Iron-sulfur Cluster Biosynthesis in Methanogens

Methanogens live in a syntrophic consortium with bacteria, taking advantage of the metabolic abilities of their syntrophic partners to overcome energetic barriers and break down compounds that they cannot digest by themselves. Interspecies electron transfer, which is a major type of microbial communication in syntrophic processes, improves methanogenesis and anaerobic oxidization of methane (AOM) processes involved in syntrophic consortia. These processes have a significant impact on the global carbon cycle. Most of the essential enzymes involved in methanogenesis are iron-sulfur proteins. Iron-sulfur clusters are one of the oldest and most versatile cofactors present in all domains of life. To date, four different Fe-S cluster assembly systems have been identified in bacteria (ISC, NIF, and SUF) and eukaryotes (ISC, CIA, and SUF). However, little is known about the Fe-S cluster assembly system in archaea. Only three proteins related to Fe-S cluster assembly are conserved in almost all sequenced archaea: SufB, SufC, and the ApbC/Nbp35 homolog. The ancestral suf operon likely only contains sufBC. Here, we provide the biochemical and spectroscopic characterizations of the Methanococcus maripaludis (Mmp) SufB, SufC, and ApbC proteins. Our major findings include: (i) The SufB and SufC form a SufB2C2 complex in methanogens; (ii) The SufB2C2 is a functional scaffold, which can assemble and transfer a [4Fe-4S] cluster; (iii) The [4Fe-4S] cluster of the SufB2C2 complex is located on three highly conserved cysteine residues, Cys218, Cys237, and Cys240, on SufC; (iv) The SufC has ATPase activity, which is not required for Fe-S cluster assembly and transfer activities; (v) The archaeal Nbp35/ApbC homolog contains a [4Fe-4S] cluster which can be transferred to activate aconitase; (vi) M. maripaludis shows no growth defect when the archaeal Nbp35/ApbC gene is deleted. Together, our results suggest that the SufB2C2 complex is a functional essential scaffold for Fe-S cluster biosynthesis in archaea and that the archaeal Nbp35/ApbC homolog is a functional, but not essential scaffold/carrier protein for Fe-S cluster maturation in M. maripaludis