The synaptic vesicle protein SV 31 assembles into a dimer and transports Zn 2+

The integral synaptic vesicle protein SV 31 has been shown to bind divalent cations. Here, we demonstrate that SV 31 protein synthesized within a cell‐free system binds Zn 2+ and to a lower extent Ni 2+ and Cu 2+ ions. Expression with Zn 2+ stabilized the protein and increased solubility. SV 31 was preferentially monomeric in detergent and revealed specific binding of Zn 2+ . When co‐translationally inserted into defined nanodisc bilayers, SV 31 assembled into dimeric complexes, resulting in increased binding of Zn 2+ . Putative Zn 2+ ‐binding motifs within SV 31 comprise aspartic acid and histidine residues. Site‐directed mutagenesis of two conserved aspartic acid residues leads to a potent decrease in Zn 2+ binding but did not affect dimerization. Chemical modification of histidine residues abolished some of the Zn 2+ ‐binding capacity. We demonstrate proton‐dependent transport of Zn 2+ as by accumulation of fluorescent FluoZin‐1 inside of SV 31‐containing proteoliposomes. Transport activity has a K m value of 44.3 μM and required external Zn 2+ and internal acidic pH . Our results demonstrate that the synaptic vesicle‐integral protein SV 31 functions as a proton‐dependent Zn 2+ transporter. SV 31 may attribute specific and yet undiscovered functions to subsets of synapses.