Ferric hydroxamate binding protein FhuD from Escherichia coli: mutants in conserved and non-conserved regions

Uptake of iron complexes into the Gram-negative bacterial cell requires highly specific outer membrane receptors and specific ATP-dependent (ATP-Binding-Cassette (ABC)) transport systems located in the inner membrane. The latter type of import system is characterized by a periplasmic binding protein (BP), integral membrane proteins, and membrane-associated ATP-hydrolyzing proteins. In Gram-positive bacteria lacking the periplasmic space, the binding proteins are lipoproteins tethered to the cytoplasmic membrane. To date, there is little structural information about the components of ABC transport systems involved in iron complex transport. The recently determined structure of the Escherichia coli periplasmic ferric siderophore binding protein FhuD is unique for an ABC transport system (Clarke et al. 2000). Unlike other BP's, FhuD has two domains connected by a long -helix. The ligand binds in a shallow pocket between the two domains. In vivo and in vitro analysis of single amino acid mutants of FhuD identified several residues that are important for proper functioning of the protein. In this study, the mutated residues were mapped to the protein structure to define special areas and specific amino acid residues in E. coli FhuD that are vital for correct protein function. A number of these important residues were localized in conserved regions according to a multiple sequence alignment of E. coli FhuD with other BP's that transport siderophores, heme, and vitamin B₁₂. The alignment and structure prediction of these polypeptides indicate that they form a distinct family of periplasmic binding proteins.     

Iron(III) hydroxamate transport across the cytoplasmic membrane ofEscherichia coli

Summary Transport of iron(III) hydroxamates across the inner membrane into the cytoplasm ofEscherichia coli is mediated by the FhuC, FhuD and FhuB proteins and displays characteristics typical of a periplasmic-binding-protein-dependent transport mechanism. In contrast to the highly specific receptor proteins in the outer membrane, at least six different siderophores of the hydroxamate type and the antibiotic albomycin are accepted as substrates. AfhuB mutant (deficient in transport of substrates across the inner membrane) which overproduced the periplasmic FhuD 30-kDa protein, bound [⁵⁵Fe] iron(III) ferrichrome. Resistance of FhuD to proteinase K in the presence of ferrichrome, aerobactin, and coprogen indicated binding of these substrates to FhuD. FhuD displays significant similarity to the periplasmic FecB, FepB, and BtuE proteins. The extremely hydrophobic FhuB 70-kDa protein is located in the cytoplasmic membrane and consists of two apparently duplicated halves. The N-and C-terminal halves [FhuB(N) and FhuB(C)] were expressed separately infhuB mutants. Only combinations of FhuB(N) and FhuB(C) polypeptides restored sensitivity to albomycin and growth on iron hydroxamate as a sole iron source, indicating that both halves of FhuB were essential for substrate translocation and that they combined to form an active permease. In addition, a FhuB derivative with a large internal duplication of 271 amino acids was found to be transport-active, indicating that the extra portion did not disturb proper insertion of the active FhuB segments into the cytoplasmic membrane. A region of considerable similarity, present twice in FhuB, was identified near the C-terminus of 20 analyzed hydrophobic proteins of periplasmic-binding-protein-dependent systems. The FhuC 30 kDa protein, most likely involved in ATP binding, contains two domains representing consensus sequences among all peripheral cytoplasmic membrane proteins of these systems. Amino acid replacements in domain I (LysGlu and Gln) and domain II (AspAsn and Glu) resulted in a transport-deficient phenotype.     

Iron-hydroxamate transport into Escherichia coli K12: localization of FhuD in the periplasm and of FhuB in the cytoplasmic membrane

Summary ThefhuB, fhuC andfhuD genes encode proteins which catalyze transport of iron(III)-hydroxamate compounds from the periplasm into the cytoplasm ofEscherichia coli. ThefhuB, C, D genes were cloned downstream of a strong phage T7 promoter and transcribed by T7 RNA polymerase. The overexpressed FhuD protein appeared in two forms of 31 and 28 kDa and was released upon conversion of vegetative cells into spheroplasts, suggesting synthesis of FhuD as a precursor and export into the periplasm. The very hydrophobic FhuB protein was found in the cytoplasmic membrane. These properties, together with the previously found homologies in the FhuC protein to ATP-binding proteins, display the characteristics of a periplasmic binding protein dependent transport system across the cytoplasmic membrane. The molecular weight of FhuB and the sequence offhuC, as previously published by us, was confirmed. FhuB exhibited double the size of most hydrophobic proteins of such systems and showed homology between the amino- and carboxy-terminal halves of the protein, indicating duplication of an original gene and subsequent fusion of the two DNA fragments.     

