Complex Formation of the Maturation Machinery in the Biosynthesis of the Lasso Peptide Microcin J25

Abstract

Microcin J25 (MccJ25) is a 21 residue antibacterial peptide with unique properties. Belonging to a class of molecules known as lasso peptides, it exhibits a threaded lasso structure consisting of an eight-residue N-terminal macrolactam ring and a C-terminal tail that loops back and threads through the ring. MccJ25 is ribosomally synthesized and produced in strains of Escherichia coli harboring a plasmid-borne fourgene cluster encoding a 58 amino acid MccJ25 precursor McjA, maturation enzymes McjB and McjC, and an exporter McjD. McjB and McjC have been reported to function interdependently and associate with the inner membrane. Thus, it is believed that a structural complex termed “microcin J25 synthetase” involving McjB, McjC, and the cytoplasmic, nucleotide-binding domain of McjD is responsible for carrying out MccJ25 maturation. Here, we present evidence in support of this putative complex. Through pulldown assays and size-exclusion chromatography, we have demonstrated protein-protein interactions between the nucleotide-binding domain of McjD and both McjB and McjC. However, we were unable to relate these interactions to any effect on the functional activity of the enzymes in vivo. Initial in vitro MccJ25 maturation attempts using purified McjB and McjC also proved ineffective. Our findings help shed new light on the MccJ25 maturation mechanism. Additionally, they hold important implications for future studies on lasso peptide biosynthesis and the application of lasso peptides as therapeutic agents.