Getting a hold on archaeal type IV pili: an expanding repertoire of cellular appendages implicates complex regulation and diverse functions

Citation: Chimileski S and Papke RT (2015) Getting a hold on archaeal type IV pili: an expanding repertoire of cellular appendages implicates complex regulation and diverse functions. Type IV pili (T4P) are a group of cell surface appendages of particular interest due to broad conservation and functional versatility across the domains Bacteria and Archaea (Albers and Meyer, 2011; Giltner et al., 2012). All T4P are composed of small protein subunits known as pilins that polymerize into helical fibers through the action of assembly ATPases (Giltner et al., 2012). This core ancestral machinery has been adapted in various lineages for many cellular processes–from adhesion and biofilm formation, to motility, horizontal gene transfer (HGT) and even electricity conduction (Giltner et al., 2012; Berry and Pelicic, 2015). When T4P structures are involved in adhesion, they are known as pili, if they no longer mediate attachment, but are associated with another function, such as scavenging macromolecules (e.g., DNA uptake by Com proteins in Bacillus subtilis), or secretion of proteins through a piston-like structure (i.e., type II secretion), they are called pseudopili (Averhoff and Friedrich, 2003; Peabody et al., 2003; Chen et al., 2005). T4P appendages may also contribute to both adhesion and another function. This dual function is sometimes true for archaella: a major group of archaeal T4P appendages characterized by the ability to rotate and enable swimming motility. Archaella are functionally analogous yet structurally and genetically unrelated to bacterial flagella (Jarrell and Albers, 2012; Shahapure et al., 2014; Albers and Jarrell, 2015). T4P have been studied to a greater extent in bacteria, in part because they are often virulence factors (Giltner et al., 2012). However, recent investigations have revealed a repertoire of archaeal T4P–highlighting implications for regulatory complexity and functional diversity. Losensky et al. (2014) demonstrated that adhesive filaments in the haloarchaeon Halobacterium salinarum R1 observed during biofilm formation (Fröls et al., 2012) are dependent on the pilus assembly ATPase gene pilB1 (Losensky et al., 2014), expanding the list of experimentally studied archaeal T4P (Table 1). Deletion of pilB1 led to a lack of pili as observed through electron microscopy and a defect in adhesion. Only 4% of a glass surface was colonized by non-piliated/non-archaellated cells (flaI/pilB1), relative to 36 and 44% for the parental and non-archaellated (flaI) strains, respectively. The molecular composition of PilB1-dependent pili has not yet been determined, however Losensky and coauthors noted that there are over 30 candidate pilins …