Iron (III) hydroxamate transport into Escherichia coli. Substrate binding to the periplasmic FhuD protein

Due to its extreme insolubility, Fe3+ is not transported as a monoatomic ion. In microbes, iron is bound to low molecular weight carriers, designated siderophores. For uptake into cells of Escherichia coli Fe3+ siderophores have to be translocated across two membranes. Transport across the outer membrane is receptor-dependent and energy-coupled; transport across the cytoplasmic membrane seems to follow a periplasmic binding protein-dependent transport mechanism. In support of this notion we demonstrate specific binding of the Fe3+ hydroxamate compounds ferrichrome, aerobactin, and coprogen, which are transported via the Fhu system, to the periplasmic FhuD protein, and no binding of the transport inactive ferrichrome A, ferric citrate, and iron sulfate. About 10(4) ferrichrome molecules were bound to the FhuD protein of cells which overproduced plasmid-encoded FhuD. Binding depended on transport across the outer membrane mediated by the FhuA receptor and the TonB protein. Binding to FhuD was supported by the exclusive resistance of FhuD to proteinase K in the presence of the transport active hydroxamates. The overproduced precursor form of the FhuD protein was not protected by the Fe3+ hydroxamates indicating a conformation different to the mature form. The FhuD protein apparently serves as a periplasmic carrier for Fe3+ hydroxamates with widely different structures.     

ATP-dependent ferric hydroxamate transport system in Escherichia coli: periplasmic FhuD interacts with a periplasmic and with a transmembrane/cytoplasmic region of the integral membrane protein FhuB, as revealed by competitive peptide mapping

The Escherichia coli iron transport system via ferrichrome belongs to the group of ATP-dependent transporters that are widely distributed in prokaryotes and eukaryotes. Transport across the cytoplasmic membrane is mediated by three proteins: FhuD in the periplasm, FhuB in the cytoplasmic membrane and FhuC (ATPase) associated with the inside of the cytoplasmic membrane. Interaction of FhuD with FhuB was studied in vitro with biotinylated synthetic 10 residue and 20–24 residue peptides of FhuB by determining the activity of β-galactosidase linked to the peptides via streptavidin. Peptides identical in sequence to only one of the four periplasmic loops (loop 2), predicted by a transmembrane model of FhuB, and peptides representing a transmembrane segment and part of the adjacent cytoplasmic loop 7 of FhuB bound to FhuD. Decapeptides were transferred into the periplasm of cells through a FhuA deletion derivative that forms permanently open channels three times as large as the porins in the outer membrane. FhuB peptides that bound to FhuD inhibited ferrichrome transport, while peptides that did not bind to FhuD did not affect transport. These data led us to propose that the periplasmic FhuD interacts with a transmembrane region and the cytoplasmic segment 7 of FhuB. The transmembrane region may be part of a pore through which a portion of FhuD inserts into the cytoplasmic membrane during transport. The cytoplasmic segment 7 of FhuB contains the conserved amino acid sequence EAA…G (in FhuB DTA…G) found in ABC transporters, which is predicted to interact with the cytoplasmic FhuC ATPase. Triggering of ATP hydrolysis by substrate-loaded FhuD may occur by physical interaction between FhuD and FhuC, which bind close to each other on loop 7. Although FhuB consists of two homologous halves, FhuB(N) and FhuB(C), the sites identified for FhuD-mediated ferrichrome transport are asymmetrically arranged.     

Active transport of iron and siderophore antibiotics

Bacteria solubilize iron (Fe(3+)) with secreted siderophores, which are then taken up as Fe(3+)-siderophore complexes. Some bacteria also use iron in heme, hemoglobin, hemopexin, transferrin and lactoferrin of eukaryotic hosts. Crystal structures of two outer membrane transport proteins, FhuA and FepA, and biochemical data reveal strong long-range conformational changes of the proteins upon binding of Fe(3+)-siderophore complexes and in response to energy transfer from the cytoplasmic membrane into the outer membrane via the TonB-ExbB-ExbD protein complex. The crystal structure of the periplasmic binding protein FhuD strongly deviates from the uniform overall structure of binding proteins hitherto determined. Sideromycins, antibiotics that contain Fe(3+)-siderophore complexes as carriers, are highly effective, as they enter cells via Fe(3+)-siderophore transport systems. In this review, recently published data is discussed to demonstrate the state of understanding of iron transport across the outer membrane and the cytoplasmic membrane.     

