Moonlighting O-acetylserine sulfhydrylase: New functions for an old protein.

O-acetylserine sulfhydrylase A (CysK) is the pyridoxal 5'-phosphate-dependent enzyme that catalyzes the final reaction of cysteine biosynthesis in bacteria. CysK was initially identified in a complex with serine acetyltransferase (CysE), which catalyzes the penultimate reaction in the synthetic pathway. This "cysteine synthase" complex is stabilized by insertion of the CysE C-terminus into the active-site of CysK. Remarkably, the CysK/CysE binding interaction is conserved in most bacterial and plant systems. For the past 40years, CysK was thought to function exclusively in cysteine biosynthesis, but recent studies have revealed a repertoire of additional "moonlighting" activities for this enzyme. CysK and its paralogs influence transcription in both Gram-positive bacteria and the nematode Caenorhabditis elegans. CysK also activates an antibacterial nuclease toxin produced by uropathogenic Escherichia coli. Intriguingly, each moonlighting activity requires a binding partner that invariably mimics the C-terminus of CysE to interact with the CysK active site. This article is part of a Special Issue entitled: Cofactor-dependent proteins: evolution, chemical diversity and bio-applications.