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

The insect pathogenic bacterium Photorhabdus luminescens secretes several i nsecticidal 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 s equences of these ORFs show a repeated motif similar to those found in the vertebrate haem scavenging molecule haemopexin. limunectin (a phosphocholin e binding protein from Limulus) and the C-terminal domains of matrix metall oproteinases (MMPs) (where they are thought to be important for cell attach ment and adhesion). We have therefore named the operon photopexin AB and th e putative encoded proteins 'photopexins' A and B (PpxA and PpxB). The pred icted amino acid sequence of PpxA was modelled onto the crystal structure o f a MMP. Our model predicts not only that PpxA and PpxB have beta-propeller domains but also that each haemopexin-like repeat corresponds to one blade of a propeller. suggesting the limunectin structure itself may also contai n two or three such haemopexin-like propellers. The overall structure of Pp xA has striking similarity to that of haemopexin suggesting that it may be used by the bacterium to scavenge iron containing compounds from insects. T he implications for the potential role of Ppx proteins in pathogenicity are discussed. This is the first finding of a haemopexin-like repeat outside p lants and animals. (C) 2000 Federation of European Microbiological Societie s. Published by Elsevier Science B.V. All rights reserved.