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

Sulfur, most abundantly found in the environment as sulfate (SO₄ ^2- ), is an essential element in metabolites required by all living cells, including amino acids, co-factors and vitamins. However, current understanding of the cellular delivery of SO₄ ^2- at the molecular level is limited. CysZ has been described as a SO₄ ^2- permease, but its sequence family is without known structural precedent. Based on crystallographic structure information, SO₄ ^2- binding and flux experiments, we provide insight into the molecular mechanism of CysZ-mediated translocation of SO₄ ^2- across membranes. CysZ structures from three different bacterial species display a hitherto unknown fold and have subunits organized with inverted transmembrane topology. CysZ from Pseudomonas denitrificans assembles as a trimer of antiparallel dimers and the CysZ structures from two other species recapitulate dimers from this assembly. Mutational studies highlight the functional relevance of conserved CysZ residues.