An extensively glycosylated archaeal pilus survives extreme conditions

Pili on the surface of Sulfolobus islandicus are used for many functions, and serve as receptors for certain archaeal viruses. The cells grow optimally at pH 3 and 80° C, exposing these extracellular appendages to a very harsh environment. These pili, when removed from cells, resist digestion by trypsin or pepsin, and survive boiling in SDS or 5M guanidinium-HCl. We have used cryo-EM to determine the structure of these filaments at 4.1 Å resolution. An atomic model was built by combining the map with bioinformatics without prior knowledge of the pilin sequence, an approach that should prove useful for assemblies where all of the components are not known. The atomic structure of the pilus was unusual, with almost a third of the residues being either threonine or serine, and with many hydrophobic surface residues. While the map showed extra density consistent with glycosylation for only three residues, mass measurements suggested extensive glycosylation. We propose that this extensive glycosylation renders these filaments soluble and provides the remarkable structural stability. We also show that the overall fold of the archaeal pilin is remarkably similar to archaeal flagellin, establishing common evolutionary origins.