The predicted structure of photopexin from Photorhabdus shows the first haemopexin-like motif in prokaryotes

Abstract

The insect pathogenic bacterium Photorhabdus luminescens secretes several insecticidal high molecular mass ‘toxin complexes’. Analysis of the putative pathogenicity island surrounding the toxin complex a ( tca) locus revealed two open reading frames (ORFs) of unknown function. The predicted protein sequences of these ORFs show a repeated motif similar to those found in the vertebrate haem scavenging molecule haemopexin, limunectin (a phosphocholine binding protein from Limulus) and the C-terminal domains of matrix metalloproteinases (MMPs) (where they are thought to be important for cell attachment and adhesion). We have therefore named the operon photopexin AB and the putative encoded proteins ‘photopexins’ A and B (PpxA and PpxB). The predicted amino acid sequence of PpxA was modelled onto the crystal structure of a MMP. Our model predicts not only that PpxA and PpxB have β-propeller domains but also that each haemopexin-like repeat corresponds to one blade of a propeller, suggesting the limunectin structure itself may also contain two or three such haemopexin-like propellers. The overall structure of PpxA has striking similarity to that of haemopexin suggesting that it may be used by the bacterium to scavenge iron containing compounds from insects. The implications for the potential role of Ppx proteins in pathogenicity are discussed. This is the first finding of a haemopexin-like repeat outside plants and animals.