation of cell size would be compromised, leading to smaller cells.

Answer 2

Inhibiting the phosphatase that acts on JJ7D using a GRG3-binding chemical is the most likely option to produce smaller cells, because it would maintain JJ7D in its active, phosphorylated state and sustain PLUS+1 expression.

Step-by-step explanation:

Among the options provided for increasing the expression of PLUS+1 and therefore influencing cell size, the last option involving a GRG3-binding chemical that inhibits the activity of the phosphatase acting on JJ7D would most likely produce smaller cells.

This is because phosphatases remove phosphate groups from proteins, typically deactivating them, and in this case would deactivate the phosphorylated JJ7D that is required for its importation into the nucleus where it activates PLUS+1 gene transcription.

By inhibiting this phosphatase, the JJ7D transcription factor would remain phosphorylated, and therefore active, leading to sustained activation of PLUS+1 expression ultimately resulting in reduced cell size, which correlates to the observed smaller cell diameter when GRG3 is activated.