Signal-strapping as a protein-sequence search method for the discovery of metalloproteins

Metalloprotein discovery is often made post hoc, in which activity studies following protein isolation reveal a metal-ion dependence. Herein we take a different approach to finding metalloproteins, by building on the discovery of copper-containing lytic polysaccharide monooxygenases (LPMOs), which include an N-terminal histidine as part of their sequence. This residue acts as a natural chelator for transition metal ions, irrespective of the structure of the protein. We report the method of signal strapping, where sequences of N-terminal signal peptides artificially appended with a histidine residue at their C-terminus are used to bootstrap a proteomic search. These searches return sequences of proteins with an N-terminal histidine capable of coordinating a metal ion. We exemplify the approach by the discovery and characterisation of four classes of bacterial metalloproteins, including two that we denote as anglerases reflecting their potential to capture transition metal ions from the bacterial environment.