The T3SS is unique because it a well-characterized protein secretion system that translocates polypeptides through both the inner and outer membranes. This is in contrast to the Sec and Tat pathways, which deliver proteins to the periplasm (Georgiou and Segatori, 2005; Wickner and Schekman, 2005). The Sec pathway threads polypeptides in an unfolded state across the inner membrane in an ATP-dependent manner (Economou and Wickner, 1994; Pohlschroder et al, 2005). The Tat pathway uses the proton motive force to drive the transport of folded proteins to the periplasm (Sargent et al, 1998; Rodrigue et al, 1999; DeLisa et al, 2003). An N-terminal signal peptide is required for both Sec and Tat export (Berks, 1996; Wickner and Schekman, 2005). Both types of signal peptides typically end with a signal peptidase I cleavage site, allowing cleavage of the tag on translocation to the periplasm (Nielsen et al, 1997; Bendtsen et al, 2005). Type II secretion can export proteins from the periplasm through the outer membrane; however, the secretion signal is difficult to identify and seems to be distributed throughout the protein, making heterologous protein secretion difficult (Pohlschroder et al, 2005). Remarkably, it has been shown that all of the genes associated with the Erwinia type II secretion system can be transferred to Escherichia coli and used to secrete Erwinia cellulases (Zhou et al, 1999). Alternatively, Gram-positive bacteria offer a single secretion event to the extracellular space and offer an attractive platform for secretion engineering.

In its natural context, the SPI-1 T3SS functions as a molecular syringe to inject effector proteins into mammalian host cells that facilitate invasion and pathogenesis (Altier, 2005). The SPI-1 T3SS forms a needle-like structure that crosses both the inner and outer membranes (Marlovits et al, 2006). A chaperone is required for secretion, as well as an N-terminal peptide tag that is not cleaved post secretion (Galan and Collmer, 1999). Secreted proteins can be folded in the cytoplasm and then are unfolded in an ATP-dependent reaction before secretion (Feldman et al, 2002; Lee and Schneewind, 2002; Akeda and Galan, 2005). It is expected based on needle dimensions that proteins must be at least partially unfolded to transit through the needle and would be required to re-fold outside the cell. The E. coli and Salmonella flagellum and Yersinia enterocolitica ysc T3SS have been shown to be able to export heterologous proteins (Russmann et al, 1998; Feldman et al, 2002; Lee and Schneewind, 2002; Majander et al, 2005; Chen et al, 2006; Konjufca et al, 2006; Vegh et al, 2006). These systems have been used to inject foreign proteins and peptides into mammalian cells as a mechanism to confer immunity (Russmann et al, 1998; Boyd et al, 2000; Konjufca et al, 2006).

A well-characterized regulatory network encoded within SPI-1 controls the dynamics of T3SS gene expression (Box 1) (Lucas and Lee, 2000). Environmental signals from two-component systems and global regulators control the expression of the HilC, HilD, and HilA transcription factors, which together form a commitment circuit for the expression of SPI-1 genes (Bajaj et al, 1996; Eichelberg and Galan, 1999; Lundberg et al, 1999; Kalir et al, 2001; Ellermeier et al, 2005). Within SPI-1, there is a genetic circuit that links the expression of effector proteins to the completion of functional needles (Darwin and Miller, 1999, 2000, 2001; Temme et al, 2008). The circuit consists of a transcription factor (InvF) that is only functional when bound to the SicA chaperone protein. Before the cell can secrete protein, the chaperone is sequestered by the SipB/C proteins. After functional needles are completed, SipB/C are secreted and SicA is free to bind InvF, thus turning on the circuit and gene expression from the sicA promoter (Darwin and Miller, 1999, 2000, 2001; Tucker and Galan, 2000).

The Salmonella SPI-1 T3SS has several properties that make it a good tool for the secretion of recombinant proteins. It is highly expressed under standard laboratory conditions (Luria-Bertani Broth at 37°C), with 10–100 needles per cell (Kubori et al, 1998). Under these conditions, effector proteins are secreted into the media in significant quantities without the need to co-culture with mammalian cells or expensive media components (Kubori and Galan, 2002). Finally, the N-terminal secretion tags, chaperone-binding domains (CBD), and chaperones have been identified (Fu and Galan, 1998; Hong and Miller, 1998; Bronstein et al, 2000; Tucker and Galan, 2000; Russmann et al, 2002; Zhang et al, 2002; Ehrbar et al, 2003; Lee and Galan, 2004; Wood et al, 2004; Karavolos et al, 2005; Higashide and Zhou, 2006; Knodler et al, 2006). On the basis of this earlier work, we have constructed a system that contains all of the necessary genetic parts to secrete heterologous proteins (Box 1). This system is demonstrated by using it to export a human protein and three spider silk monomers.

