Novel Role for PilNO in Type IV Pilus Retraction Revealed by Alignment Subcomplex Mutations.

UNLABELLED: Type IV pili (T4P) are dynamic protein filaments that mediate bacterial adhesion, biofilm formation, and twitching motility. The highly conserved PilMNOP proteins form an inner membrane alignment subcomplex required for function of the T4P system, though their exact roles are unclear. Three potential interaction interfaces for PilNO were identified: core-core, coiled coils (CC), and the transmembrane segments (TMSs). A high-confidence PilNO heterodimer model was used to select key residues for mutation, and the resulting effects on protein-protein interactions were examined both in a bacterial two-hybrid (BTH) system and in their native Pseudomonas aeruginosa context. Mutations in the oppositely charged CC regions or the TMS disrupted PilNO heterodimer formation in the BTH assay, while up to six combined mutations in the core failed to disrupt the interaction. When the mutations were introduced into the P. aeruginosa chromosome at the pilN or pilO locus, specific changes at each of the three interfaces--including core mutations that failed to disrupt interactions in the BTH system--abrogated surface piliation and/or impaired twitching motility. Unexpectedly, specific CC mutants were hyperpiliated but nonmotile, a hallmark of pilus retraction defects. These data suggest that PilNO participate in both the extension and retraction of T4P. Our findings support a model of multiple, precise interaction interfaces between PilNO; emphasize the importance of studying protein function in a minimally perturbed context and stoichiometry; and highlight potential target sites for development of small-molecule inhibitors of the T4P system. IMPORTANCE: Pseudomonas aeruginosa is an opportunistic pathogen that uses type IV pili (T4P) for host attachment. The T4P machinery is composed of four cell envelope-spanning subcomplexes. PilN and PilO heterodimers are part of the alignment subcomplex and essential for T4P function. Three potential PilNO interaction interfaces (the core-core, coiled-coil, and transmembrane segment interfaces) were probed using site-directed mutagenesis followed by functional assays in an Escherichia coli two-hybrid system and in P. aeruginosa. Several mutations blocked T4P assembly and/or motility, including two that revealed a novel role for PilNO in pilus retraction, while other mutations affected extension dynamics. These critical PilNO interaction interfaces represent novel targets for small-molecule inhibitors with the potential to disrupt T4P function.

PilMNOPQ from the Pseudomonas aeruginosa type IV pilus system form a transenvelope protein interaction network that interacts with PilA.

Pseudomonas aeruginosa type IV pili (T4P) are virulence factors that promote infection of cystic fibrosis and immunosuppressed patients. As the absence of T4P impairs colonization, they are attractive targets for the development of novel therapeutics. Genes in the pilMNOPQ operon are important for both T4P assembly and a form of bacterial movement, called twitching motility, that is required for pathogenicity. The type II membrane proteins, PilN and PilO, dimerize via their periplasmic domains and anchor this complex in the inner membrane. Our earlier work showed that PilNO binds PilP, a periplasmic lipoprotein (S. Tammam, L. M. Sampaleanu, J. Koo, P. Sundaram, M. Ayers, P. A. Chong, J. D. Forman-Kay, L. L. Burrows, and P. L. Howell, Mol. Microbiol. 82:1496-1514, 2011). Here, we show that PilP interacts with the N0 segment of the outer membrane secretin PilQ via its C-terminal domain, and that the N-terminal cytoplasmic tail of PilN binds to the actin-like protein PilM, thereby connecting all cellular compartments via the PilMNOPQ protein interaction network. We show that PilA, the major pilin subunit, interacts with PilNOPQ. The results allow us to propose a model whereby PilA makes extensive contacts with the transenvelope complex, possibly to increase local concentrations of PilA monomers for polymerization. The PilNOP complex could provide a stable anchor in the inner membrane, while the PilMNOPQ transenvelope complex facilitates transit of the pilus through the periplasm and clamps the pilus in the cell envelope. The PilMN interaction is proposed to be responsible for communicating signals from the cytoplasmic to periplasmic components of this complex macromolecular machine.

PilF is an outer membrane lipoprotein required for multimerization and localization of the Pseudomonas aeruginosa Type IV pilus secretin.

