The first 28 amino acids of mature LamB are required for rapid and efficient export from the cytoplasm

The role of signal sequence proximal residues in the mature region of bacterial secreted proteins in E. coli

Secreted proteins contain an N-terminal signal peptide to guide them through the secretion pathway. Once the protein is translocated, the signal peptide is removed by a signal peptidase, such as signal peptidase I. The signal peptide has been extensively studied and reviewed; however, the mature region has not been the focus of review. Here we cover the experimental evidence that highlights the important role of the mature region amino acid residues in both the efficiency and the ability of secreted proteins to be successfully exported via secretion pathways and cleaved by signal peptidase I.

Contribution of single amino acid and codon substitutions to the production and secretion of a lipase by Bacillus subtilis

Background

Bacillus subtilis produces and secretes proteins in amounts of up to 20 g/l under optimal conditions. However, protein production can be challenging if transcription and cotranslational secretion are negatively affected, or the target protein is degraded by extracellular proteases. This study aims at elucidating the influence of a target protein on its own production by a systematic mutational analysis of the homologous B. subtilis model protein lipase A (LipA). We have covered the full natural diversity of single amino acid substitutions at 155 positions of LipA by site saturation mutagenesis excluding only highly conserved residues and qualitatively and quantitatively screened about 30,000 clones for extracellular LipA production. Identified variants with beneficial effects on production were sequenced and analyzed regarding B. subtilis growth behavior, extracellular lipase activity and amount as well as changes in lipase transcript levels.

Results

In total, 26 LipA variants were identified showing an up to twofold increase in either amount or activity of extracellular lipase. These variants harbor single amino acid or codon substitutions that did not substantially affect B. subtilis growth. Subsequent exemplary combination of beneficial single amino acid substitutions revealed an additive effect solely at the level of extracellular lipase amount; however, lipase amount and activity could not be increased simultaneously.

Conclusions

Single amino acid and codon substitutions can affect LipA secretion and production by B. subtilis. Several codon-related effects were observed that either enhance lipA transcription or promote a more efficient folding of LipA. Single amino acid substitutions could improve LipA production by increasing its secretion or stability in the culture supernatant. Our findings indicate that optimization of the expression system is not sufficient for efficient protein production in B. subtilis. The sequence of the target protein should also be considered as an optimization target for successful protein production. Our results further suggest that variants with improved properties might be identified much faster and easier if mutagenesis is prioritized towards elements that contribute to enzymatic activity or structural integrity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-017-0772-z) contains supplementary material, which is available to authorized users.

Systematic Screening of All Signal Peptides from Bacillus subtilis: A Powerful Strategy in Optimizing Heterologous Protein Secretion in Gram-positive Bacteria

Efficient protein secretion is very important in biotechnology as it provides active and stable enzymes, which are an essential prerequisite for successful biocatalysis. Therefore, optimizing enzyme-producing bacterial strains is a major challenge in the field of biotechnology and protein production. In this study, the Gram-positive model bacterium Bacillus subtilis was optimized for heterologous protein secretion using a novel approach. Two lipolytic enzymes, cutinase from Fusarium solani pisi and a cytoplasmatic esterase of metagenomic origin, were chosen as reporters for heterologous protein secretion. In a systematic screening approach, all naturally occurring (non-lipoprotein) Sec-type signal peptides (SPs) from B. subtilis were characterized for their potential in heterologous protein secretion. Surprisingly, optimal SPs in cutinase secretion were inefficient in esterase secretion and vice versa, indicating the importance of an optimal fit between the SP and the respective mature part of the desired secretion target proteins. These results highlight the need for individually optimal signal peptides for every heterologous secretion target. Therefore, the SP library generated in this study represents a powerful tool for secretion optimization in Gram-positive expression hosts.

Targeting and assembly of periplasmic and outer-membrane proteins in Escherichia coli

Escherichia coli must actively transport many of its proteins to extracytoplasmic compartments such as the periplasm and outer membrane. To perform this duty, E. coli employs a collection of Sec (secretion) proteins that catalyze the translocation of various polypeptides through the inner membrane. After translocation across the inner membrane, periplasmic and outer-membrane proteins are folded and targeted to their appropriate destinations. Here we review our knowledge of protein translocation across the inner membrane. We also discuss the various signal transduction systems that monitor extracytoplasmic protein folding and targeting, and we consider how these signal transduction systems may ultimately control these processes.

