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Reuptake, or re-uptake, is the reabsorption of a neurotransmitter by a neurotransmitter transporter of a pre-synaptic neuron after it has performed its function of transmitting a neural impulse. Reuptake is necessary for normal synaptic physiology because it allows for the recycling of neurotransmitters and regulates the level of neurotransmitter present in the synapse, thereby controlling how long a signal resulting from neurotransmitter release lasts. Because neurotransmitters are too large and hydrophilic to diffuse through the membrane, specific transport proteins are necessary for the reabsorption of neurotransmitters. Much research, both biochemical and structural, has been performed to obtain clues about the mechanism of reuptake. ==Protein structure== The first primary sequence of a reuptake protein was published in 1990. The technique for protein sequence determination relied upon the purification, sequencing, and cloning of the transporter protein in question, or expression cloning strategies in which transport function was used as an assay for cDNA species coding for that transporter. After separation, it was realized that there were many similarities between the two DNA sequences. Further exploration in the field of reuptake proteins found that many of the transporters associated with important neurotransmitters within the body were also very similar in sequence to the GABA and norepinephrine transporters. The members of this new family include transporters for dopamine, norepinephrine, serotonin, glycine, proline and GABA. They were called Na+/Cl− dependent neurotransmitter transporters. Sodium and Chloride ion dependence will be discussed later in the mechanism of action. Using the commonalities among sequences and hydropathy plot analyses, it was predicted that there are 12 hydrophobic membrane spanning regions in the ‘Classical’ transporter family. In addition to this, the N- and C-termini exist in the intracellular space. These proteins also all have an extended extracellular loop between the third and fourth transmembrane sequences. Site-directed chemical labeling experiments verified the predicted topological organization of the serotonin transporter. In addition to neurotransmitter transporters, many other proteins in both animals and prokaryotes were found with similar sequences, indicating a larger family of Neurotransmitter:Sodium Symporters (NSS). One of these proteins, LeuT, from ''Aquifex aeolicus'', was crystallized by Yamashita et al. with very high resolution, revealing a molecule of leucine and two Na+ ions bound near the center of the protein. They found that the transmembrane (TM) helices 1 and 6 contained unwound segments in the middle of the membrane. Along with these two helices, TM helices 3 and 8 and the areas surrounding the unwound sections of 1 and 6 formed the substrate and sodium ion binding sites. The crystal structure revealed pseudo-symmetry in LeuT, in which the structure of TM helices 1-5 is reflected in the structure of helices 6-10. There is an extracellular cavity in the protein, into which protrudes a helical hairpin formed by extracellular loop EL4. In TM1, an aspartate distinguishes monoamine NSS transporters from amino acid transporters which contain a glycine at the same position. External and internal “gates” were assigned to pairs of negatively and positively charged residues in the extracellular cavity and near the cytoplasmic ends of TM helices 1 and 8. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Reuptake」の詳細全文を読む スポンサード リンク
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