Structure and conservation of amyloid spines from the Candida albicans Als5 adhesin including similarity to human LARKS

Candida Als family adhesins mediate adhesion to biological and abiotic substrates, as well as fungal cell aggregation and fungal-bacterial co-aggregation. The activity of at least two family members, Als5 and Als1, is dependent on amyloid-like protein aggregation that is initiated by shear force. Each Als adhesin has a ~300-residue N-terminal Ig-like/invasin region. The following 108-residue, low complexity, threonine-rich (T) domain unfolds under shear to expose a critical amyloid-forming segment 322SNGIVIVATTRTV334 at the interface between the Ig-like/invasin domain 2 and the T domain of Candida albicans Als5. Amyloid prediction programs identified six potential amyloidogenic sequences in the Ig/invasin region and three others in the T domain of C. albicans Als5. Peptides derived from four of these sequences formed fibrils that bound thioflavin T, the amyloid indicator dye, and three of these revealed atomic-resolution structures of cross-b spines. These are the first atomic-level structures for fungal adhesins. One of these segments, from the T domain, revealed kinked b-sheets, similarly to LARKS (Low-complexity, Amyloid-like, Reversible, Kinked segments) found in human functional amyloids. Based on the cross-b structures in Als proteins, we use evolutionary arguments to identify functional amyloidogenic sequences in other fungal adhesins. Thus, cross-b structures are often involved in fungal pathogenesis and potentially in antifungal therapy.

Characterisation, Manipulation and Directed Evolution of Non-Ribosomal Peptide Synthetase Enzymes

<p><b>Non-ribosomal peptide synthetases (NRPS) are large, modular enzymes that synthesisebiologically active secondary metabolites from amino acid precursors without the need for anucleic acid template. NRPS play an integral role in microbial physiology and also havepotential applications in the synthesis of novel peptide molecules. Both of these aspects areexamined in this thesis.</b></p> <p>Under conditions of iron starvation Pseudomonas syringae synthesises siderophores for activeuptake of iron. The primary siderophore of P. syringae is pyoverdine, a fluorescent moleculethat is assembled from amino acid (aa) precursors by NRPS. Five putative pyoverdine NRPSgenes in P. syringae pv. phaseolicola 1448a (Ps1448a) were identified and characterised insilico and their role in pyoverdine biosynthesis was confirmed by gene knockout. Creation ofpyoverdine null Ps1448a enabled identification of a previously uncharacterised temperatureregulatedsecondary siderophore, achromobactin, which is NRPS independent and has loweraffinity for iron. Pyoverdine and achromobactin null mutants were characterised in regard toiron uptake, virulence and growth in iron-limited conditions. Determination of the substratespecificity for the seven adenylation (A) domains of the Ps1448a pyoverdine sidechain NRPSwas also attempted. Although ultimately unsuccessful, these attempts provided a rigorousassessment of methods for the expression, purification and biochemical characterisation of Adomains.</p> <p>The Ps1448a NRPS were subsequently employed in domain swapping experiments to testcondensation (C) domain specificity for aa substrates during peptide formation in vivo.</p> <p>Experiments in which the terminal C- and/or A-domain of the Pseudomonas aeruginosa(PAO1) pyoverdine NRPS system were replaced with alternative domains from Ps1448a andPAO1 were consistent with previous in vitro observations that C-domains exhibit strongsidechain and stereo-selectivity at the downstream aa position, but only stereo-selectivity atthe upstream aa position.</p> <p>These results prompted investigation into the role of inter-domain communication in NRPSfunction, to test the hypothesis that the thiolation (T) domain enters into specific interactionswith other domains, which might provide an alternative explanation for the diminished activityof recombinant NRPS enzymes. A recently characterised single-module NRPS, bpsA, waschosen as a reporter gene for these experiments based on its ability to generate blue pigment inEscherichia coli. Substitution of the native bpsA T-domain consistently impaired function,consistent with the hypothesis. It was shown that directed evolution could be applied to restorefunction in substituted T-domains. Mutations that restored function were mapped in silico, anda structural model for interaction between the thioester (TE) and T-domain of BpsA wasderived.</p> <p>The utility of bpsA for discovery and characterisation of phosphopantetheinyl transferase(PPTase) enzymes was also investigated. In vivo and in vitro assays for determination ofPPTase activity were developed and a high-throughput screen for discovery of new PPTases inenvironmental DNA libraries was successfully implemented.</p>