Folding-based suppression of extracytoplasmic toxicity conferred by processing-defective LamB

We have utilized processing-defective derivatives of the outer membrane maltoporin, LamB, to study protein trafficking functions in the cell envelope of Escherichia coli. Our model proteins contain amino acid substitutions in the consensus site for cleavage by signal peptidase. As a result, the signal sequence is cleaved with reduced efficiency, effectively tethering the precursor protein to the inner membrane. These mutant porins are toxic when secreted to the cell envelope. Furthermore, strains producing these proteins exhibit altered outer membrane permeability, suggesting that the toxicity stems from some perturbation of the cell envelope (J. H. Carlson and T. J. Silhavy, J. Bacteriol. 175:3327-3334, 1993). We have characterized a multicopy suppressor of the processing-defective porins that appears to act by a novel mechanism. Using fractionation experiments and conformation-specific antibodies, we found that the presence of this multicopy suppressor allowed the processing-defective LamB precursors to be folded and localized to the outer membrane. Analysis of the suppressor plasmid revealed that these effects are mediated by the presence of a truncated derivative of the polytopic inner membrane protein, TetA. The suppression mediated by TetA' is independent of the CpxA/CpxR regulon and the sigma E regulon, both of which are involved in regulating protein trafficking functions in the cell envelope.

Characterization of the C-terminal propeptide involved in bacterial wall spanning of alpha-amylase from the psychrophile Alteromonas haloplanctis

The antarctic psychrophile Alteromonas haloplanctis secretes a Ca2+- and Cl--dependent alpha-amylase. The nucleotide sequence of the amy gene and the amino acid sequences of the gene products indicate that the alpha-amylase precursor is a preproenzyme composed by the signal peptide (24 residues), the mature alpha-amylase (453 residues, 49 kDa), and a long C-terminal propeptide or secretion helper (192 residues, 21 kDa). In cultures of the wild-type strain, the 70-kDa precursor is secreted at the mid-exponential phase and is cleaved by a nonspecific protease into the mature enzyme and the propeptide. The purified C-terminal propeptide displays several features common to beta-pleated transmembrane proteins. It has no intramolecular chaperone function because active alpha-amylase is expressed by Escherichia coli in the absence of the propeptide coding region. In E. coli, the 70-kDa precursor is directed toward the supernatant. When the alpha-amylase coding region is excised from the gene, the secretion helper can still promote its own membrane spanning. It can also accept a foreign passenger, as shown by the extracellular routing of a beta-lactamase-propeptide fusion protein.

A neural network method for identification of prokaryotic and eukaryotic signal peptides and prediction of their cleavage sites

We have developed a new method for the identification of signal peptides and their cleavage sites based on neural networks trained on separate sets of prokaryotic and eukaryotic sequences. The method performs significantly better than previous prediction schemes, and can easily be applied to genome-wide data sets. Discrimination between cleaved signal peptides and uncleaved N-terminal signal-anchor sequences is also possible, though with lower precision. Predictions can be made on a publicly available WWW server: http://www.cbs.dtu.dk/services/SignalP/.

Amino acid residues in the pro region of Escherichia coli heat-stable enterotoxin I that affect efficiency of translocation across the inner membrane

Escherichia coli heat-stable enterotoxin Ip (STIp), which is a typical extracellular toxin consisting of 18 amino acid residues, is synthesized as a precursor consisting of pre (amino acid residues 1 to 19), pro (amino acid residues 20 to 54), and mature (amino acid residues 55 to 72) regions. Though the pre region functions as a conventional leader peptide that guides the following region to cross the inner membrane, the role of the pro region in the maturation pathway remains to be elucidated. We previously indicated that the sequence from residues 29 to 38 in the pro region increases the efficiency of STI translocation across the inner membrane (H. Yamanaka, Y. Fuke, S. Hitotsubashi, Y. Fujii, and K. Okamoto, Microbiol. Immunol. 37:195-205, 1993). We therefore examined the amino acid residues in the sequence that are responsible for this function. We substituted several amino acid residues in the sequence by means of oligonucleotide-directed site-specific mutagenesis. We then evaluated the effect of the substitution on the efficiency of STI translocation across the inner membrane by determining the enterotoxic activity of the culture supernatant, the amount of a fusion protein consisting of STI and nuclease A released into the periplasm, and the amount of the labeled ST released into the periplasm after pulse-labeling with [35S]cysteine. Substitution of the charged amino acid residues at positions 29 to 31 (K-E-K) with hydrophobic (I-V-L, F-W-F, or F-W-Q) or basic (K-K-K) residues significantly reduced these values in every assay. In contrast, the substitution of these amino acid residues with acidic amino acid residues (E-E-E) increased these values in all assays. This means that the negative charge near position 30 is important for STI to translocate efficiently across the inner membrane. A similar substitution of lysine residues at positions 37 and 38 showed that they are not involved in the translocation of STI across the inner membrane.