Investigation of the impact of Tat export pathway enhancement on E. coli culture, protein production and early stage recovery

The twin arginine translocation (Tat) pathway occurs naturally in E. coli and has the distinct ability to translocate folded proteins across the inner membrane of the cell. It has the potential to export commercially useful proteins that cannot be exported by the ubiquitous Sec pathway. To better understand the bioprocess potential of the Tat pathway, this article addresses the fermentation and downstream processing performances of E. coli strains with a wild‐type Tat system exporting the over‐expressed substrate protein FhuD. These were compared to strains cell‐engineered to over‐express the Tat pathway, since the native export capacity of the Tat pathway is low. This low capacity makes the pathway susceptible to saturation by over‐expressed substrate proteins, and can result in compromised cell integrity. However, there is concern in the literature that over‐expression of membrane proteins, like those of the Tat pathway, can impact negatively upon membrane integrity itself. Under controlled fermentation conditions E. coli cells with a wild‐type Tat pathway showed poor protein accumulation, reaching a periplasmic maximum of only 0.5 mg L^?1 of growth medium. Cells over‐expressing the Tat pathway showed a 25% improvement in growth rate, avoided pathway saturation, and showed 40‐fold higher periplasmic accumulation of FhuD. Moreover, this was achieved whilst conserving the integrity of cells for downstream processing: experimentation comparing the robustness of cells to increasing levels of shear showed no detrimental effect from pathway over‐expression. Further experimentation on spheroplasts generated by the lysozyme/osmotic shock method—a scaleable way to release periplasmic protein—showed similar robustness between strains. A scale‐down mimic of continuous disk‐stack centrifugation predicted clarifications in excess of 90% for both intact cells and spheroplasts. Cells over‐expressing the Tat pathway performed comparably to cells with the wild‐type system. Overall, engineering E. coli cells to over‐express the Tat pathway allowed for greater periplasmic yields of FhuD at the fermentation scale without compromising downstream processing performance. Biotechnol. Bioeng. 2012; 109:983–991. © 2011 Wiley Periodicals, Inc.     

Iron-hydroxamate uptake systems in Bacillus subtilis: identification of a lipoprotein as part of a binding protein-dependent transport system

Bacillus subtilis was shown to utilize three types of hydroxamate siderophores, ferrichromes, ferrioxamines and shizokinen, each of which is taken up by different transport systems. Mutants deficient in the uptake of ferrichrome and/or ferrioxamine B were isolated. The gene fhuD, which was able to complement a mutant defective in ferrichrome uptake, was cloned. The deduced sequence of FhuD showed low but significant homology to the binding proteins FepB, FecB and FhuD of Escherichia coli, which are all components of binding protein-dependent, ferric siderophore transport systems. The first 23 amino acids of FhuD of B. subtilis possessed all characteristics of a lipoprotein signal sequence. The processing of FhuD in E. coli was inhibited by globomycin. Inhibition by globomycin indicated a lipid modification at the N-terminal cysteine in E. coli. It is highly likely that this step may also take place in B. subtilis. As in other binding protein-dependent transport systems of Gram-positive organisms it is proposed that the lack of a periplasm is compensated for by the lipid through which the binding protein is anchored to the cytoplasmic membrane.     

The structure of the ferric siderophore binding protein FhuD complexed with gallichrome

Siderophore binding proteins play a key role in the uptake of iron in many gram-positive and gram-negative bacteria. FhuD is a soluble periplasmic binding protein that transports ferrichrome and other hydroxamate siderophores. The crystal structure of FhuD from Escherichia coli in complex with the ferrichrome homolog gallichrome has been determined at 1.9 ? resolution, the first structure of a periplasmic binding protein involved in the uptake of siderophores. Gallichrome is held in a shallow pocket lined with aromatic groups; Arg and Tyr side chains interact directly with the hydroxamate moieties of the siderophore. FhuD possesses a novel fold, suggesting that its mechanisms of ligand binding and release are different from other structurally characterized periplasmic ligand binding proteins.     

