Jiantao Guo

A Facile System for Encoding Unnatural Amino Acids in Mammalian Cells

A shuttle system has been developed to genetically encode unnatural amino acids in mammalian cells using aminoacyl-tRNA synthetases (aaRSs) evolved in E. coli. A pyrrolysyl-tRNA synthetase (PylRS) mutant was evolved in E. coli that selectively aminoacylates a cognate nonsense suppressor tRNA with a photocaged lysine derivative. Transfer of this orthogonal tRNA-aaRS pair into mammalian cells made possible the selective incorporation of this unnatural amino acid into proteins.

Genetic incorporation of a small, environmentally sensitive, fluorescent probe into proteins in Saccharomyces cerevisiae.

Here, we report that the fluorescent amino acid, 3-(6-acetylnaphthalen-2-ylamino)-2-aminopropanoic acid (Anap), can be genetically incorporated into proteins in yeast with excellent selectivity and efficiency by means of an orthogonal tRNA/aminoacyl-tRNA synthetase pair. This small, environmentally sensitive fluorophore was site-specifically incorporated into Escherichia coli glutamine binding protein and used to directly probe local structural changes caused by ligand binding. The small size of Anap and the ability to introduce it by simple mutagenesis at defined sites in the proteome make it a useful local probe of protein structure, molecular interactions, protein folding, and localization.

Evolution of Amber Suppressor tRNAs for Efficient Bacterial Production of Proteins Containing Nonnatural Amino Acids

Open in a separate window Regions of the M. jannaschii tyrosyl tRNACUA thought to interact with elongation factor Tu were randomized, and the resulting tRNA libraries were subjected to in vitro evolution. The tRNAs identified resulted in significantly improved unnatural amino acid-containing protein yields. In some cases, the degree of improvement varied in an amino acid-dependent manner.

A Genetically Encoded Fluorescent Probe in Mammalian Cells

Fluorescent reporters are useful in vitro and in vivo probes of protein structure, function, and localization. Here we report that the fluorescent amino acid, 3-(6-acetylnaphthalen-2-ylamino)-2-aminopropanoic acid (Anap), can be site-specifically incorporated into proteins in mammalian cells in response to the TAG codon with high efficiency using an orthogonal amber suppressor tRNA/aminoacyl-tRNA synthetase (aaRS) pair. We further demonstrate that Anap can be used to image the subcellular localization of proteins in live mammalian cells. The small size of Anap, its environment-sensitive fluorescence, and the ability to introduce Anap at specific sites in the proteome by simple mutagenesis make it a unique and valuable tool in eukaryotic cell biology.

An expanded genetic code in mammalian cells with a functional quadruplet codon.

We have utilized in vitro evolution to identify tRNA variants with significantly enhanced activity for the incorporation of unnatural amino acids into proteins in response to a quadruplet codon in both bacterial and mammalian cells. This approach will facilitate the creation of an optimized and standardized system for the genetic incorporation of unnatural amino acids using quadruplet codons, which will allow the biosynthesis of biopolymers that contain multiple unnatural building blocks.

Genetic Incorporation of Unnatural Amino Acids into Proteins in Mycobacterium tuberculosis

New tools are needed to study the intracellular pathogen Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), to facilitate new drug discovery and vaccine development. We have developed methodology to genetically incorporate unnatural amino acids into proteins in Mycobacterium smegmatis, BCG and Mtb, grown both extracellularly in culture and inside host cells. Orthogonal mutant tRNATyr/tyrosyl-tRNA synthetase pairs derived from Methanococcus jannaschii and evolved in Escherichia coli incorporate a variety of unnatural amino acids (including photocrosslinking, chemically reactive, heavy atom containing, and immunogenic amino acids) into proteins in response to the amber nonsense codon. By taking advantage of the fidelity and suppression efficiency of the MjtRNA/pIpaRS pair in mycobacteria, we are also able to use p-iodophenylalanine to induce the expression of proteins in mycobacteria both extracellularly in culture and inside of mammalian host cells. This provides a new approach to regulate the expression of reporter genes or mycobacteria endogenous genes of interest. The establishment of the unnatural amino acid expression system in Mtb, an intracellular pathogen, should facilitate studies of TB biology and vaccine development.