Defining a similarity threshold for a functional protein sequence pattern: the signal peptide cleavage site

When preparing data sets of amino acid or nucleotide sequences it is necessary to exclude redundant or homologous sequences in order to avoid overestimating the predictive performance of an algorithm. For some time methods for doing this have been available in the area of protein structure prediction. We have developed a similar procedure based on pair-wise alignments for sequences with functional sites. We show how a correlation coefficient between sequence similarity and functional homology can be used to compare the efficiency of different similarity measures and choose a nonarbitrary threshold value for excluding redundant sequences. The impact of the choice of scoring matrix used in the alignments is examined. We demonstrate that the parameter determining the quality of the correlation is the relative entropy of the matrix, rather than the assumed (PAM or identity) substitution mode. Results are presented for the case of prediction of cleavage sites in signal peptides. By inspection of the false positives, several errors in the database were found. The procedure presented may be used as a general outline for finding a problem-specific similarity measure and threshold value for analysis of other functional amino acid or nucleotide sequence patterns.

Thirty-three amino acids of the mature moiety of an unprocessed maltose-binding protein are sufficient for export in Escherichia coli

Maltose-binding protein (MBP) is translocated across the cytoplasmic membrane of Escherichia coli; successful export depends on information in both the signal peptide and the mature moiety of the protein. To determine the shortest portion of the mature region that would maintain detectable entry of MBP into the export pathway, we took advantage of the properties of an MBP species with proline substituted in the +1 position relative to the cleavage site (MBP27-P). This protein efficiently crosses the cytoplasmic membrane but is not processed and acts as a competitive inhibitor of signal peptidase I (leader peptidase). Export of MBP27-P is measured by the inhibition of processing of other proteins, such as ribose-binding protein (RBP). A series of truncated derivatives of MBP27-P were tested for the ability to inhibit processing of RBP. An MBP27-P species with only 33 amino acids of the mature moiety inhibited processing of RBP, indicating that this truncated polypeptide was probably exported and interacted with signal peptidase I.

Signal sequence processing is required for the assembly of LamB trimers in the outer membrane of Escherichia coli

Proteins destined for either the periplasm or the outer membrane of Escherichia coli are translocated from the cytoplasm by a common mechanism. It is generally assumed that outer membrane proteins, such as LamB (maltoporin or lambda receptor), which are rich in beta-structure, contain additional targeting information that directs proper membrane insertion. During transit to the outer membrane, these proteins may pass, in soluble form, through the periplasm or remain membrane associated and reach their final destination via sites of inner membrane-outer membrane contact (zones of adhesion). We report lamB mutations that slow signal sequence cleavage, delay release of the protein from the inner membrane, and interfere with maltoporin biogenesis. This result is most easily explained by proposing a soluble, periplasmic LamB assembly intermediate. Additionally, we found that such lamB mutations confer several novel phenotypes consistent with an abortive attempt by the cell to target these tethered LamB molecules. These phenotypes may allow isolation of mutants in which the process of outer membrane protein targeting is altered.

Combinatorial mutagenesis of the lamB gene: residues 41 through 43, which are conserved in Escherichia coli outer membrane proteins, are informationally important in maltoporin structure and function

A new strategy for combinatorial mutagenesis was developed and applied to residues 40 through 60 of LamB protein (maltoporin), with the aim of identifying amino acids important for LamB structure and function. The strategy involved a template containing a stop codon in the target sequence and a pool of random degenerate oligonucleotides covering the region. In vitro mutagenesis followed by selection for function (Dex+, ability to utilize dextrins) corrected the nonsense mutation and simultaneously forced incorporation of a random mutation(s) within the region. The relative importance of each residue within the target was indicated by the frequency and nature of neutral and deleterious mutations recovered at each position. Residues 41 through 43 in LamB accepted few neutral substitutions, whereas residues 55 through 57 were highly flexible in this regard. Consistent with this finding was that the majority of defective mutants were altered at residues 41 to 43. Characterization of these mutants indicated that the nature of residues 41 to 43 influenced the amount of stable protein in the outer membrane. These results, as well as the conserved nature of this stretch of residues among outer membrane proteins, suggest that residues 41 to 43 of LamB play an important role in the process of outer membrane localization.

