High Resolution Cryo-Em Structure of the EspA Filament from EPEC

Abstract

Antimicrobial resistance is a growing concern for the global population, predominantly in animal husbandry, hospitals, and developing countries. Several pathogens are of concern, including Escherichia coli. The rise in resistance has contributed to the decline in development of novel antibiotics, thus pushing us closer to a pre-antibiotic era. Virulence mechanisms that bacteria use are often not required for their survival and therefore are promising targets for the development of anti-virulence compounds. The T3SS is a proteinaceous channel that spans the inner and outer membranes of the bacterial cell, projecting into the extracellular medium where it interacts with the host cell membrane to deliver virulence factors. The main components of the T3SS are: (1) the basal body, spanning the inner and outer membranes of the bacterial cell; (2) the needle that projects from the basal body, and (3) the tip and pore-forming translocon that completes the channel from bacteria to host. Entero-pathogenic and -hemorrhagic E. coli (EPEC/EHEC) possess a distinct T3SS from that of other species; EPEC/EHEC possess a needle extension, or filament, in place of the ‘tip’ component, called EspA. The function of EspA is required for colonization of EPEC within the gut and espA EPEC knockouts are associated with decreased virulence. Here we describe the cryo-EM structure of natively-sheared EspA filaments from EPEC, determined to 3.6 Å resolution. Within the filament lumen, a pattern of positively charged residues adjacent to a hydrophobic groove lines the lumen of the filament in a spiral manner, suggesting a mechanism of translocation through the channel mediated via electrostatics. Using structure-guided mutagenesis of these residues, in vivo studies corroborate the role of these residues in secretion function. The high-resolution structure of the EspA filament will aid in structure-guided drug design of novel antivirulence therapeutics.