Nucleotide sequences of the fecBCDE genes and locations of the proteins suggest a periplasmic-binding-protein-dependent transport mechanism for iron(III) dicitrate in Escherichia coli.

The fec region of the Escherichia coli chromosome determines a citrate-dependent iron(III) transport system. The nucleotide sequence of fec revealed five genes, fecABCDE, which are transcribed from fecA to fecE. The fecA gene encodes a previously described outer membrane receptor protein. The fecB gene product is formed as a precursor protein with a signal peptide of 21 amino acids; the mature form, with a molecular weight of 30,815, was previously found in the periplasm. The fecB genes of E. coli B and E. coli K-12 differed in 3 nucleotides, of which 2 gave rise to conservative amino acid exchanges. The fecC and fecD genes were found to encode very hydrophobic polypeptides with molecular weights of 35,367 and 34,148, respectively, both of which are localized in the cytoplasmic membrane. The fecE product was a rather hydrophilic but cytoplasmic membrane-bound protein of Mr 28,189 and contained regions of extensive homology to ATP-binding proteins. The number, structural characteristics, and locations of the FecBCDE proteins were typical for a periplasmic-binding-protein-dependent transport system. It is proposed that after FecA- and TonB-dependent transport of iron(III) dicitrate across the outer membrane, uptake through the cytoplasmic membrane follows the binding-protein-dependent transport mechanism. FecC and FecD exhibited homologies to each other, to the N- and C-terminal halves of FhuB of the iron(III) hydroxamate transport system, and to BtuC of the vitamin B12 transport system. FecB showed some homology to FhuD, suggesting that the latter may function in the same manner as a binding protein in iron(III) hydroxamate transport. The close homology between the proteins of the two iron transport systems and of the vitamin B12 transport system indicates a common evolution for all three systems.     

Characterization of a periplasmic ATP-binding cassette iron import system of Brachyspira (Serpulina) hyodysenteriae

The nucleotide sequence of the pathogenic spirochete Brachyspira hyodysenteriae bit (for "Brachyspira iron transport") genomic region has been determined. The bit region is likely to encode an iron ATP-binding cassette transport system with some homology to those encountered in gram-negative bacteria. Six open reading frames oriented in the same direction and physically linked have been identified. This system possesses a protein containing ATP-binding motifs (BitD), two hydrophobic cytoplasmic membrane permeases (BitE and BitF), and at least three lipoproteins (BitA, BitB, and BitC) with homology to iron periplasmic binding proteins. These periplasmic binding proteins exhibit lipoprotein features. They are labeled by [(3)H]palmitate when tested in recombinant Escherichia coli, and their signal peptides are typical for substrates of the type II secretory peptidase. The FURTA system and Congo red assay indicate that BitB and BitC are involved in iron binding. The Bit system is detected only in B. hyodysenteriae and is absent from B. innocens and B. pilosicoli.     

Ferrichrome transport in Escherichia coli K-12: altered substrate specificity of mutated periplasmic FhuD and interaction of FhuD with the integral membrane protein FhuB.

FhuD is the periplasmic binding protein of the ferric hydroxamate transport system of Escherichia coli. FhuD was isolated and purified as a His-tag-labeled derivative on a Ni-chelate resin. The dissociation constants for ferric hydroxamates were estimated from the concentration-dependent decrease in the intrinsic fluorescence intensity of His-tag-FhuD and were found to be 0.4 microM for ferric aerobactin, 1.0 microM for ferrichrome, 0.3 microM for ferric coprogen, and 5.4 microM for the antibiotic albomycin. Ferrichrome A, ferrioxamine B, and ferrioxamine E, which are poorly taken up via the Fhu system, displayed dissociation constants of 79, 36, and 42 microM, respectively. These are the first estimated dissociation constants reported for a binding protein of a microbial iron transport system. Mutants impaired in the interaction of ferric hydroxamates with FhuD were isolated. One mutated FhuD, with a W-to-L mutation at position 68 [FhuD(W68L)], differed from wild-type FhuD in transport activity in that ferric coprogen supported promotion of growth of the mutant on iron-limited medium, while ferrichrome was nearly inactive. The dissociation constants of ferric hydroxamates were higher for FhuD(W68L) than for wild-type FhuD and lower for ferric coprogen (2.2 microM) than for ferrichrome (156 microM). Another mutated FhuD, FhuD(A150S, P175L), showed a weak response to ferrichrome and albomycin and exhibited dissociation constants two- to threefold higher than that of wild-type FhuD. Interaction of FhuD with the cytoplasmic membrane transport protein FhuB was studied by determining protection of FhuB degradation by trypsin and proteinase K and by cross-linking experiments. His-tag-FhuD and His-tag-FhuD loaded with aerobactin specifically prevented degradation of FhuB and were cross-linked to FhuB. FhuD loaded with substrate and also FhuD free of substrate were able to interact with FhuB.     