Regulation by proteolysis: energy-dependent proteases and their targets

A number of critical regulatory proteins in both prokaryotic and eukaryotic cells are subject to rapid, energy-dependent proteolysis. Rapid degradation combined with control over biosynthesis provides a mechanism by which the availability of a protein can be limited both temporally and spatially. Highly unstable regulatory proteins are involved in numerous biological functions, particularly at the commitment steps in developmental pathways and in emergency responses. The proteases involved in energy-dependent proteolysis are large proteins with the ability to use ATP to scan for appropriate targets and degrade complete proteins in a processive manner. These cytoplasmic proteases are also able to degrade many abnormal proteins in the cell.

Secretion of mutant leucine-specific binding proteins with internal deletions in Escherichia coli

The leucine-specific binding protein, encoded by the livK gene, is located in the periplasm of E. coli. The present study is an attempt to identify intragenic regions that determine the efficiency of its secretion into the periplasm. C-terminal deletions or fusions of the livK gene to trpA (encoding the alpha subunit of tryptophan synthetase) were secreted with little loss of efficiency [1]. A series of deletions was constructed at the unique Sphl site within livK, near the 5' end of the region coding for the mature protein. Between 16 and 113 amino acids were deleted in the amino-terminal one-third of the protein. A few of these deletions were located within a few amino acids of the signal sequence processing site. Deletions extending within thirteen residues of the processing site were processed and secreted more slowly than normal. Secondary structure predictions suggested that the alpha-helical core region of the signal sequence extends into the mature protein in the case of the slow processing mutants, perhaps interfering with the recognition site for leader peptidase or other secretory components. These results suggest that the conformation around the signal processing site may be a critical factor in determining the efficiency of secretion. During the course of this study, it was found that the difference in molecular weight between precursor and mature forms of some binding protein mutants, as judged by SDS-PAGE, was much greater than could be accounted for by processing of the signal sequence. This anomalous mobility on gels, however, could be eliminated by performing SDS-PAGE in the presence of 6 M urea.

Conformational and membrane-binding properties of a signal sequence are largely unaltered by its adjacent mature region

We have synthesized a peptide corresponding to the 25-residue signal sequence plus the first 28 residues of the Escherichia coli outer membrane protein LamB in order to explore the properties of a signal sequence in the presence of the N-terminal region of its passenger. In the last few years, there have been several observations of differing efficiencies of export when signal sequences are attached to different passenger proteins or when the first part of a passenger protein undergoes mutation. In the LamB case, gene fusions with lacZ have shown that the signal sequence plus the first 28 residues of mature LamB are necessary to direct beta-galactosidase into the export pathway [Rasmussen, B. A. & Silhavy, T. J. (1987) Genes Dev. 1, 185-196]. The origin of these observations and whether there is an influence of the mature region on the properties of the signal sequence have not been known. We find that the conformational and membrane-binding properties of the LamB signal sequence manifest in a 25-residue peptide are essentially unaltered in the context of the 53-residue peptide corresponding to this signal sequence plus the first 28 residues of the mature LamB protein. CD spectra show that the signal peptide and passenger domains are conformationally independent of each other in micelle or bilayer environments. Furthermore, the signal sequence leads to the spontaneous association of the 53-residue peptide with a lipid bilayer; alone, the mature domain does not interact with lipid bilayers. Fluorescence results show that the mode of interaction of the signal peptide with a bilayer is essentially unaltered by the presence of its mature region. This lack of influence of the mature domain on the behavior of the signal sequence is unexpected for juxtaposed polypeptides of comparable length and may be of physiological importance: N-terminal regions of secreted proteins may be selected to be passive, by comparison with their cognate signal sequences, which themselves must engage the export apparatus and actively interact with its components.

