Variation in type IV pilus stability modulates DNA-uptake and biofilm formation.

Abstract

Type IV pili are helical filaments composed of protein subunits which are produced by numerous taxa of bacteria, including Acinetobacter. Type IV pili are extended out from the cell by extension enzyme complexes, which extract subunits from the membrane and insert them into the base of the filament, but can also be retracted by reverse rotation catalyzed by a retraction enzyme. Type IV pili have diverse functions, including twitching motility and DNA-uptake, which require retraction, and host adhesion and bacterial aggregation, which do not. Acinetobacter bacteria, including International Clone I (IC-I) and International Clone II (IC-II) strains, show variable phenotypes in assays of type IV pilus-dependent functions. Here, we show this variation is the result of differentiation of type IV pilus subtypes in Acinetobacter, which we defined based on the sequence of the major subunit, PilA. These subtypes show variable efficiency in pilus retraction between pilus subtypes, and from that, a differential balance between retraction-dependent and retraction-independent functions. In both naturally-occurring pilA variants from the IC-I and IC-II groups and isogenic strains complemented with IC-I or IC-II pilA, the IC-I pilus subtype promotes greater twitching motility and DNA-uptake while the IC-II pilus subtype promotes biofilm formation while showing reduced capacity for DNA-uptake and twitching motility, similar to a retraction-deficient mutant and consistent with the hypothesis that pilus retraction of the IC-II pilus is naturally deficient. This defect in retraction was sufficient to increase the level of piliation on the cell surface when we compared the yields of T4P sheared from the cell surface from IC-I pilA and IC-II pilA complements in an isogenic background. Complementation with IC-II pilA results in greater levels of surface PilA per cell than equivalent complementation with an IC-I pilA gene. Additionally, direct comparisons of pilus stability between type IV pili isolated from IC-I pilA and IC-II pilA complements show greater thermostability for the IC-II pili, supporting the hypothesis that pilus stability can impede retraction and increase piliation.