Correction to: Genetically encoded photochemical covalent crosslinking within the Hcp-1 self-assembling bacterial secretion machinery.

The authors declare that the contributing author, Antonczak A.K., had no active academic address at or after the time of publication.

Genetically encoded photochemical covalent crosslinking within the Hcp-1 self-assembling bacterial secretion machinery.

The target protein, Hcp1, was first described as part of the bacterial Type VI secretion system from Pseudomonas aeruginosa. The protein first self-assembles into a hexamer and then the hexamers further stack into a nanotubular structure. Hcp1 monomers were targeted for mutagenesis with two widely used photoactivatable amino acids: para-benzoyl phenylalanine or para-azidophenylalanine. The ability of these amino acids to form covalent adducts within the Hcp1 self-assembled system was investigated. Multiple residues, putatively of equal distance between the monomer-monomer interface were targeted. The efficiency of each amino acid to covalently link self-assembled hexamers was determined. The results demonstrate the choice and role of genetically encoded tools applied to complicated biological processes such as self-assembly and also suggested some structural dynamics of the Hcp-1 protein not obvious from crystallographic structures.

Advances in the mechanism and understanding of site-selective noncanonical amino acid incorporation.

There are many approaches to introduce non-native functionality into proteins either translationally or post-translationally. When a noncanonical amino acid (NAA) is incorporated translationally, the host organism's existing translational machinery is relied upon to insert the amino acid by the same well-established mechanisms used by the host to achieve high fidelity insertion of its canonical amino acids. Research into the in vivo incorporation of NAAs has typically concentrated on evolving or engineering aminoacyl tRNA synthetases (aaRSs); however, new studies have increasingly focused on other members of the translational apparatus, for example entire ribosomes, in attempts to increase the fidelity and efficiency of incorporation of ever more structurally diverse NAAs. As the biochemical methods of NAA systems increase in complexity, it is informative to ask whether the 'rules' for canonical translation (i.e. aaRSs, tRNA, ribosomes, elongation factors, amino acid uptake, and metabolism) hold for NAA systems, or whether new rules are warranted. Here, recent advances in introducing novel chemical functionality into proteins are highlighted.

Probing eudesmane cation-π interactions in catalysis by aristolochene synthase with non-canonical amino acids.

Stabilization of the reaction intermediate eudesmane cation (3) through interaction with Trp 334 during catalysis by aristolochene synthase from Penicillium roqueforti was investigated by site-directed incorporation of proteinogenic and non-canonical aromatic amino acids. The amount of germacrene A (2) generated by the mutant enzymes served as a measure of the stabilization of 3. 2 is a neutral intermediate, from which 3 is formed during PR-AS catalysis by protonation of the C6,C7 double bond. The replacement of Trp 334 with para-substituted phenylalanines of increasing electron-withdrawing properties led to a progressive accumulation of 2 that showed a good correlation with the interaction energies of simple cations such as Na(+) with substituted benzenes. These results provide compelling evidence for the stabilizing role played by Trp 334 in aristolochene synthase catalysis for the energetically demanding transformation of 2 to 3.

Importance of single molecular determinants in the fidelity of expanded genetic codes.

The site-selective encoding of noncanonical amino acids (NAAs) is a powerful technique for the installation of novel chemical functional groups in proteins. This is often achieved by recoding a stop codon and requires two additional components: an evolved aminoacyl tRNA synthetase (AARS) and a cognate tRNA. Analysis of the most successful AARSs reveals common characteristics. The highest fidelity NAA systems derived from the Methanocaldococcus jannaschii tyrosyl AARS feature specific mutations to two residues reported to interact with the hydroxyl group of the substrate tyrosine. We demonstrate that the restoration of just one of these determinants for amino acid specificity results in the loss of fidelity as the evolved AARSs become noticeably promiscuous. These results offer a partial explanation of a recently retracted strategy for the synthesis of glycoproteins. Similarly, we reinvestigated a tryptophanyl AARS reported to allow the site-selective incorporation of 5-hydroxy tryptophan within mammalian cells. In multiple experiments, the enzyme displayed elements of promiscuity despite its previous characterization as a high fidelity enzyme. Given the many similarities of the TyrRSs and TrpRSs reevaluated here, our findings can be largely combined, and in doing so they reinforce the long-established central dogma regarding the molecular basis by which these enzymes contribute to the fidelity of translation. Thus, our view is that the central claims of fidelity reported in several NAA systems remain unproven and unprecedented.

A critical examination of Escherichia coli esterase activity.

The ability of Escherichia coli to grow on a series of acetylated and glycosylated compounds has been investigated. It is surmised that E. coli maintains low levels of nonspecific esterase activity. This observation may have ramifications for previous reports that relied on nonspecific esterases from E. coli to genetically encode nonnatural amino acids. It had been reported that nonspecific esterases from E. coli deacetylate tri-acetyl O-linked glycosylated serine and threonine in vivo. The glycosylated amino acids were reported to have been genetically encoded into proteins in response to the amber stop codon. However, it is our contention that such amino acids are not utilized in this manner within E. coli. The current results report in vitro analysis of the original enzyme and an in vivo analysis of a glycosylated amino acid. It is concluded that the amber suppression method with nonnatural amino acids may require a caveat for use in certain instances.