Characterisation, Manipulation and Directed Evolution of Non-Ribosomal Peptide Synthetase Enzymes

<p><b>Non-ribosomal peptide synthetases (NRPS) are large, modular enzymes that synthesisebiologically active secondary metabolites from amino acid precursors without the need for anucleic acid template. NRPS play an integral role in microbial physiology and also havepotential applications in the synthesis of novel peptide molecules. Both of these aspects areexamined in this thesis.</b></p> <p>Under conditions of iron starvation Pseudomonas syringae synthesises siderophores for activeuptake of iron. The primary siderophore of P. syringae is pyoverdine, a fluorescent moleculethat is assembled from amino acid (aa) precursors by NRPS. Five putative pyoverdine NRPSgenes in P. syringae pv. phaseolicola 1448a (Ps1448a) were identified and characterised insilico and their role in pyoverdine biosynthesis was confirmed by gene knockout. Creation ofpyoverdine null Ps1448a enabled identification of a previously uncharacterised temperatureregulatedsecondary siderophore, achromobactin, which is NRPS independent and has loweraffinity for iron. Pyoverdine and achromobactin null mutants were characterised in regard toiron uptake, virulence and growth in iron-limited conditions. Determination of the substratespecificity for the seven adenylation (A) domains of the Ps1448a pyoverdine sidechain NRPSwas also attempted. Although ultimately unsuccessful, these attempts provided a rigorousassessment of methods for the expression, purification and biochemical characterisation of Adomains.</p> <p>The Ps1448a NRPS were subsequently employed in domain swapping experiments to testcondensation (C) domain specificity for aa substrates during peptide formation in vivo.</p> <p>Experiments in which the terminal C- and/or A-domain of the Pseudomonas aeruginosa(PAO1) pyoverdine NRPS system were replaced with alternative domains from Ps1448a andPAO1 were consistent with previous in vitro observations that C-domains exhibit strongsidechain and stereo-selectivity at the downstream aa position, but only stereo-selectivity atthe upstream aa position.</p> <p>These results prompted investigation into the role of inter-domain communication in NRPSfunction, to test the hypothesis that the thiolation (T) domain enters into specific interactionswith other domains, which might provide an alternative explanation for the diminished activityof recombinant NRPS enzymes. A recently characterised single-module NRPS, bpsA, waschosen as a reporter gene for these experiments based on its ability to generate blue pigment inEscherichia coli. Substitution of the native bpsA T-domain consistently impaired function,consistent with the hypothesis. It was shown that directed evolution could be applied to restorefunction in substituted T-domains. Mutations that restored function were mapped in silico, anda structural model for interaction between the thioester (TE) and T-domain of BpsA wasderived.</p> <p>The utility of bpsA for discovery and characterisation of phosphopantetheinyl transferase(PPTase) enzymes was also investigated. In vivo and in vitro assays for determination ofPPTase activity were developed and a high-throughput screen for discovery of new PPTases inenvironmental DNA libraries was successfully implemented.</p>

Genetic manipulation of pyoverdine non-ribosomal peptide synthetases to identify genetic constraints to effective domain recombination

