Abstract Pili are ubiquitous filamentous surface extensions that play crucial roles for bacterial and archaeal cellular processes such as adhesion, biofilm formation, motility, cell-cell communication, DNA uptake and horizontal gene transfer to name a few. Here we report on the discovery and structure of the archaeal thread – a remarkably stable archaeal pilus that belongs to a so-far largely unknown class of protein filaments. We find that the filament is highly glycosylated and interconnected via donor strand complementation, as well as isopeptide bonds, reminiscent of bacterial type I pili. Despite striking structural similarity with bacterial type-1 pili, archaeal threads appear to have evolved independently and are likely assembled by a markedly distinct mechanism.

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