Unusual cell surfaces, pili, and archaella of Thermoplasmatales archaea

Archaea of the order Thermoplasmatales push the boundaries of our current knowledge of prokaryotic life. They show distinct cellular plasticity, heterogenous cell morphologies, and lack a paracrystalline S-layer. As the S-layer has previously been implicated in acting as a stator scaffold for filaments driving cellular propulsion, particularly archaella, we asked whether the absence of an S-layer precludes the formation of functional archaella or pili in Thermoplasmatales. Using cryoEM, we investigated the two Thermoplasmatales species Cuniculiplasma divulgatum and Oxyplasma meridianum. We found that these species indeed generate pili and archaella, and that the latter likely function in cellular propulsion. Whereas C. divulgatum produces pili with terminal hooks using a unique assembly machinery, O. meridianum generates wide, "barbed" archaella with an unusually high degree of glycosylation. Our results show that, for the generation of functional archaella and pili, a canonical S-layer is not necessary.