Archaeal bundling pili of Pyrobaculum calidifontis reveal similarities between archaeal and bacterial biofilms

While biofilms formed by bacteria have received great attention due to their importance in pathogenesis, much less research has been focused on the biofilms formed by archaea. It has been known that extracellular filaments in archaea, such as Type IV pili, hami and cannulae, play a part in the formation of archaeal biofilms. We have used cryo-electron microscopy to determine the atomic structure of a previously uncharacterized class of archaeal surface filaments from hyperthermophilic Pyrobaculum calidifontis . These filaments, which we call archaeal bundling pili (ABP), assemble into highly ordered bipolar bundles. The bipolar nature of these bundles most likely arises from the association of filaments from at least two different cells. The component protein shows homology, both at the sequence and structural level, to the bacterial protein TasA, a major component of the extracellular matrix in bacterial biofilms, contributing to biofilm stability. We show that ABP forms very stable filaments in a manner similar to the donor-strand exchange of bacterial TasA fibers and chaperone-usher pathway pili where a β-strand from one subunit is incorporated into a β-sheet of the next subunit. Our results reveal mechanistic similarities and evolutionary connection between bacterial and archaeal biofilms, and suggest that there could be many other archaeal surface filaments that are as yet uncharacterized. Significance Biofilms are communities of microbes where cells attach to each other as well as to surfaces, and bacterial biofilms have been intensively studied due to their importance in many infections. Much less has been known about archaeal biofilms, where archaea are a third domain of life. Using cryo-electron microscopy, we have determined the atomic structure of a surface filament in archaea that forms bi-polar bundles connecting cells. We show that this protein has common ancestry with a protein known to be an important component of bacterial biofilms. This adds to our understanding of the evolutionary relationship between bacteria and archaea and may provide new insights into bacterial biofilms.