Which of the following statements is true of signal molecules? Question 3 options: When signal molecules first bind to receptor tyrosine kinases, the receptors phosphorylate a number of nearby molecules. In response to some G protein-mediated signals, a special type of lipid molecule associated with the plasma membrane is cleaved to form IP3 and calcium. In most cases, signal molecules interact with the cell at the plasma membrane and then enter the cell and eventually the nucleus. Toxins such as those that cause botulism and cholera interfere with the ability of activated G proteins to hydrolyze GTP to GDP, resulting in phosphodiesterase activity in the absence of an appropriate signal molecule. Protein kinase A activation is one possible result of signal molecules binding to G protein-coupled receptors.