LAUSR

The integral synaptic vesicle protein SV31 has been shown to bind divalent cations. Here, we demonstrate that SV31 protein synthesized within a cell‐free system binds Zn2+ and to a lower extent Ni2+ and Cu2+ ions. Expression with Zn2+ stabilized the protein and increased solubility. SV31 was preferentially monomeric in detergent and revealed specific binding of Zn2+. When co‐translationally inserted into defined nanodisc bilayers, SV31 assembled into dimeric complexes, resulting in increased binding of Zn2+. Putative Zn2+‐binding motifs within SV31 comprise aspartic acid and histidine residues. Site‐directed mutagenesis of two conserved aspartic acid residues leads to a potent decrease in Zn2+ binding but did not affect dimerization. Chemical modification of histidine residues abolished some of the Zn2+‐binding capacity. We demonstrate proton‐dependent transport of Zn2+ as by accumulation of fluorescent FluoZin‐1 inside of SV31‐containing proteoliposomes. Transport activity has a Km value of 44.3 μM and required external Zn2+ and internal acidic pH. Our results demonstrate that the synaptic vesicle‐integral protein SV31 functions as a proton‐dependent Zn2+ transporter. SV31 may attribute specific and yet undiscovered functions to subsets of synapses.