Structure-based analysis of CysZ-mediated cellular uptake of sulfate

Sulfur, most abundantly found in the environment as sulfate (SO 4 2- ), is an essential element in metabolites required by all living cells, including amino acids, co-factors and vitamins. Current understanding of the cellular delivery of SO 4 2- at the molecular level is limited however. CysZ has been described as a SO 4 2- permease, but its sequence family is without known structural precedent. Based on crystallographic structure information, SO 4 2- binding and uptake experiments in cells and proteoliposomes, and single-channel conductance measurements, we provide insight into the molecular mechanism of CysZ-mediated translocation of SO 4 2- across membranes. CysZ properties differ markedly from those of known transporters and ion channels. The structures display a hitherto unknown fold with dual topology, assembling in CysZ from Pseudomonas denitrificans as a trimer of antiparallel dimers in the membrane. CysZ structures from two other species recapitulate dimers from this assembly. Mutational studies highlight the functional relevance of conserved CysZ residues.