Signal peptide mutants of Escherichia coli

Numerous secretory proteins of the Gram-negative bacteria E. coli are synthesized as precursor proteins which require an amino terminal extension known as the signal peptide for translocation across the cytoplasmic membrane. Following translocation, the signal peptide is proteolytically cleaved from the precursor to produce the mature exported protein. Signal peptides do not exhibit sequence homology, but invariably share common structural features: (1) The basic amino acid residues positioned at the amino terminus of the signal peptide are probably involved in precursor protein binding to the cytoplasmic membrane surface. (2) A stretch of 10 to 15 nonpolar amino acid residues form a hydrophobic core in the signal peptide which can insert into the lipid bilayer. (3) Small residues capable of beta-turn formation are located at the cleavage site in the carboxyl terminus of the signal peptide. (4) Charge characteristics of the amino terminal region of the mature protein can also influence precursor protein export. A variety of mutations in each of the structurally distinct regions of the signal peptide have been constructed via site-directed mutagenesis or isolated through genetic selection. These mutants have shed considerable light on the structure and function of the signal peptide and are reviewed here.

Export and sorting of the Escherichia coli outer membrane protein OmpA

Results of studies, mostly using the outer membrane, 325 residue protein OmpA, are reviewed which concern its translocation across the plasma membrane and incorporation into the outer membrane of Escherichia coli. For translocation, neither a unique export signal, acting in a positive fashion within the mature part of the precursor, nor a unique conformation of the precursor is required. Rather, the mature part of a secretory protein has to be export-compatible. Export-incompatibility can be caused by a stretch of 16 (but not 8 or 12) hydrophobic residues, too low a size of the polypeptide (smaller than 75 residue precursors), net positive charge at the N-terminus, or lack of a turn potential at the same site. It is not yet clear whether binding sites for chaperonins (SecB, trigger factor, GroEL) within OmpA are important in vivo. The mechanism of sorting of outer membrane proteins is not yet understood. The membrane part of OmpA, encompassing residues 1 to about 170, it thought to traverse the membrane eight times in antiparallel beta-sheet conformation. At least the structure of the last beta-strand (residues 160-170) is of crucial importance for membrane assembly. It must be amphiphilic or hydrophobic, these properties must extend over at least nine residues, and it must not contain a proline residue at or near its center. Membrane incorporation of OmpA involves a conformational change of the protein and it could be that the last beta-strand initiates folding and assembly in the outer membrane.

Export of the periplasmic maltose-binding protein of Escherichia coli

The export of the maltose-binding protein (MBP), the malE gene product, to the periplasm of Escherichia coli cells has been extensively investigated. The isolation of strains synthesizing MalE-LacZ hybrid proteins led to a novel genetic selection for mutants that accumulate export-defective precursor MBP (preMBP) in the cytoplasm. The export defects were subsequently shown to result from alterations in the MBP signal peptide. Analysis of these and a variety of mutants obtained in other ways has provided considerable insight into the requirements for an optimally functional MBP signal peptide. This structure has been shown to have multiple roles in the export process, including promoting entry of preMBP into the export pathway and initiating MBP translocation across the cytoplasmic membrane. The latter has been shown to be a late event relative to synthesis and can occur entirely posttranslationally, even many minutes after the completion of synthesis. Translocation requires that the MBP polypeptide exist in an export-competent conformation that most likely represents an unfolded state that is not inhibitory to membrane transit. The signal peptide contributes to the export competence of preMBP by slowing the rate at which the attached mature moiety folds. In addition, preMBP folding is thought to be further retarded by the binding of a cytoplasmic protein, SecB, to the mature moiety of nascent preMBP. In cells lacking this antifolding factor, MBP export represents a race between delivery of newly synthesized, export-competent preMBP to the translocation machinery in the cytoplasmic membrane and folding of preMBP into an export-incompetent conformation. SecB is one of three E. coli proteins classified as "molecular chaperones" by their ability to stabilize precursor proteins for membrane translocation.

The role of the mature part of secretory proteins in translocation across the plasma membrane and in regulation of their synthesis in Escherichia coli

Presently available data are reviewed which concern the role of the mature parts of secretory precursor proteins in translocation across the plasma membrane of Escherichia coli. The following conclusions can be drawn; i) signals, acting in a positive fashion and required for translocation do not appear to exist in the mature polypeptides; ii) a number of features have been identified which either affect the efficiency of translocation or cause export incompatibility. These are: alpha) protein folding prior to translocation; beta) restrictions regarding the structure of N-terminus; gamma) presence of lipophilic anchors; delta) too low a size of the precursor. Efficiency of translocation is also enhanced by binding of chaperonins (SecB, trigger factor, GroEL) to precursors. Binding sites for chaperonins appear to exist within the mature parts of the precursors but the nature of these sites has remained rather mysterious. Mutant periplasmic proteins with a block in release from the plasma membrane have been described, the mechanism of this block is not known. The mature parts of secretory proteins can also be involved in the regulation of their synthesis. It appears that exported proteins are already recognized as such before they are channelled into the export pathway and that their synthesis can be feed-back inhibited at the translational level.