FhuD1, a ferric hydroxamate-binding lipoprotein in Staphylococcus aureus: a case of gene duplication and lateral transfer

Staphylococcus aureus can utilize ferric hydroxamates as a source of iron under iron-restricted growth conditions. Proteins involved in this transport process are: FhuCBG, which encodes a traffic ATPase; FhuD2, a post-translationally modified lipoprotein that acts as a high affinity receptor at the cytoplasmic membrane for the efficient capture of ferric hydroxamates; and FhuD1, a protein with similarity to FhuD2. Gene duplication likely gave rise to fhuD1 and fhuD2. While the genomic locations of fhuCBG and fhuD2 in S. aureus strains are conserved, both the presence and the location of fhuD1 are variable. The apparent redundancy of FhuD1 led us to examine the role of this protein. We demonstrate that FhuD1 is expressed only under conditions of iron limitation through the regulatory activity of Fur. FhuD1 fractions with the cell membrane and binds hydroxamate siderophores but with lower affinity than FhuD2. Using small angle x-ray scattering, the solution structure of FhuD1 resembles that of FhuD2, and only a small conformational change is associated with ferrichrome binding. FhuD1, therefore, appears to be a receptor for ferric hydroxamates, like FhuD2. Our data to date suggest, however, that FhuD1 is redundant to FhuD2 and plays a minor role in hydroxamate transport. However, given the very real possibility that we have not yet identified the proper conditions where FhuD1 does provide an advantage over FhuD2, we anticipate that FhuD1 serves an enhanced role in the transport of untested hydroxamate siderophores and that it may play a prominent role during the growth of S. aureus in its natural environments.     

Point mutations in two conserved glycine residues within the integral membrane protein FhuB affect iron(II) hydroxamate transport

Summary A region of substantial homology, comprising 32 amino acids around a highly conserved glycine residue, is located near the C-terminal ends of the hydrophobic Fhu, Fec, Fep, Fat, and Btu transport proteins involved in the uptake of ferrisiderophores and vitamin B₁₂ into Escherichia coli and Vibrio anguillarum. Furthermore, a region similar in location and sequence containing an invariant glycine at an equivalent position was identified in the hydrophobic component of all other periplasmic binding protein-dependent (PBT) systems. In the FhuB protein, which is twice the size of the other PBT-related inner membrane proteins and which displays an internal homology, two conserved glycine residues are present. Alteration of Gly at positions 226 and 559 to Ala, Val, or Glu reduced iron(III) hydroxamate uptake, suggesting that this homologous region may play a general role in the mechanism of PBT-dependent transport.     

Holo- and apo-bound structures of bacterial periplasmic heme-binding proteins

An essential component of heme transport in Gram-negative bacterial pathogens is the periplasmic protein that shuttles heme between outer and inner membranes. We have solved the first crystal structures of two such proteins, ShuT from Shigella dysenteriae and PhuT from Pseudomonas aeruginosa. Both share a common architecture typical of Class III periplasmic binding proteins. The heme binds in a narrow cleft between the N- and C-terminal binding domains and is coordinated by a Tyr residue. A comparison of the heme-free (apo) and -bound (holo) structures indicates little change in structure other than minor alterations in the heme pocket and movement of the Tyr heme ligand from an "in" position where it can coordinate the heme iron to an "out" orientation where it points away from the heme pocket. The detailed architecture of the heme pocket is quite different in ShuT and PhuT. Although Arg(228) in PhuT H-bonds with a heme propionate, in ShuT a peptide loop partially takes up the space occupied by Arg(228), and there is no Lys or Arg H-bonding with the heme propionates. A comparison of PhuT/ShuT with the vitamin B(12)-binding protein BtuF and the hydroxamic-type siderophore-binding protein FhuD, the only two other structurally characterized Class III periplasmic binding proteins, demonstrates that PhuT/ShuT more closely resembles BtuF, which reflects the closer similarity in ligands, heme and B(12), compared with ligands for FhuD, a peptide siderophore.     

