Abstract
Vesicular monoamine transporters (VMAT) catalyze transport of serotonin, dopamine, epinephrine, and norepinephrine into subcellular storage organelles in a variety of cells. Accumulation of the neurotransmitter depends on the proton electrochemical gradient (Delta micro H+) across the organelle membrane and involves VMAT-mediated exchange of two lumenal protons with one cytoplasmic amine. Mutagenic analysis of the role of two conserved Asp residues located in transmembrane segments X and XI of rat VMAT type I reveals an important role of these two residues in catalysis. Replacement of Asp 431 with either Glu or Ser inhibits VMAT-mediated [3H]serotonin transport. The mutated proteins are unimpaired in ligand recognition as measured with the high affinity ligand [3H]reserpine or coupling to the proton electrochemical gradient as judged by its ability to accelerate [3H]reserpine binding. Therefore, the Asp residue is needed as such in this position and even a conservative replacement with Glu generates a protein that can catalyze only partial reactions but cannot complete the transport cycle. Replacement of Asp 404 with either Ser or Cys inhibits all VMAT-mediated reactions measured. However, replacement with Glu generated a protein that catalyzed [3H]serotonin transport with modified properties. Whereas the mutated protein binds [3H]reserpine to normal levels and the pH optimum of this reaction is only slightly affected, the optimum pH for transport activity shifted to the acid side and became very sharp; in addition the sensitivity to the inhibitor tetrabenazine increased significantly in this mutated protein. The results point to the need of a carboxyl moiety in position 404. A slight change in its relative location or in the environment around it has a significant effect on the pK of group(s) involved in steps after ligand recognition and coupling to the first H+.
Highlights
The results suggested that either proton transport or a conformational change induced by proton transport was inhibited by diethyl pyrocarbonate (DEPC)
Asp-431 and Asp-404 residues are charged residues located in the middle of two putative transmembrane segments in rVMAT1, and they are fully conserved in all the vesicular neurotransmitter transporters sequences available [5]
Replacement of Asp-431 with Either Glu, Cys, or Ser Completely Abolishes Transport Catalyzed by rVMAT1—When Asp431 is replaced with either Glu, Cys, or Ser the effect on [3H]serotonin transport activity is very dramatic
Summary
DEPC inhibited transport, it had no effect on binding of reserpine, indicating that the inhibition of transport was not due to a direct interaction with either of the known binding sites. The results suggested that either proton transport or a conformational change induced by proton transport was inhibited by DEPC. Identical results were obtained with phenylglyoxal, a reagent specific for Arg residues. Cloning of VMAT [11, 12] made it feasible to try to identify the residue(s) modified by DEPC. One His (His-419) is Classical neurotransmitters are stored in synaptic vesicles and storage organelles of secretory cells.
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