An Archaeal Pilus Survives the Most Extreme Conditions Through Extensive Glycosylation

Pili on the surface of Sulfolobus islandicus are used for a host of functions, and serve as receptors for certain archaeal viruses. We find that 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 when looking at assemblies where all of the components may not be known. The atomic structure of the archaeal pilus was unusual due to almost a third of the residues being either threonine or serine and many hydrophobic surface residues. While the map showed specific glycosylation of only three residues, mass per unit length measurements suggested extensive glycosylation. We show 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 quite similar to archaeal flagellin, establishing common evolutionary origins.