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

Abstract

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.SignificanceBiofilms 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.

Donor strand complementation, isopeptide bonds and glycosylation reinforce highly resilient archaeal thread filaments

“…Similar observations were made for the TasA filament formed by B. subtilis, where TapA a helps nucleating filament assembly of TasA, as well as the recently characterized archaeal bundle pili 46,61 . Interestingly, we find that unlike Saci_0406, the putative cap protein Saci_0405 does not have a predicted SEC signal sequence or a transmembrane domain.…”

“…Donor strand complementation has previously been found in various types of bacterial pili, such as T1P, P-pili and T5P 2 , as well as in the very recently published structure of the archaeal bundling pili of Pyrobaculum calidifontis 46 . All of these filaments have been implicated in adhesion or biofilm formation.…”

“…As no isopeptide bonds were reported in the recently solved archaeal bundle pili of P. calidifonitis, they may be a unique feature of threads 18,19,46 . In addition, compared to the archaeal bundle pili, Saci_0406 does not contain a PFAM domain, which appears to be confined to Caldarchaeales and an uncharacterized lineage 46 . These differences suggest that threads form a separate class of archaeal cell surface filaments.…”

“…The filament polymerization is initialized by an accessory protein TapA which can bind to the strand accepting groove of TasA, so TasA can initiate binding to the next subunit 61 . Structural prediction of the archaeal bundle pili subunit revealed a similar mechanism 46 . In contrast, we did not observe auto complementation for Saci_0406, potentially highlighting a distinct assembly mechanism.…”

Electron cryo-microscopy reveals the structure of the archaeal thread filament

“…Threads likely gain particular tensile strength and resilience against heat and acidic pH through three structural adaptions that we have discovered: DSC, the proposed intermolecular covalent isopeptide bonds, as well as a high degree of glycosylation. DSC has been found previously in various types of bacterial pili, such as T1P, P-pili (CU pili), T5P 59 , as well as in the very recently published structure of the archaeal bundling pili of Pyrobaculum calidifontis 62 . All these filaments have been implicated in adhesion or biofilm formation.…”