Tinkering with transporters: periplasmic binding protein-dependent maltose transport in E. coli

Identification of a heterologous cellulase and its N-terminus that can guide recombinant proteins out of Escherichia coli

Background

The Gram-negative bacterium Escherichia coli has been widely used as a cell factory for the production of proteins and specialty chemicals because it is the best characterized host with many available expression and regulation systems. However, recombinant proteins produced in Escherichia coli are generally intracellular and often found in the form of inclusion bodies. Extracellular production of proteins is advantageous compared with intracellular production because extracellular proteins can be purified more easily and can avoid protease attack, which results in higher product quality. In this study, we found a catalytic domain of a cellulase (Cel-CD) and its N-terminus can be employed as carriers for extracellular production of recombinant proteins.

Results

In this report, we identified the catalytic domain of a cellulase (Cel-CD) from Bacillus sp. that can be secreted into the medium from recombinant E. coli BL21 (DE3) in large quantities without its native signal peptide. By subcellular location analysis, we proved that the secretion was a two-step process and the N-terminal sequence of the full length Cel-CD played a crucial function in secretion. Both the Cel-CD and its N-terminal sequence can serve as carriers for efficient extracellular production of select target proteins.

Conclusions

Fusion of heterologous proteins with N20 from Cel-CD can carry the target proteins out of the cells with a concentration from 101 to 691 mg/L in flask cultivation. The extracellular recombinant proteins with a relative high purity. The results suggested that this system has a potential application in plant biomass conversion and industrial production of enzymes and therapeutic proteins.

Maltose/maltodextrin system of Escherichia coli: transport, metabolism, and regulation

The maltose system of Escherichia coli offers an unusually rich set of enzymes, transporters, and regulators as objects of study. This system is responsible for the uptake and metabolism of glucose polymers (maltodextrins), which must be a preferred class of nutrients for E. coli in both mammalian hosts and in the environment. Because the metabolism of glucose polymers must be coordinated with both the anabolic and catabolic uses of glucose and glycogen, an intricate set of regulatory mechanisms controls the expression of mal genes, the activity of the maltose transporter, and the activities of the maltose/maltodextrin catabolic enzymes. The ease of isolating many of the mal gene products has contributed greatly to the understanding of the structures and functions of several classes of proteins. Not only was the outer membrane maltoporin, LamB, or the phage lambda receptor, the first virus receptor to be isolated, but also its three-dimensional structure, together with extensive knowledge of functional sites for ligand binding as well as for phage lambda binding, has led to a relatively complete description of this sugar-specific aqueous channel. The periplasmic maltose binding protein (MBP) has been studied with respect to its role in both maltose transport and maltose taxis. Again, the combination of structural and functional information has led to a significant understanding of how this soluble receptor participates in signaling the presence of sugar to the chemosensory apparatus as well as how it participates in sugar transport. The maltose transporter belongs to the ATP binding cassette family, and although its structure is not yet known at atomic resolution, there is some insight into the structures of several functional sites, including those that are involved in interactions with MBP and recognition of substrates and ATP. A particularly astonishing discovery is the direct participation of the transporter in transcriptional control of the mal regulon. The MalT protein activates transcription at all mal promoters. A subset also requires the cyclic AMP receptor protein for transcription. The MalT protein requires maltotriose and ATP as ligands for binding to a dodecanucleotide MalT box that appears in multiple copies upstream of all mal promoters. Recent data indicate that the ATP binding cassette transporter subunit MalK can directly inhibit MalT when the transporter is inactive due to the absence of substrate. Despite this wealth of knowledge, there are still basic issues that require clarification concerning the mechanism of MalT-mediated activation, repression by the transporter, biosynthesis and assembly of the outer membrane and inner membrane transporter proteins, and interrelationships between the mal enzymes and those of glucose and glycogen metabolism.

Subunit interactions in ABC transporters: a conserved sequence in hydrophobic membrane proteins of periplasmic permeases defines an important site of interaction with the ATPase subunits

The cytoplasmic membrane proteins of bacterial binding protein-dependent transporters belong to the superfamily of ABC transporters. The hydrophobic proteins display a conserved, at least 20 amino acid EAA---G---------I-LP region exposed in the cytosol, the EAA region. We mutagenized the EAA regions of MalF and MalG proteins of the Escherichia coli maltose transport system. Substitutions at the same positions in MalF and MalG have different phenotypes, indicating that EAA regions do not act symmetrically. Mutations in malG or malF that slightly affect or do not affect transport, determine a completely defective phenotype when present together. This suggests that EAA regions of MalF and MalG may interact during transport. Maltose-negative mutants fall into two categories with respect to the cellular localization of the MalK ATPase: in the first, MalK is membrane-bound, as in wild-type strains, while in the second, it is cytosolic, as in strains deleted in the malF and malG genes. From maltose-negative mutants of the two categories, we isolated suppressor mutations within malK that restore transport. They map mainly in the putative helical domain of MalK, suggesting that EAA regions may constitute a recognition site for the ABC ATPase helical domain.

