Possible Important Pair of Acidic Residues in Vesicular Acetylcholine Transporter

Invariant E309 is in contact with critical and invariant D398 in a three-dimensional homology model of vesicular acetylcholine transporter (VAChT, TC 2.A.1.2.13) [Vardy, E., et al. (2004) Protein Sci. 13, 1832-1840]. In the work reported here, E309 and D398 in human VAChT were mutated singly and together to test their functions, assign pK values to them, and determine whether the residues are close to each other in three-dimensional space. Mutants were stably expressed in the PC12A(123,7) cell line, and transport and binding properties were characterized at different pH values using radiolabeled ligands and filtration assays. Contrary to a prior conclusion, the results demonstrate that most D398 mutants do not bind the allosteric inhibitor vesamicol even weakly. Earlier work showed that most D398 mutants do not transport ACh. D398 therefore probably is the residue that must deprotonate with a pK of 6.5 for binding of vesamicol and with a pK of similar to 5.9 for transport of ACh. Because E309Q has no effect on VAChT functions at physiological pH, E309 has no apparent critical role. However, radical mutations in E309 cause decreases in ACh and vesamicol affinities and total loss of ACh transport. Unlike wild-type VAChT, which exhibits a peak of [H-3]vesamicol binding centered at pH 7.4, mutants E309Q, E309D, E309A, and E309K all exhibit peaks of binding centered at pH >= 9. The combination of high pH and mutated E309 apparently produces a relaxed (in contrast to tense) conformation of VAChT that binds vesamicol exceptionally tightly. No compensatory interactions between E309 and D398 in double mutants were discovered. Proof of a close spatial relationship between E309 and D398 was not found. Nevertheless, the data are more consistent with the homology model than an alternative hydropathy model of VAChT that likely locates E309 far from D398 and the ACh binding site in three-dimensional space. Also, a probable network of interactions involving E309 and an unknown residue having a pK of 10 has been revealed.