Question

The lab you work in has discovered a previously unidentified extracellular signal molecule called QGF, a 75,000-dalton protein. You add purified QGF to different types of cells to determine its effect on these cells. When you add QGF to heart muscle cells, you observe an increase in cell contraction. When you add it to fibroblasts, they undergo cell division. When you add it to nerve cells, they die. When you add it to glial cells, you do not see any effect on cell division or survival. Given these observations, which of the following statements is most likely to be true? And why?

(a) Because it acts on so many diverse cell types, QGF probably diffuses across the plasma membrane into the cytoplasm of these cells.
(b) Glial cells do not have a receptor for QGF.
(c) QGF activates different intracellular signaling pathways in heart muscle cells, fibroblasts, and nerve cells to produce the different responses observed.
(d) Heart muscle cells, fibroblasts, and nerve cells must all have the same receptor for QGF.

Answer 1

Option A is not possible because a 75 kDa protein is too large to diffuse through the plasma membrane.

Option B also cannot be assumed without more information. Even though the addition of QGF does not affect the cell division or survival of glial cells, there may be other signaling pathways that are triggered.

Option D is also not possible because the same receptor would give rise to the same signaling cascade.

Option C is correct - QGF activates different intracellular signaling pathways in heart muscle cells, fibroblasts, and nerve cells to produce the different responses observed.

Assuming it was the same cascading pathway, the same responses would be observed. However, this is not the case (heart muscle cells - cell contraction; fibroblasts - cell division; nerve cells - death; glial cells - no effect). Therefore, the different responses are due to different pathways.