Structure and function of a YeeE-YeeD complex for sophisticated thiosulfate uptake

Abstract

From bacteria to eukaryotes, sulfur is a vital element for cellular activities. For example, sulfur34 containing biomolecules, such as L-cysteine, L-methionine, thiamine, glutathione, and biotin, play a 35 variety of essential roles in cells (1). Bacteria and plants can utilize L-cysteine as a source of sulfate, 36 but they also have sulfur assimilation pathways to synthesize L-cysteine from inorganic sulfur 37 compounds. In bacteria, L-cysteine is important not only as a component for protein synthesis but also 38 as a reducing agent against oxidative stress (2). There are two pathways for bacterial L-cysteine 39 synthesis, which use O-acetylserine as a precursor, called the sulfate and thiosulfate pathways (3). In 40 the sulfate pathway, sulfate ion is first decomposed into sulfide ion through phosphorylation by two 41 molecules of ATP and reduction by four molecules of NADPH. Subsequently, L-cysteine is 42 synthesized from sulfide ion and O-acetylserine by O-acetylserine sulfhydrylase-A (CysK). In the 43 thiosulfate pathway, S-sulfocysteine is synthesized from thiosulfate ion and O-acetylserine by O44 acetylserine sulfhydrylase-B (CysM); S-sulfocysteine is then reduced by one NADPH molecule, and 45 L-cysteine is synthesized. The sulfate and thiosulfate ions used in these pathways are taken up from 46 the environment by transporters on the cytoplasmic membrane. Sulfate and thiosulfate ions are trapped 47 by periplasmic proteins Sbp (4) and CysP (5), respectively. Both sulfate and thiosulfate ions are then 48 passed on to a complex formed by the inner membrane proteins CysU and CysW and the cytosolic 49 protein CysA (CysUWA) (also called CysTWA) (3) and transported into the cytoplasm by the 50 CysUWA complex using the energy from ATP hydrolysis.