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Type IV Pilins Regulate Their Own Expression Via Direct Intramembrane Interactions with the Sensor Kinase Pils

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Type IV Pilins Regulate Their Own Expression Via Direct Intramembrane Interactions with the Sensor Kinase Pils Type IV pilins regulate their own expression via direct intramembrane interactions with the sensor kinase PilS Sara L. N. Kilmurya,b and Lori L. Burrowsa,b,1 aDepartment of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, ON, Canada L8S 4K1; and bMichael G. DeGroote Institute for Infectious Diseases Research, McMaster University, Hamilton, ON, Canada L8S 4K1 Edited by Scott J. Hultgren, Washington University School of Medicine, St. Louis, MO, and approved April 12, 2016 (received for review July 1, 2015) Type IV pili are important virulence factors for many pathogens, regulator, which regulates gene expression in response to the stim- including Pseudomonas aeruginosa. Transcription of the major pilin ulus (18). gene—pilA—is controlled by the PilS-PilR two-component system in In the PilS-PilR TCS, PilR is the cytoplasmic response regulator response to unknown signals. The absence of a periplasmic sensing that activates pilA transcription (19). Its cognate SK PilS is atyp- domain suggested that PilS may sense an intramembrane signal, pos- ical, with six TM segments connected by very short loops and no sibly PilA. We suggest that direct interactions between PilA and PilS in obvious external signal input domain (20). When PilS is absent, the inner membrane reduce pilA transcription when PilA levels are pilA transcription is significantly reduced, whereas loss of PilR high. Overexpression in trans of PilA proteins with diverse and/or trun- abrogates pilA transcription (13). Interestingly, overexpression of cated C termini decreased native pilA transcription, suggesting that the full-length PilS also decreases pilin expression, whereas expression highly conserved N terminus of PilA was the regulatory signal. Point of its cytoplasmic kinase domain alone leads to constitutive pilA transcription, a common result of decoupling sensor kinases from mutations in PilA or PilS that disrupted their interaction prevented their signal input domains (19, 21, 22). autoregulation of pilA transcription. A subset of PilA point mutants Recently, a conserved E/DxxN/T phosphatase motif adjacent to retained the ability to interact with PilS but could no longer decrease the phosphorylated His was identified in the HisKA family of SKs pilA transcription, suggesting that interaction between the pilin and to which PilS belongs, indicating that many SKs can have dual sensor kinase is necessary but not sufficient for pilA autoregulation. kinase and phosphatase activities to fine tune regulation (23). PilS Furthermore, PilS’s phosphatase motif was required for the autoregu- has a canonical ExxN motif at position 320–323, beside the H319 lation of pilA transcription, suggesting that under conditions where phosphorylation site. Coupled with the observation that PilS PilA is abundant, the PilA–PilS interaction promotes PilR dephosphory- overexpression reduces pilA transcription, the presence of this lation and thus down-regulation of further pilA transcription. These motif suggests that PilS could have intrinsic phosphatase activity data reveal a clever bacterial inventory control strategy in which the on phospho-PilR. major subunit of an important P. aeruginosa virulence factor controls Among the most significant challenges in the TCS field is its own expression. identification of the specific signal(s) to which SKs respond. The observation that pilA transcription is inversely correlated type IV pili | two-component system | transcriptional regulation | with levels of PilA in the inner membrane, coupled with the Pseudomonas aeruginosa | Geobacter sulfurreducens unusual six TM topology of PilS, suggested that it recognizes an MICROBIOLOGY intramembrane signal, possibly PilA itself. Here, we show that direct intramembrane interactions between the conserved, hy- wide variety of bacteria, including the opportunistic pathogen drophobic N terminus of PilA and one or more TM of PilS Pseudomonas aeruginosa, use type IV Pili (T4P) for attachment A down-regulate pilA transcription, and define the sequence el- to surfaces and host tissues, biofilm formation, DNA uptake, and ements on PilA involved in interaction and regulation. Based – twitching motility (1 4). T4P are retractile surface appendages on analysis of PilS mutants, we propose that PilA–PilS inter- comprised predominantly of thousands of subunits of the major pilin actions likely maintain PilS in a phosphatase state when protein, PilA (5), which is rapidly polymerized and depolymerized by membrane pools of PilA are high, providing a sensitive feed- a complex assembly machine. Also incorporated into T4P are small back mechanism for pilin