Sulfolobus acidocaldarius adhesion pili power twitching motility in the absence of a dedicated retraction ATPase.

Abstract

Type IV pili are ancient and widespread filamentous organelles found in most bacterial and archaeal phyla. They support a wide range of fundamental functions including adhesion to substrates, DNA uptake, self-aggregation and motility. In particular, disassembly of type IV pili mediated by PilT-family ATPases allows pilus retraction, which is responsible for twitching motility that is important for surface colonization in bacteria. Archaea do not possess PilT retraction ATPases, and it was therefore hypothesized that archaeal cells are not capable of twitching motility. Yet, recent microscopic observations of live Sulfolobus acidocaldarius showed that cells exhibit short-range surface-motility that resembles bacterial twitching motility. S. acidocaldarius is a thermoacidophilic crenarcheon that encodes three different type IV pili systems with non-redundant functions. While archaella are rotating type IV pili that allow swimming motility, UV-inducible pili promote cell aggregation and DNA exchange upon exposure to UV light, and adhesion pili are responsible for surface attachment and biofilm formation. Here, using a combination of automated single cell tracking at high temperature, high-temperature fluorescence imaging, and genetic manipulations, we demonstrate that S. acidocaldarius exhibits bona fide twitching motility, and that this behavior depends specifically on adhesion pili. We also find that twitching motility is conserved in other Sulfolobales species. Our results show that adhesion pili are capable of retraction in the absence of a PilT retraction ATPase. While PilT-independent pilus retraction was previously observed in bacteria such as C. crescentus, our study suggests that in archaea, which do not encode a dedicated retraction ATPase, this might be a common mechanism for pilus retraction. Our results also suggest that the ancestral type IV pilus machinery (maybe in LUCA) was capable of dynamic retraction and extension cycles in the absence of a dedicated retraction ATPase.