The Vibrio cholerae VctPDGC system transports catechol siderophores and a siderophore-free iron ligand

Vibrio cholerae, the causative agent of cholera, has an absolute requirement for iron. It transports the catechol siderophores vibriobactin, which it synthesizes and secretes, and enterobactin. These siderophores are transported across the inner membrane by one of two periplasmic binding protein-dependent ABC transporters, VctPDGC or ViuPDGC. We show here that one of these inner membrane transport systems, VctPDGC, also promotes iron acquisition in the absence of siderophores. Plasmids carrying the vctPDGC genes stimulated growth in both rich and minimal media of a Shigella flexneri mutant that produces no siderophores. vctPDGC also stimulated the growth of an Escherichia coli enterobactin biosynthetic mutant in low iron medium, and this effect did not require feoB, tonB or aroB. A tyrosine to phenylalanine substitution in the periplasmic binding protein VctP did not alter enterobactin transport, but eliminated growth stimulation in the absence of a siderophore. These data suggest that the VctPDGC system has the capacity to transport both catechol siderophores and a siderophore-free iron ligand. We also show that VctPDGC is the previously unidentified siderophore-independent iron transporter in V. cholerae, and this appears to complete the list of iron transport systems in V. cholerae.     

大肠杆菌的蛋白分泌机制

大肠杆菌的分泌蛋白定位于内膜、外膜、周质空间和胞外环境,它们在N端或C端带有一定的结构包含着分泌信号,这两类分泌蛋白在各自特定的一组蛋白因子的协助下跨越内膜,再通过目前尚不清楚的方式实现其最终定位.N端带有信号肽的分子在跨越内膜时得到Sec家族蛋白因子协助,信号肽在跨膜过程中可能被切除,该过程由ATP和电化学势提供能量.C端带分泌信号的分子主要受到Hly家族分子协助,一次穿过内膜和外膜而不经过周质空间.     

Iron(III) hydroxamate transport of Escherichia coli: restoration of iron supply by coexpression of the N- and C-terminal halves of the cytoplasmic membrane protein FhuB cloned on separate plasmids

Summary Transport of iron(III) hydroxamates across the inner membrane into the cytoplasm of Escherichia coli cells is mediated by the FhuC, FhuD and FhuB proteins. We studied the extremely hydrophobic FhuB protein (70 kDa) which is located in the cytoplasmic membrane. The N- and C-terminal halves of the protein [FhuB(N) and FhuB(C)] show homology to each other and to the equivalent polypeptides involved in uptake of ferric dicitrate and of vitamin B₂. Various plasmids carrying only one-half of the fhuB gene were expressed in fhuB ^– mutants. Only combinations of FhuB(N) and FhuB(C) polypeptides restored sensitivity to albomycin and growth on iron hydroxamates as sole iron source; no activity was obtained with either half of FhuB alone. These results indicate that both halves of FhuB are essential for substrate translocation and that they combine to form an active permease when expressed separately. In addition, a FhuB derivative with a large internal duplication of 271 amino acids was found to be partially active in transport, indicating that the extra portion did not perurb proper insertion of the active FhuB segments into the cytoplasmic membrane.     

fhuC and fhuD genes for iron (III)-ferrichrome transport into Escherichia coli K-12.

The nucleotide sequence for a 1,900-base-pair region of the Escherichia coli chromosome that includes the genes fhuC and fhuD was determined. Within this sequence are two open reading frames: nucleotides 127 to 921 and nucleotides 924 to 1811. These coding regions specify a FhuC protein with an Mr of 28,423 and a mature FhuD protein with an Mr of 29,610. The deduced amino acid sequence of FhuC shows extensive homology with those of components of some bacterial transport systems which are peripheral proteins of the cytoplasmic membrane. Because the FhuD protein contains a typical signal sequence of 30 amino acids at the amino terminus and displays characteristics of a soluble protein, it may be exported into the periplasm.