Question

Your friend is studying mouse fur color and has isolated the GPCR responsible for determining its color, as well as the extracellular signal that activates the receptor. She finds that, on addition of the signal to pigment cells (cells that produce the pigment determining fur color), cAMP levels rise in the cell. She starts a biotech company, and the company isolates more components of the signaling pathway responsible for fur color. Using transgenic mouse technology, the company genetically engineers mice that are defective in various proteins involved in determining fur color. The company obtains the following results.

Normal mice have beige (very light brown) fur color.
Mice lacking the extracellular signal have white fur.
Mice lacking the GPCR have white fur.
Mice lacking cAMP phosphodiesterase have dark brown fur.

Your friend has also made mice that are defective in the α subunit of the G protein in this signaling pathway. The defective α subunit works normally except that, once it binds GTP, it cannot hydrolyze GTP to GDP. What color do you predict that the fur of these mice will be? Why?

Answer 1

Mice with a defective α subunit of the G protein that cannot hydrolyze GTP to GDP will likely have dark brown fur, as the mutation leads to continuous signaling and increased cAMP levels, stimulating greater pigment production.

Step-by-step explanation:

If a transgenic mouse is genetically engineered to have a defective α subunit of the G protein that cannot hydrolyze GTP to GDP, we can predict the color of the fur of these mice. Normally, the α subunit of the G protein turns itself off by hydrolyzing GTP to GDP. When it cannot perform this hydrolysis, the α subunit remains active, continuously signaling downstream effects in the pathway. This is similar to the case where mice lacking cAMP phosphodiesterase, which normally degrades cAMP, have dark brown fur. The persistent signal would result in elevated cAMP levels in the pigment cells, thus we can predict that mice with the defective α subunit will also have dark brown fur. This is because uninterrupted signaling would stimulate pigment production just as in mice with uncontrolled cAMP levels.