<p>Non-ribosomal peptide synthetases (NRPSs) synthesise small highly diverse peptides with a wide range of activities, such as antibiotics, anticancer drugs, and immunosuppressants. NRPS synthesis often resembles an assembly line, in which each module acts in a linear order to add one monomer to the growing peptide chain. In the basic mechanism of synthesis, an adenylation (A) domain within each module activates a specific monomer. Once activated, the monomer is attached to an immediately downstream thiolation (T) domain via a prosthetic phosphopantheine group, which acts as a flexible arm to pass the substrate between catalytic domains. A condensation (C) domain, upstream to the A-T domains, catalyses peptide bond formation between an acceptor substrate attached to the T domain and a donor substrate attached to the T domain of the upstream module. The peptide remains attached to the T domain of the acceptor substrate, and then acts as the donor substrate for the next C domain. When peptide synthesis reaches the final module, the peptide is released by a thioesterase (TE) domain. The linear mode of synthesis and discrete functional domains within each module gives the potential to generate new products by substituting domains or entire modules with ones that activate alternative substrates. Attempts to create new products using domain and module substitution often result in a loss of activity. The work in this thesis focuses on identifying barriers to effective domain substitution. The NRPS enzyme pvdD, which adds the final residue to the eleven residue non-ribosomal peptide pyoverdine, was developed as a model for domain substitution. The primary benefit for using this model is that pyoverdine creates easily detectible fluorescent products. The first set of experiments focused on testing the limitations of A domain and C-A domain substitutions to alter pyoverdine. Nine A domain and nine C-A domain substitution pvdD variants were constructed and used to complement a P. aeruginosa PAO1 pvdD deletion strain. The A domain substitutions that specified the wild type substrate were highly functional, whereas A domains that specified other substrates resulted in low levels of wild type pyoverdine production. This suggests the acceptor site substrate specificity of the C domain limited the success of A domain substitutions, rather than disruption of the C/A domain junction. In contrast, although C-A domain substitutions in pvdD in some cases synthesised novel pyoverdines, the majority lost function for unknown reasons. The high success rate A domain substitutions (when not limited by the acceptor site specificity of the C domain) suggested the addition of new C domains was a likely cause for loss of function. The second set of experiments investigated whether disrupting the protein interface between C domains and their upstream T domains may cause a loss in function of C-A domain substitutions. However, domain substitutions of T domains were found to have a high rate of success. Therefore, the results thus far confirmed that disrupting interactions of the C domain with A domains or T domains does not have a large affect on enzyme activity. An alternative explanation for the loss in function with C-A domain substitutions is that C domains translocated to a new enzyme are unable to process the new incoming donor peptide chain because of substrate specificity or steric constraints. To develop methods to circumvent limitations caused by the C domain, the final part of this thesis examined acceptor substrate specificity of C domains. Acceptor site substrate specificity was chosen over donor site specificity as it acts on only an amino acid rather than peptide chain. The substrate specificity was narrowed down to a small subsection of the C domain. This was an initial study of C domain substrate specificity, which may guide future development of relaxed specificity C domains.</p>

Genetic manipulation of pyoverdine non-ribosomal peptide synthetases to identify genetic constraints to effective domain recombination

<p>Non-ribosomal peptide synthetases (NRPSs) synthesise small highly diverse peptides with a wide range of activities, such as antibiotics, anticancer drugs, and immunosuppressants. NRPS synthesis often resembles an assembly line, in which each module acts in a linear order to add one monomer to the growing peptide chain. In the basic mechanism of synthesis, an adenylation (A) domain within each module activates a specific monomer. Once activated, the monomer is attached to an immediately downstream thiolation (T) domain via a prosthetic phosphopantheine group, which acts as a flexible arm to pass the substrate between catalytic domains. A condensation (C) domain, upstream to the A-T domains, catalyses peptide bond formation between an acceptor substrate attached to the T domain and a donor substrate attached to the T domain of the upstream module. The peptide remains attached to the T domain of the acceptor substrate, and then acts as the donor substrate for the next C domain. When peptide synthesis reaches the final module, the peptide is released by a thioesterase (TE) domain. The linear mode of synthesis and discrete functional domains within each module gives the potential to generate new products by substituting domains or entire modules with ones that activate alternative substrates. Attempts to create new products using domain and module substitution often result in a loss of activity. The work in this thesis focuses on identifying barriers to effective domain substitution. The NRPS enzyme pvdD, which adds the final residue to the eleven residue non-ribosomal peptide pyoverdine, was developed as a model for domain substitution. The primary benefit for using this model is that pyoverdine creates easily detectible fluorescent products. The first set of experiments focused on testing the limitations of A domain and C-A domain substitutions to alter pyoverdine. Nine A domain and nine C-A domain substitution pvdD variants were constructed and used to complement a P. aeruginosa PAO1 pvdD deletion strain. The A domain substitutions that specified the wild type substrate were highly functional, whereas A domains that specified other substrates resulted in low levels of wild type pyoverdine production. This suggests the acceptor site substrate specificity of the C domain limited the success of A domain substitutions, rather than disruption of the C/A domain junction. In contrast, although C-A domain substitutions in pvdD in some cases synthesised novel pyoverdines, the majority lost function for unknown reasons. The high success rate A domain substitutions (when not limited by the acceptor site specificity of the C domain) suggested the addition of new C domains was a likely cause for loss of function. The second set of experiments investigated whether disrupting the protein interface between C domains and their upstream T domains may cause a loss in function of C-A domain substitutions. However, domain substitutions of T domains were found to have a high rate of success. Therefore, the results thus far confirmed that disrupting interactions of the C domain with A domains or T domains does not have a large affect on enzyme activity. An alternative explanation for the loss in function with C-A domain substitutions is that C domains translocated to a new enzyme are unable to process the new incoming donor peptide chain because of substrate specificity or steric constraints. To develop methods to circumvent limitations caused by the C domain, the final part of this thesis examined acceptor substrate specificity of C domains. Acceptor site substrate specificity was chosen over donor site specificity as it acts on only an amino acid rather than peptide chain. The substrate specificity was narrowed down to a small subsection of the C domain. This was an initial study of C domain substrate specificity, which may guide future development of relaxed specificity C domains.</p>