Type IV pili (T4P) are retractile appendages that contribute to the virulence of bacterial pathogens. PilF is a Pseudomonas aeruginosa lipoprotein that is essential for T4P biogenesis. Phenotypic characterization of a pilF mutant confirmed that T4P-mediated functions are abrogated: T4P were no longer present on the cell surface, twitching motility was abolished, and the mutant was resistant to infection by T4P retraction-dependent bacteriophage. The results of cellular fractionation studies indicated that PilF is the outer membrane pilotin required for the localization and multimerization of the secretin, PilQ. Mutation of the putative PilF lipidation site untethered the protein from the outer membrane, causing secretin assembly in both inner and outer membranes. T4P-mediated twitching motility and bacteriophage susceptibility were moderately decreased in the lipidation site mutant, while cell surface piliation was substantially reduced. The tethering of PilF to the outer membrane promotes the correct localization of PilQ and appears to be required for the formation of stable T4P. Our 2.0-A structure of PilF revealed a superhelical arrangement of six tetratricopeptide protein-protein interaction motifs that may mediate the contacts with PilQ during secretin assembly. An alignment of pseudomonad PilF sequences revealed three highly conserved surfaces that may be involved in PilF function.

Functional role of conserved residues in the characteristic secretion NTPase motifs of the Pseudomonas aeruginosa type IV pilus motor proteins PilB, PilT and PilU.

Type IV pili are retractable protein fibres used by many bacterial pathogens for adherence, twitching motility, biofilm development and host colonization. In Pseudomonas aeruginosa, PilB and PilT are bipolar proteins belonging to the secretion NTPase superfamily, and power pilus extension and retraction, respectively, while the unipolar PilT paralogue PilU supports pilus retraction in an unknown manner. Assay of purified 6xHis-tagged PilB, PilT and PilU from P. aeruginosa showed that all three proteins have ATPase activities in vitro. Conserved residues in the Walker A (WA), Walker B (WB), Asp Box and His Box motifs characteristic of secretion NTPases were mutated, and complementation of twitching motility was tested. Mutation of conserved WA or WB residues in any of the three ATPases abrogated twitching motility, and for the WA mutant of PilT caused loss of polar localization. The requirement for three invariant acidic residues in the Asp Box motif, and for two invariant His residues in the His Box motif varied, with PilB being the least tolerant of changes. In all three proteins, the third acidic residue in the Asp Box and the second His of the His Box were crucial for function; mutation of these residues caused loss of PilT ATPase activity in vitro. Modelling of the effects of these mutations on the crystal structures of Aquifex aeolicus PilT and Vibrio cholerae EpsE (a PilB homologue) showed that the critical Asp Box and His Box residues contribute to a catalytic pocket that surrounds the ligand. These results provide experimental evidence differentiating widely conserved Asp and His Box residues that are essential for function from those whose roles are modulated by specific local environments.

A duck delta1 crystallin double loop mutant provides insight into residues important for argininosuccinate lyase activity.

Delta-crystallin is directly related to argininosuccinate lyase (ASL), and catalyzes the reversible hydrolysis of argininosuccinate to arginine and fumarate. Two delta-crystallin isoforms exist in duck lenses, delta1 and delta2, which are 94% identical in amino acid sequence. Although the sequences of duck delta2-crystallin (ddeltac2) and duck delta1-crystallin (ddeltac1) are 69 and 71% identical to that of human ASL, respectively, only ddeltac2 has maintained ASL activity. Domain exchange experiments and comparisons of various delta-crystallin structures have suggested that the amino acid substitutions in the 20's (residues 22-31) and 70's (residues 74-89) loops of ddeltac1 are responsible for the loss of enzyme activity in this isoform. To test this hypothesis, a double loop mutant (DLM) of ddeltac1 was constructed in which all the residues that differ between the two isoforms in the 20's and 70's loops were mutated to those of ddeltac2. Contrary to expectations, kinetic analysis of the DLM found that it was enzymatically inactive. Furthermore, binding of argininosuccinate by the DLM, as well as the ddeltac1, could not be detected by isothermal titration calorimetry (ITC). To examine the conformation of the 20's and 70's loops in the DLM, and to understand why the DLM is unable to bind the substrate, its structure was determined to 2.5 A resolution. Comparison of this structure with both wild-type ddeltac1 and ddeltac2 structures reveals that the conformations of the 20's and 70's loops in the DLM mutant are very similar to those of ddeltac2. This suggests that the five amino acid substitutions in domain 1 which lie outside of the two loop regions and which are different in the DLM, and ddeltac2, must be important enzymatically. The structure of the DLM in complex with sulfate was also determined to 2.2 A resolution. This structure demonstrates that the conformational changes of the 280's loop and domain 3, previously observed in ddeltac1, also occur in the DLM upon sulfate binding, reinforcing the hypothesis that these events may occur in the active ddeltac2 protein during catalysis.

