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 immunofluores- cence 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.

Required:
In which subcellular membrane or organelle would you expect most of the TransB to be found? Explain your answer.

Most neuronal cells have vesicles in their interior, which are organelles that store neurotransmitters for exportation -by exocytosis- or from recycling -by endocytosis-. These vesicles also protect the neurotransmitter from the enzymatic action.

Vesicles form in the cellular soma, from where they are transported to nervous terminals. Once the vesicle releases the neurotransmitter to the intercellular space, their membrane remains available in the plasmatic membrane to be reused.

The neurotransmitter concentration in the vesicle interior is related to the storage system and the transport system. There are specialized transporter proteins in the vesicle membrane that are involved with the introduction of the molecule to the organelle.

In the exposed example, transporter Trans B might be located in the vesicle membranes, and hence could be found in the cytosol of the cells.