Unnatural Amino Acid Mutagenesis of Fluorescent Proteins

Fluorescent proteins are useful probes of protein localization and function both in vitro and in vivo. As a result, considerable effort has focused on engineering new properties into fluorescent proteins. 4, 5] Both directed evolution and rational design have resulted in mutant proteins with altered photophysical properties, stabilities, and sensitivity to pH value, exogenous ligands, phosphorylation and the like. To further explore the effects of steric/electronic perturbations to the fluorophore of a fluorescent protein, we previously substituted tyrosine 66 of the fluorophore in green fluorescent protein (GFP) with a series of unnatural amino acids. Furthermore, others have reported the use of unnatural amino acid mutagenesis to study the spectral properties and folding behavior of GFP, and to generate biosensors. Herein we report the properties of four GFP (GFPUV, a GFP variant optimized for maximal fluorescence when excited by standard UV light; Clontech) mutants in which the hydroxy substituent of Tyr66 is replaced with boronate, azido, keto, and nitro substituents (Scheme 1).

Expanding the chemistry of fluorescent protein biosensors through genetic incorporation of unnatural amino acids.

Fluorescent proteins are essential tools in biological research, ranging from the study of individual biological components to the interrogation of complex cellular systems. Fluorescent protein derived biosensors are increasingly applied to the study of biological molecules and events in living cells. The present review focuses on a specific class of fluorescent protein biosensors in which a genetically installed unnatural amino acid (UAA*) acts as the sensing element. Upon direct interaction with the analyte of interest, the chemical and/or physical properties of UAA* are altered, which triggers fluorescence property changes of the biosensor and generates readouts. In comparison to mutagenesis approaches within the standard genetic code, introduction of UAA*s with a unique functionality and chemical reactivity could broaden the scope of analytes and improve the specificity of biosensors. Nonconventional functional groups in fluorescent proteins enable sensor designs that are not readily accessible using the common twenty amino acids. Recent reports of UAA*-containing fluorescent protein sensors serve as excellent examples for the utility of such sensor design. We envisage that the integration of the two powerful chemical biology tools, fluorescent protein sensors and genetic incorporation of UAA*s, will lead to novel biosensors that can expand and deepen current understanding of cellular processes.

Construction of a Live‐Attenuated HIV‐1 Vaccine through Genetic Code Expansion

A safe and effective vaccine against human immunodeficiency virus type 1 (HIV-1) is urgently needed to combat the worldwide AIDS pandemic, but still remains elusive. The fact that uncontrolled replication of an attenuated vaccine can lead to regaining of its virulence creates safety concerns precluding many vaccines from clinical application. We introduce a novel approach to control HIV-1 replication, which entails the manipulation of essential HIV-1 protein biosynthesis through unnatural amino acid (UAA*)-mediated suppression of genome-encoded blank codon. We successfully demonstrate that HIV-1 replication can be precisely turned on and off in vitro.

Biotechnological production of 1,2,4-butanetriol: An efficient process to synthesize energetic material precursor from renewable biomass

1,2,4-Butanetriol (BT) is a valuable chemical with extensive applications in many different fields. The traditional chemical routes to synthesize BT suffer from many drawbacks, e.g., harsh reaction conditions, multiple steps and poor selectivity, limiting its industrial production. In this study, an engineered Escherichia coli strain was constructed to produce BT from xylose, which is a major component of the lignocellulosic biomass. Through the coexpression of a xylose dehydrogenase (CCxylB) and a xylonolactonase (xylC) from Caulobacter crescentus, native E. coli xylonate dehydratase (yjhG), a 2-keto acid decarboxylase from Pseudomonas putida (mdlC) and native E. coli aldehyde reductase (adhP) in E. coli BL21 star(DE3), the recombinant strain could efficiently convert xylose to BT. Furthermore, the competitive pathway responsible for xylose metabolism in E. coli was blocked by disrupting two genes (xylA and EcxylB) encoding xylose isomerase and xyloluse kinase. Under fed-batch conditions, the engineered strain BL21ΔxylAB/pE-mdlCxylBC&pA-adhPyjhG produced up to 3.92 g/L of BT from 20 g/L of xylose, corresponding to a molar yield of 27.7%. These results suggest that the engineered E. coli has a promising prospect for the large-scale production of BT.