Featured Article - October 2012
Short description: The crystal structure of MtfA reveals a self-inhibited member of the lethal factor family of endopeptidases.
MtfA was originally identified in Escherichia coli as a regulator of bacterial glucose metabolism through the phosphoenolpyruvate (PEP)-dependent glucose phosphotransferase system. In this phospho-relay system, the downstream component EIIBGlc (encoded by ptsG) ultimately becomes dephosphorylated through phosphate transfer to incoming glucose. Mlc, a global transcription repressor, is sequestered by unphosphorylated EIIBGlc to allow increased expression of glucose-utilizing genes. While the precise mechanism remains unknown, MtfA inactivates Mlc through direct binding, leading to increased ptsG expression and glucose utilization.
Wilson, Jahreis and colleagues (PSI JCSG) have now solved the crystal structures of the full-length MtfA apoenzyme from Klebsiella pneumoniae at 2.2-Å resolution (PDB 3DL1). Although this structure did not contain zinc, the presence of a conserved HEXXH zinc-binding motif commonly found in metallopeptidases led the authors to crystallize MtfA in the presence of ZnCl2 and determine the structure of the holoenzyme at 1.9-Å resolution (PDB 3KHI).
The overall structure of the holoenzyme is similar to that of the apoenzyme, with the notable exception being that zinc binding confers partial order to a region (residues 99 to 123) that is largely disordered in the apo structure. In addition, a segment of the protein within the C-terminal lobe is rearranged to optimize zinc coordination. The conformation of the strictly conserved residues of the HEXXH motif within the holoenzyme structure is essentially identical to that of anthrax lethal factor (LF), a thermolysin-like enzyme. However, the limited sequence similarities between MtfA and the LF family suggest that MtfA and its homologs might define a novel family of gluzincins.
Although water is normally the fourth ligand for the bound zinc, in the case of MtfA, the imidazole of a highly conserved His completes the coordination. This results in the protein chain blocking access to the active site in a non-productive, self-inhibited conformation. Although MtfA inhibits Mlc function through direct binding, Mlc is not a substrate of MtfA, but rather stimulates the cleavage of peptide substrates. This observation supports the hypothesis that MtfA becomes activated upon interactions with one or several partners, and that self-inhibition is part of a mechanism regulating this family of endopeptidases.
Q. Xu et al. The Structure of Mlc Titration Factor A (MtfA/YeeI) Reveals a Prototypical Zinc Metallopeptidase Related to Anthrax Lethal Factor. J Bacteriol. 194, 2987-2999 (2012). doi:10.1128/JB.00038-12