Crystal structure of the pilotin from the enterohemorrhagic Escherichia coli type II secretion system

Bacteria contain several sophisticated macromolecular machineries responsible for translocating proteins across the cell envelope. One prominent example is the type II secretion system (T2SS), which contains a large outer membrane channel, called the secretin. These gated channels require specialized proteins, so-called pilotins, to reach and assemble in the outer membrane. Here we report the crystal structure of the pilotin GspS from the T2SS of enterohemorrhagic Escherichia coli (EHEC), an important pathogen that can cause severe disease in cases of food poisoning. In this four-helix protein, the straight helix alpha 2, the curved helix alpha 3 and the bent helix alpha 4 surround the central N-terminal helix alpha 1. The helices of GspS create a prominent groove, mainly formed by side chains of helices alpha 1, alpha 2 and alpha 3. In the EHEC GspS structure this groove is occupied by extra electron density which is reminiscent of an a-helix and corresponds well with a binding site observed in a homologous pilotin. The residues forming the groove are well conserved among homologs, pointing to a key role of this groove in this class of T2SS pilotins. At the same time, T2SS pilotins in different species can be entirely different in structure, and the pilotins for secretins in non-T2SS machineries have yet again unrelated folds, despite a common function. It is striking that a common complex function, such as targeting and assembling an outer membrane multimeric channel, can be performed by proteins with entirely different folds. (c) 2013 Elsevier Inc. All rights reserved.