Recombinant Escherichia coli Inner membrane ABC transporter permease protein yejB (yejB)

Physical and Biochemical Properties

Commercial recombinant YejB proteins are typically available with the following specifications:

Source: Compiled from multiple commercial product listings

The YejABEF Transporter System

YejB functions as an integral component of the YejABEF transporter system, which belongs to the ABC transporter superfamily. The yej cluster in E. coli consists of four genes (yejABEF) transcribed in one direction, likely forming an operon, and a downstream gene, yejG, transcribed in the opposite direction .

Components and Their Functions

The YejABEF system comprises:

1. YejA: Functions as the periplasmic receptor component

2. YejB and YejE: Serve as transmembrane components (permeases)

3. YejF: Acts as the ATPase that provides energy for transport through ATP hydrolysis

This integrated transport system represents a typical ABC transporter architecture, with each component performing a specialized function in the substrate transport process. Notably, the YejG protein has no apparent functional relationship to the YejABEF components, despite its genomic proximity .

Functional Roles of YejB and the YejABEF System

Research has identified several critical functional roles for the YejABEF transporter system, with YejB serving as a key transmembrane component.

Antibiotic Transport and Resistance

One of the most well-characterized functions of the YejABEF system is its role in the uptake of Microcin C (McC), a peptide-nucleotide antibiotic that targets aspartyl-tRNA synthetase. Studies using random transposon libraries identified that mutations in the yejABEF locus confer resistance to McC in E. coli .

Experimental evidence established that all four components of the transporter (YejA, YejB, YejE, and YejF) are required for McC sensitivity. Since aspartyl-tRNA synthetase in yej mutant extracts remained fully sensitive to McC, researchers concluded that yej mutations interfere with McC uptake, indicating that YejABEF is the only inner membrane transporter responsible for McC uptake in E. coli under laboratory conditions .

Role in Antimicrobial Peptide Resistance

Beyond McC transport, the YejABEF system has been implicated in resistance to host antimicrobial peptides (AMPs). Studies with Brucella melitensis demonstrated that yejA1, yejA2, yejB, yejE, yejF, and whole yej operon deletion mutants showed increased sensitivity to polymyxin B, with expression of yej operon genes induced by polymyxin B exposure .

Cell and mouse infection assays indicated that Brucella melitensis strains with yejE and yejABEF deletions exhibited restricted invasion and replication abilities inside macrophages and were rapidly cleared from the spleens of infected mice. These findings suggest that the ABC transporter YejABEF is required for virulence, with resistance to host antimicrobials being a key mechanism for persistent survival in vivo .

Potential Role in Peptide Transport

The YejABEF system has been annotated as a putative peptide-Ni transporter, although bioinformatic analysis suggests limited similarity to known nickel transporters . An addendum to one study noted that Salmonella enterica serovar Typhimurium with genetic lesions in the Yej transporter induces a superior CD8+ T-cell response due to increased peptide presentation on MHC-1 class molecules, suggesting a role for Yej in oligopeptide transport in this organism .

Phylogenetic Distribution

Phylogenetic analysis has revealed conserved operons similar to yejABEF in 126 bacterial species belonging to alpha-, beta-, gamma-, delta-, and epsilonproteobacteria, with fifteen species containing two paralogous yejABEF-like operons . The YejA protein and its homologs form a distinct branch on the phylogenetic tree, well-separated from all known dipeptide or oligopeptide transporters .

Recombinant Production of YejB

Recombinant YejB protein is produced using various expression systems to facilitate research and potential applications.

Expression Systems

Commercial recombinant YejB proteins are produced in several host systems:

Source: Compiled from commercial listings

Protein Tags and Modifications

The most common modification to recombinant YejB is the addition of a His-tag, which facilitates purification through affinity chromatography. Some commercially available variants include:

1. N-terminal His-tagged YejB

2. Avi-tag Biotinylated YejB (biotinylated in vivo by AviTag-BirA technology)

Comparison with Other ABC Transporters

The YejABEF system shares structural similarities with other ABC transporters but has distinct functional characteristics. Despite being annotated as a putative nickel/peptide transporter, the Yej transport system has little similarity to known nickel transporters from the nickel/peptide/opine transporter family, and no NikR-binding motifs have been found upstream of the yejABEF-like operons .

Phylogenetic analysis places YejA orthologs and close paralogs in a distinct branch on the evolutionary tree, separate from known dipeptide or oligopeptide transporters . Similar phylogenetic distinctiveness has been observed for other components of the YejABEF transporter, suggesting unique evolutionary and functional adaptations.

Interestingly, not all bacteria possessing yejABEF homologs display sensitivity to Microcin C. For example, Pseudomonas aeruginosa, which contains two yejABEF homologs, is McC resistant, suggesting that either its Yej homologs do not recognize McC or they are not expressed under laboratory conditions .

Dual Functionality in Related ABC Transporters

While the primary focus of this article is on YejB, it's worth noting that related ABC transporters in E. coli have demonstrated remarkable functional versatility. For instance, YbhFSR, another putative efflux transporter of the ABC family, exhibits dual functions as both a drug efflux pump and a Na+(Li+)/H+ antiporter .

This functional duality suggests that ABC transporters, including potentially YejABEF, may have evolved multiple roles that contribute to bacterial survival under various environmental conditions. The implication is that YejB and its associated transporter complex might have additional functions beyond what is currently documented.

Research Applications and Future Directions

Recombinant YejB protein has several important research applications:

1. Antibiotic resistance studies: Understanding the mechanisms of antibiotic uptake and resistance mediated by ABC transporters

2. Structural biology: Investigating the structure-function relationships of membrane transport proteins

3. Drug discovery: Potential target for novel antimicrobial agents that could inhibit or exploit bacterial transport systems

4. Bacterial pathogenesis research: Examining the role of transport systems in bacterial virulence and persistence during infection

Future research directions might include:

1. Detailed structural characterization of YejB and the entire YejABEF complex

2. Identification of additional natural substrates beyond Microcin C

3. Exploration of species-specific variations in YejB function

4. Development of inhibitors targeting the YejABEF system as potential antimicrobial agents