Architecture of the biofilm-associated archaic CupE pilus from Pseudomonas aeruginosa
Chaperone-Usher Pathway (CUP) pili are major adhesins in Gram-negative bacteria, mediating bacterial adherence to biotic and abiotic surfaces. While classical CUP pili have been extensively characterized, little is known about so-called archaic CUP pili, which are phylogenetically widespread and promote biofilm formation by several human pathogens. In this study, we present the electron cryomicroscopy structure of the archaic CupE pilus from the opportunistic human pathogen Pseudomonas aeruginosa. We show that CupE pili consist of CupE1 subunits arranged in a zigzag architecture, with an N-terminal donor {beta}-strand extending from each subunit into the next, where it is anchored by hydrophobic interactions, resulting in an overall flexible pilus arrangement. Imaging CupE pili on the surface of P. aeruginosa cells using electron cryotomography shows that CupE pili adopt variable curvatures in response to their environment, which may facilitate their role in promoting cohesion between bacterial cells. Finally, bioinformatic analysis shows the widespread abundance of cupE genes in isolates of P. aeruginosa and the co-occurrence of cupE with other cup clusters, suggesting interdependence of cup pili in regulating bacterial adherence within biofilms. Taken together, our study provides insights into the architecture of archaic CUP pili and their role in promoting cellular adhesion and biofilm formation in P. aeruginosa.