The functions of NifS-like proteins in plant sulfur and selenium metabolism

NifS-like proteins were originally studied in bacteria, where they play an important role in sulfur (S) and selenium (Se) metabolism. NifS-like proteins, now thought to exist in all organisms, are best known for their cysteine desulfurase activity that catalyzes the conversion of cysteine into alanine and elemental S needed for various cofactors: iron–sulfur clusters, thiamine, biotin and molybdenum cofactor. Plants contain three NifS-like proteins that are localized to the mitochondria (mtNifS), the chloroplast (cpNifS), and the cytosol (ABA3). mtNifS likely provides the S for the formation of biotin and Fe–S cluster assembly for mitochondrial and cytosolic proteins. cpNifS is necessary for Fe–S clusters assembled in the chloroplast, and may also be required for thiamine synthesis. The third NifS-like protein, ABA3, is cytosolic and probably does not participate in Fe–S cluster formation, but rather is required for the sulfuration of molybdenum cofactor. In addition to cysteine desulfurase activity, NifS-like proteins also possess selenocysteine lyase activity that converts selenocysteine into alanine and elemental Se. In contrast to many bacteria, animals, and some green algae that require selenocysteine lyase activity for essential selenoproteins, plants are not known to require Se. However, the selenocysteine lyase activity found in cpNifS may prevent Se toxicity in plants growing in high concentrations of selenate. This review summarizes what is known about NifS-like proteins in plants and discusses other potential roles that still need to be examined.