Cryo-EM structural determination of Met18, a scaffold protein for iron-sulfur protein maturation

Iron-sulfur ([Fe-S]) clusters are essential cofactors for sustaining life. Because [Fe-S] clusters play a role in a wide range of cellular processes, defects in their biosynthesis, trafficking, and incorporation into target proteins lead to human diseases. The molecular details of how iron-sulfur clusters are assembled and transferred to their target proteins are not well understood. Therefore, the goal of this work is to structurally and biophysically characterize proteins involved in the Cytosolic Iron-Sulfur Cluster Assembly (CIA) pathway. One of these proteins, Met18, is in the CIA targeting complex, and this complex is believed to receive a mature [4Fe-4S] cluster and transfer it to target proteins downstream. To better understand its function, we determined a 3.3 Å resolution cryo-EM structure of Met18. In the absence of other CIA targeting complex proteins and target proteins, we observed that Met18 forms a hexamer with its N-terminus exposed and its C-terminus buried. Conserved sequences within Met18 assist in forming the hexamer. Upon the addition of Cia2, a CIA targeting complex protein, the hexamer appears to be broken up. We hypothesize that the hexamer is a storage form of Met18 that protects itself from ubiquitination and proteosomal degradation. Here we present the cryo-EM structure determination of the Met18 hexamer.