Structural Basis for Fe–S Cluster Assembly and tRNA Thiolation Mediated by IscS Protein–Protein Interactions

Abstract

Author SummarySulfur is incorporated into the backbone of almost all proteins in the form of the amino acids cysteine and methionine. In some proteins, sulfur is also present as iron–sulfur clusters, sulfur-containing vitamins, and cofactors. What's more, sulfur is important in the structure of tRNAs, which are crucial for translation of the genetic code from messenger RNA for protein synthesis. The biosynthetic pathways for assembly of these sulfur-containing molecules are generally well known, but the molecular details of how sulfur is delivered from protein to protein are less well understood. In bacteria, one of three pathways for sulfur delivery is the isc (iron-sulfur clusters) system. First, an enzyme called IscS extracts sulfur atoms from cysteine. This versatile enzyme can then interact with several proteins to deliver sulfur to various pathways that make iron–sulfur clusters or transfer sulfur to cofactors and tRNAs. This study describes in atomic detail precisely how IscS binds in a specific and yet distinct way to two different proteins: IscU (a scaffold protein for iron–sulfur cluster formation) and TusA (which delivers sulfur for tRNA modification). Furthermore, by introducing mutations into IscS, we have identified the region on the surface of this protein that is involved in binding its target proteins. These findings provide a molecular view of the protein–protein interactions involved in sulfur transfer and advance our understanding of how sulfur is delivered from one protein to another during biosynthesis of iron–sulfur clusters.