Introducing Photox: A Novel Actin-Targeting Mono-ADP-Ribosyltransferase

Abstract

Photorhabdus luminescens is a pathogenic bacterium that produces many toxic proteins. Previously primarily known to target insects, Photorhabdus has been studied for its potential use in agriculture and the control of pests. However, Photorhabdus infections of humans are now beginning to be seen in the United States and Australia. The mono-ADP-ribosyltransferases (mARTs) are an enzyme class produced by numerous pathogenic bacteria and participate in diseases in plants and animals, including humans. We have discovered and characterized a novel mART from P. luminescens, which has been named Photox. This 46 kDa toxin shows high homology to other actin-targeting mARTs in key catalytic regions and a similar core catalytic fold. Furthermore, Photox shows in vivo cytotoxic activity against yeast, and growth recovery with the substitution of alanine for catalytic residues. In vitro, enzymatic activity is quite high (kcat, 2235 ± 270 min−1) and comparable with that of iota toxin from Clostridium perfringens. Substitutions of hallmark catalytic residues within Photox result in decreases in mART activity up to 20,000-fold. This toxin specifically ADP-ribosylates actin at Arg177, targeting each of alpha-, beta-, and gamma-actin isoforms, and inhibiting regular polymerization of actin filaments. By epifluorescent microscopy, Photox has been seen to associate with actin within yeast cells. After nearly a decade since the last addition to this enzyme family, Photox is the newest actin-targeting ADP-ribosyltransferase.