inventory control. amounts of the minor pilins FimU, PilV, PilW, PilX, and PilE (P. aeruginosa nomenclature), which prime pilus assembly (6, 7). Significance PilA and the minor pilins share a similar lollipop-like topology, α with a highly conserved, hydrophobic N-terminal -helix packed Although two-component systems are a ubiquitous means of β ∼ against a variable C-terminal antiparallel -sheet (8, 9). The first 24 rapid bacterial adaptation to changing environments, identifica- residues of mature pilins anchor the subunits in the inner membrane tion of the specific signals detected by sensor kinases can be until they are polymerized by the assembly machinery (10). When pili challenging. Also, little is known about the diverse, poorly char- are retracted, subunits are disassembled from the base of the pilus acterized family of sensor kinases that detect intramembrane and returned to the inner membrane for reuse in subsequent cycles of signals. We show that the major type IV pilin, PilA, is an inhibitory extension. In P. aeruginosa, high levels of PilA in the inner membrane intramembrane ligand for the PilS sensor kinase that controls pilA lead to decreased pilA transcription, whereas depletion of PilA inner expression and we characterize the mechanism of signal trans- membrane pools significantly elevates pilA transcription (11, 12). duction. Because the conserved N-terminal domain of PilA alone These data suggest that pilA expression is responsive to levels of in- can repress pilA expression, peptides corresponding to this short tracellular PilA, although the sensory mechanism is unknown. region could have potential as therapeutic agents to suppress pilA transcription in P. aeruginosa and many other species is de- type IV Pili (T4P) biogenesis. pendent on the PilS-PilR two-component regulatory system (13–16). Two-component systems (TCSs) allow bacteria to rapidly detect and Author contributions: S.L.N.K. and L.L.B. designed research; S.L.N.K. performed research; adapt to changes in their environment (17). The sensor kinase (SK) S.L.N.K. and L.L.B. analyzed data; and S.L.N.K. and L.L.B. wrote the paper. detects physical or chemical signals, typically via an extracytoplasmic The authors declare no conflict of interest. domain flanked by two transmembrane (TM) segments (17). On This article is a PNAS Direct Submission. detection of an activating signal, the protein undergoes autophos- 1To whom correspondence should be addressed. Email: [email protected]. phorylation on a conserved His residue in the cytoplasmic kinase This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. domain. The phosphate is transferred to a cytoplasmic response 1073/pnas.1512947113/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1512947113 PNAS | May 24, 2016 | vol. 113 | no. 21 | 6017–6022 Downloaded by guest on September 25, 2021 supported near-WT levels of PilA expression, overexpression of the heterologous group III PilA in this background failed to reduce native PilA levels (Fig. 1B). Although overexpression of heterologous pilins in the pres- ence of WT PilS reduced native PilA levels, T4P function was unaffected if the heterologous subunits were competent for as- sembly. GS can assemble P. aeruginosa pilins (31), but the re- verse was not true, even when pilus retraction was blocked to maximize the capture of assembled pili (Fig. S2A). When the assembly-incompetent GS pilins were expressed from an in- ducible vector in P. aeruginosa, twitching motility decreased in an inducer concentration-dependent manner (Fig. S2B). To address whether overexpression of other pilin-like proteins with divergent N termini could impact PilA levels, each of the minor pilins—FimU, PilV, PilW, PilX, or PilE—was overex- pressed in strain PAK. However, the levels of PilW and PilX were lower than those of other minor pilins and they were therefore excluded. Of the three remaining minor pilins, only PilE reduced native PilA levels, to ∼60% of the vector control. Neither FimU nor PilV had an effect on PilA levels when overexpressed (Fig. Fig. 1. Overexpression of heterologous pilins reduces native PilA levels. 1C). Thus, specific sequences in the pilin N terminus are required (A) V5 epitope tagged major pilins from PA1244 (group I), PA14 (group III), for pilA regulation. PA5196 (group IV), PA1457 (group V), and the G. sulfurreducens PilA were To determine if PilA autoregulation occurred at the level of overexpressed in the PAK (group II) strain. Heterologous PilAs overexpressed transcription, the pilA promoter was cloned upstream of a luciferase to similar extents (as demonstrated by α-V5 Western blot) reduced native (lux) reporter (pMS402-ppilA). pilA promoter activity was moni- PilA levels. (B) In the absence of the TM segments of PilS (pilScyt), over- tored in PAK carrying the pBADGr vector or pBADGr-pilA, expression of group III PilA no longer reduces native
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