Performance of Generalized Receiver Employed by Broadband Multicarrier DS-CDMA System Using Space-Time Spreading-Assisted Transmit Diversity

In this paper the multicarrier direct-sequence code-division multiple access (MC DS-CDMA) using space-time spreading assisted transmit diversity is investigated in the context of broadband wireless communi-cations systems constructed based on the generalized approach to signal processing in noise over frequency-se-lective Rayleigh fading channels. We consider the issue of parameter design for the sake of achieving high-ef-ficiency communications in various dispersive environments. In contrast to the conventional MC DS-CDMA wireless communication system employing the time (T)-domain spreading only, in the present paper the broad-band wireless MC DS-CDMA wireless communication schemes employ both the time (T)-domain and frequen-cy (F)-domain spreading, i.e., employ the TF-domain spreading. The bit-error rate (BER) performance of the space-time spreading assisted broadband MC DS-CDMA wireless communications system is investigated for down-link transmissions associated with the single user and multiuser generalized detectors and is compared with that of the single user correlation detector and the multiuser decorrelating detector. Our study demonstra-tes that with appropriately selecting the system parameters, the broadband MC DS-CDMA wireless communi-cation system using the space-time spreading assisted transmit diversity constitutes a promising downlink tran-smission scheme. This scheme is capable to support ubiquitous communications over diverse communication environments without the BER performance degradation

An Engineered Nonribosomal Peptide Synthetase Shows Opposite Amino Acid Loading and Condensation Specificity

<div> <p>Engineering of nonribosomal peptide synthetases (NRPS) has faced numerous obstacles despite being an attractive path towards novel bioactive molecules. Specificity filters in the nonribosomal peptide assembly line determine engineering success, but the relative contribution of adenylation (A-) and condensation (C-)domains is under debate. In the engineered, bimodular NRPS sdV-GrsA/GrsB1, the first module is a subdomain-swapped chimera showing substrate promiscuity. On sdV-GrsA and evolved mutants, we have employed kinetic modelling to investigate product specificity under substrate competition. Our model contains one step, in which the A-domain acylates the thiolation (T-)domain, and one condensation step deacylating the T-domain. The simplified model agrees well with experimentally determined acylation preferences and shows that the condensation specificity is mismatched with the engineered acylation specificity. Our model predicts changing product specificity in the course of the reaction due to dynamic T-domain loading, and that A-domain overrules C-domain specificity when T-domain loading reaches a steady-state. Thus, we have established a tool for investigating poorly accessible C-domain specificity through nonlinear kinetic modeling and gained critical insights how the interplay of A- and C-domains determines the product specificity of NRPSs.</p> </div>

An Engineered Nonribosomal Peptide Synthetase Shows Opposite Amino Acid Loading and Condensation Specificity

<div> <p>Engineering of nonribosomal peptide synthetases (NRPS) has faced numerous obstacles despite being an attractive path towards novel bioactive molecules. Specificity filters in the nonribosomal peptide assembly line determine engineering success, but the relative contribution of adenylation (A-) and condensation (C-)domains is under debate. In the engineered, bimodular NRPS sdV-GrsA/GrsB1, the first module is a subdomain-swapped chimera showing substrate promiscuity. On sdV-GrsA and evolved mutants, we have employed kinetic modelling to investigate product specificity under substrate competition. Our model contains one step, in which the A-domain acylates the thiolation (T-)domain, and one condensation step deacylating the T-domain. The simplified model agrees well with experimentally determined acylation preferences and shows that the condensation specificity is mismatched with the engineered acylation specificity. Our model predicts changing product specificity in the course of the reaction due to dynamic T-domain loading, and that A-domain overrules C-domain specificity when T-domain loading reaches a steady-state. Thus, we have established a tool for investigating poorly accessible C-domain specificity through nonlinear kinetic modeling and gained critical insights how the interplay of A- and C-domains determines the product specificity of NRPSs.</p> </div>