Localized frameshift mutation generates selective, high-frequency phase variation of a surface lipoprotein encoded by a mycoplasma ABC transporter operon

The wall-less mycoplasmas have revealed unusual microbial strategies for adaptive variation of antigenic membrane proteins exposed during their surface colonization of host cells. In particular, high-frequency mutations affecting the expression of selected surface lipoproteins have been increasingly documented for this group of organisms. A novel manifestation of mutational phase variation is shown here to occur in Mycoplasma fermentans, a chronic human infectious agent and possible AIDS-associated pathogen. A putative ABC type transport operon encoding four gene products is identified. The 3' distal gene encoding P78, a known surface-exposed antigen and the proposed substrate-binding lipoprotein of the transporter, is subject to localized hypermutation in a short homopolymeric tract of adenine residues located in the N-terminal coding region of the mature product. High-frequency, reversible insertion/deletion frameshift mutations lead to selective phase variation in P78 expression, whereas the putative nucleotide-binding protein, P63, encoded by the most 5' gene of the operon, is continually expressed. Mutation-based phase variation in specific surface-exposed microbial transporter components may provide an adaptive advantage for immune evasion, while continued expression of other elements of the same transporter may preserve essential metabolic functions and confer alternative substrate specificity. These features could be critical in mycoplasmas, where limitations in both transcriptional regulators and transport systems may prevail. This study also documents that P63 contains an uncharacteristic hydrophobic sequence between predicted nucleotide binding motifs and displays an amphiphilic character in detergent fractionation. Both features are consistent with an evolutionary adaptation favoring integral association of this putative energy-transducing component with the single mycoplasma membrane.

High-affinity maltose/trehalose transport system in the hyperthermophilic archaeon Thermococcus litoralis

The hyperthermophilic marine archaeon Thermococcus litoralis exhibits high-affinity transport activity for maltose and trehalose at 85 degrees C. The K(m) for maltose transport was 22 nM, and that for trehalose was 17 nM. In cells that had been grown on peptone plus yeast extract, the Vmax for maltose uptake ranged from 3.2 to 7.5 nmol/min/mg of protein in different cell cultures. Cells grown in peptone without yeast extract did not show significant maltose or trehalose uptake. We found that the compound in yeast extract responsible for the induction of the maltose and trehalose transport system was trehalose. [14C]maltose uptake at 100 nM was not significantly inhibited by glucose, sucrose, or maltotriose at a 100 microM concentration but was completely inhibited by trehalose and maltose. The inhibitor constant, Ki, of trehalose for inhibiting maltose uptake was 21 nM. In contrast, the ability of maltose to inhibit the uptake of trehalose was not equally strong. With 20 nM [14C]trehalose as the substrate, a 10-fold excess of maltose was necessary to inhibit uptake to 50%. However, full inhibition was observed at 2 microM maltose. The detergent-solubilized membranes of trehalose-induced cells contained a high-affinity binding protein for maltose and trehalose, with an M(r) of 48,000, that exhibited the same substrate specificity as the transport system found in whole cells. We conclude that maltose and trehalose are transported by the same high-affinity membrane-associated system. This represents the first report on sugar transport in any hyperthermophilic archaeon.

Characterization of transmembrane domains 6, 7, and 8 of MalF by mutational analysis

Oligonucleotide mutagenesis was used to isolate mutations in membrane-spanning segments 6, 7, and 8 of MalF. MalF is a cytoplasmic membrane component of the binding protein-dependent maltose transport system in Escherichia coli. The current structural model predicts eight transmembrane domains for MalF. Membrane-spanning segments 6, 7, and 8 of MalF flank or are part of the EAA-X3-G-X9-I-X-LP consensus region present in the cytoplasmic membrane subunits of the bacterial ABC transporter superfamily members. Mutations with two novel phenotypes with respect to substrate specificity of the maltose transport system were isolated. One mutant grew on minimal maltose media but not on media containing either maltoheptaose or maltoheptaose plus maltose and was thus termed dextrin dominant negative. The other class of mutations led to a maltose minus but maltoheptaose plus phenotype. Nine of the isolated mutations leading to changes in substrate specificity were tightly clustered on one face of the postulated transmembrane helix 6. A similar clustering of mutations was detected in transmembrane domain 7. The majority of mutations in membrane-spanning segment 7 led to a protease-sensitive or a conditional phenotype with respect to MalF function or both. Mutations in transmembrane domain 8 appeared to be more randomly distributed. The majority of mutations in membrane-spanning segment 8 caused a Mal+ Dex- phenotype. Six Mal+ suppressor mutations isolated to two mutations in transmembrane domain 7 changed amino acid residues in membrane-spanning segment 6 or 8.