The mature portion of Escherichia coli maltose-binding protein (MBP) determines the dependence of MBP on SecB for export

The product of the secB gene is required for export of a subset of secreted proteins to the outer membrane and periplasm of Escherichia coli. Precursor maltose-binding protein (MBP) accumulates in the cytoplasm of secB-carrying mutants, but export of alkaline phosphatase is only minimally affected by secB mutations. When export of MBP-alkaline phosphatase hybrid proteins was analyzed in wild-type and secB-carrying mutant strains, the first third of mature MBP was sufficient to render export of the hybrid proteins dependent on SecB. Substitution of a signal sequence from a SecB-independent protein had no effect on SecB-dependent export. These findings show that the first third of mature MBP is capable of conferring export incompetence on an otherwise competent protein.

Isolation, molecular characterization and expression of the ushB gene of Salmonella typhimurium which encodes a membrane-bound UDP-sugar hydrolase

The UDP-sugar hydrolase of Salmonella typhimurium has previously been reported to be located in both the inner and the outer membrane. We have cloned the gene, designated ushB, encoding this enzyme and determined its nucleotide sequence. No significant sequence homology with the periplasmic UDP-sugar hydrolase of Escherichia coli was found at either the DNA or protein level. However, a sequence is detectable, in the E. coli genome, which weakly hybridizes with a specific ushB probe. Polypeptide analysis has allowed the identification of the Salmonella hydrolase which has an Mr of 28,349 as compared to an Mr of 60,767 for the E. coli hydrolase. Most of the protein (approximately 90%) is located in the inner membrane. Two independent membrane fractionation procedures indicate that the remainder may be associated with the outer membrane. The deduced primary structure indicates the presence of an N-terminal signal peptide, although certain features of the region surrounding the putative processing site indicate that processing may be inefficient, or may not occur. Experiments with several inhibitors of signal peptidase function fail to demonstrate the appearance of a precursor form.

Alteration of the amino terminus of the mature sequence of a periplasmic protein can severely affect protein export in Escherichia coli

Escherichia coli alkaline phosphatase, coded for by the phoA gene, is normally translocated across the cytoplasmic membrane into the periplasm with high efficiency. We have constructed a series of derivatives of the phoA gene that code for a wild-type signal sequence but result in altered amino acid sequences at the amino terminus of the mature alkaline phosphatase. Our results suggest that the presence of two positively charged amino acids very early in the mature sequence interferes significantly with protein export. In one case, phoA2AB, the presence of the sequence Arg-Ile-Arg at the amino terminus of alkaline phosphatase results in a 50-times reduction in the export of the protein. By using oligonucleotide-directed mutagenesis, we have constructed mutant derivatives of phoA2AB that are greatly enhanced for export. In all cases, these derivatives reduce the net positive charge in the region. Our results may explain the failure of E. coli to export a number of proteins coded for by artificial constructs and suggest a way to improve export in these cases.

On the role of the mature part of an Escherichia coli outer membrane protein (OmpA) in translocation across the plasma membrane

The 325-residue OmpA protein, which is synthesized as a precursor with a 21-residue signal sequence, is a polypeptide of the outer membrane of Escherichia coli K-12. The signal peptide is able to direct translocation across the plasma membrane of virtually any fragment of this protein. It had, therefore, been concluded that information required for this translocation does not exist within the mature part of the protein. This view has been criticized and it was suggested that our data showed that both the signal sequence and residues within the first 44 amino acid residues of the mature protein contributed to an optimal translocation mechanism. It is shown that, at least as far as is detectable, this is not so. The apparent rates of processing of various pro-OmpA constructs were measured. It was found that these rates did not depend on the presence of amino acid residues 4 through 45 but on the size of the polypeptides; the processing rate decreased with decreasing size. A possible explanation for this phenomenon is offered. While the results do not exclude the possibility that a defined area of the mature protein is involved in optimizing translocation, there is so far no evidence for it.