Structural studies of duck delta2 crystallin mutants provide insight into the role of Thr161 and the 280s loop in catalysis.

Delta crystallin, a taxon-specific crystallin present in avian eye lenses, is homologous to the urea cycle enzyme ASL (argininosuccinate lyase). Although there are two delta crystallin isoforms in duck lenses, ddeltac1 (duck delta1 crystallin) and ddeltac2 (duck delta2 crystallin), only ddeltac2 is catalytically active. Previous structural studies have suggested that residues Ser283 and His162 in the multi-subunit active site of ddeltac2/ASL are the putative catalytic acid/base, while the highly conserved, positively charged Lys289 is thought to help stabilize the carbanion intermediate. The strict conservation of a small hydroxy-containing residue (Thr or Ser) at position 161 adjacent to the putative catalytic base, as well as its proximity to the substrate in the S283A ddeltac2 enzyme-substrate complex, prompted us to investigate further the role this residue. Structures of the active T161S and inactive T161D ddeltac2 mutants, as well as T161D complexed with argininosuccinate, have been determined to 2.0 A resolution. The structures suggest that a hydroxy group is required at position 161 to help correctly position the side chain of Lys289 and the fumarate moiety of the substrate. Threonine is probably favoured over serine, because the interaction of its methyl group with Leu206 would restrict its conformational flexibility. Residues larger than Thr or Ser interfere with substrate binding, supporting previous suggestions that correct positioning of the substrate's fumarate moiety is essential for catalysis to occur. The presence of the 280s loop (i.e. a loop formed by residues 270-290) in the 'open' conformation suggests that loop closure, thought to be essential for sequestration of the substrate, may be triggered by the formation of the carbanion or aci-carboxylate intermediates, whose charge distribution more closely mimics that of the sulphate ion found in the active-site region of the inactive ddeltac1. The 280s loop in ddeltac1 is in the closed conformation.

Mutational analysis of duck delta 2 crystallin and the structure of an inactive mutant with bound substrate provide insight into the enzymatic mechanism of argininosuccinate lyase.

The major soluble avian eye lens protein, delta crystallin, is highly homologous to the housekeeping enzyme argininosuccinate lyase (ASL). ASL is part of the urea and arginine-citrulline cycles and catalyzes the reversible breakdown of argininosuccinate to arginine and fumarate. In duck lenses, there are two delta crystallin isoforms that are 94% identical in amino acid sequence. Only the delta2 isoform has maintained ASL activity and has been used to investigate the enzymatic mechanism of ASL. The role of the active site residues Ser-29, Asp-33, Asp-89, Asn-116, Thr-161, His-162, Arg-238, Thr-281, Ser-283, Asn-291, Asp-293, Glu-296, Lys-325, Asp-330, and Lys-331 have been investigated by site-directed mutagenesis, and the structure of the inactive duck delta2 crystallin (ddeltac2) mutant S283A with bound argininosuccinate was determined at 1.96 A resolution. The S283A mutation does not interfere with substrate binding, because the 280's loop (residues 270-290) is in the open conformation and Ala-283 is more than 7 A from the substrate. The substrate is bound in a different conformation to that observed previously indicating a large degree of conformational flexibility in the fumarate moiety when the 280's loop is in the open conformation. The structure of the S283A ddeltac2 mutant and mutagenesis results reveal that a complex network of interactions of both protein residues and water molecules are involved in substrate binding and specificity. Small changes even to residues not involved directly in anchoring the argininosuccinate have a significant effect on catalysis. The results suggest that either His-162 or Thr-161 are responsible for proton abstraction and reinforce the putative role of Ser-283 as the catalytic acid, although we cannot eliminate the possibility that arginine is released in an uncharged form, with the solvent providing the required proton. A detailed enzymatic mechanism of ASL/ddeltac2 is presented.