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?