You are studying a cloned line of neuronal cells that grow in culture and form synapses with a cloned line of muscle cells. These synapses use serotonin as the neurotransmitter, and you have determined that, as expected, these neuronal cells synthesize serotonin from tryptophan. Using sophisticated molecular cloning techniques, you have isolated the genes for two serotonin transporters, called TransA and TransB, which are expressed in these neuronal cells. A preliminary immunofluorescence localization of fixed and permeabilized cells found very bright staining of TransA at the nerve cell plasma membrane and little staining elsewhere. TransB, in contrast, stained only very faintly at the nerve cell plasma membrane, but there was intense staining within the cytosol. A. In which subcellular membrane or organelle would you expect most of the TransB to be found? Explain your answer.
Yesterday you received a frantic phone call from the Director of Research at Newgene, a major pharmaceutical company. She tells you that they have generated two compounds, termed alpha and beta, that she knows both diffuse efficiently through all cell membranes and also powerfully inhibit serotonin transporters. Due to confidentiality, she cannot tell you the evidence for this assertion. You immediately obtain these compounds from Oldgene, Inc., and add them to the medium of your co-culture of neuronal and muscle cell lines. In the first experiment, you find that addition of alpha inhibits the ability of the neuronal cells to stimulate the muscle cells, while application of beta has the opposite effect, and causes the neuronal cells to induce a more prolonged contraction of the muscle cells.
B. Which transporter – TransA or TransB – is likely to be inhibited by alpha? Explain your answer.
C. Which transporter – TransA or TransB – is likely to be inhibited by beta? Explain your answer.
D. Which transporter – TransA or TransB – would you expect to find in clathrin-coated vesicles? Explain your answer.