Natural silks are abundant biomaterials that span a remarkable diversity of physical properties (Vollrath and Knight, 2001; Swanson et al, 2006). For example, the dragline silk of spider webs is extremely strong yet remains highly elastic. These materials have a number of uses including medical device implants, high strength fibers, advanced composites, and drug delivery systems (Lewis, 2006; Wang et al, 2006a, 2006b; Hofmann et al, 2007; Lee et al, 2009). Spiders are not conducive to scalable agriculture, therefore requiring silk production to be done in a recombinant host (Lewis, 2006). Using a solution of natural or recombinant silk monomers, it has been shown that threads can be artificially spun, producing materials with properties approaching natural silks (Seidel et al, 2000; Lazaris et al, 2002). One of the key limitations in creating materials that match or exceed the natural properties is the lack of practical approaches for recombinant protein expression (Kluge et al, 2008).

The production of recombinant native silk proteins has been complicated by several factors. First, the genes themselves are often unstable because of highly repetitive regions of DNA that results in frequent homologous recombination (Arcidiacono et al, 1998). Second, the codon usage in silk genes is not optimized for expression outside of the specialized cells in the silk gland (Prince et al, 1995; Rising et al, 2005). Rare codons result in ribosome pausing and early truncation producing incomplete protein products (Fahnestock and Irwin, 1997). Third, if the proteins are highly expressed in the confined cell volume they can self-assemble to produce fibrils, which have been visualized when recombinantly expressed in insect cells (Huemmerich et al, 2004b).

We constructed a genetic system to secrete heterologous proteins into the culture media (Box 1). This system consists of four genetic parts. A circuit is harnessed from the natural SPI-1 regulatory pathway that controls gene expression based on the capacity of the cell to export protein (Darwin and Miller, 1999, 2000, 2001; Temme et al, 2008). The output of this circuit (PsicA) drives the transcription of an operon containing the heterologous protein fused to an N-terminal secretion tag and the associated chaperone. The human DH domain (Hussain et al, 2001) was chosen as an initial target because it is small (24 kDa), easy to manipulate, and it expresses well in Salmonella without affecting cell growth. A protease cleavage site is included between the tag and exported protein so that the tag can be removed after secretion. All of these parts are combined onto a plasmid (Box 1) and are available in the Registry of Standard Biological Parts (http://www.partsregistry.org/). The part numbers (BBa_) are provided throughout the text.

Using the SicA:InvF circuit to control expression has two advantages. First, it restricts expression until the T3SS is built and is functional (Darwin and Miller, 1999, 2000, 2001). This links expression to secretion capacity and can reduce the build-up of protein before secretion. The second advantage is that no exogenous inducer is required. The circuit can be maintained in the off state when cells are grown in low salt L Broth (LB) and then turned on after the cells are induced by high salt LB (inducing medium) and the needle is constructed (Figure 2A) (Materials and methods) (Tartera and Metcalf, 1993). The sicA promoter has a low basal transcription rate and increases 200-fold in activity when induced. Additionally, the sicA promoter has no activity in E. coli under inducing conditions, which is a useful trait for cloning purposes (Figure 3A).

The sicA promoter turns on within 3 h after cells are shifted into inducing media (Figure 2A) (Temme et al, 2008). At this point, secreted protein begins to accumulate in the media (Figure 2A). This can be clearly visualized in a Coomassie gel (Figure 3D) along with other proteins naturally secreted by Salmonella. There was no observable lysis or outer membrane shedding in the secretion assay, as determined using an antibody against the MalE periplasmic protein (Majander et al, 2005).

There are multiple chaperones that target proteins to the SPI-1 T3SS and they interact with different N-terminal tags (Table I). A secretion assay was performed for the known tag/chaperone pairs to compare the amount of protein that is exported to the supernatant. This was repeated for each protein in this study to identify the optimal combination (Figure 2B; Supplementary information S2) (Materials and methods) (Collazo and Galan, 1996). The SptP/SicP combination was found to yield the largest amount of secreted protein for DH, ADF-2 (Figure 2B), and ADF-3 (Supplementary information S2). For ADF-1, the optimum pair was SopB/SigE.

The SptP tag consists of a 15 amino-acid signal sequence and 152 amino-acid CBD (Lee and Galan, 2004). The SptP CBD interacts with SicP chaperone dimers, which direct the SptP-tagged protein to the SPI-1 T3SS (Stebbins and Galan, 2001; Akeda and Galan, 2005). It has been shown that the signal sequence directs the protein generally to T3SSs and flagella. The CBD causes the protein to be directed specifically to the SPI-1 T3SS (Lee and Galan, 2004). The T3SS secretion signal is not self-cleaving. A tobacco etch virus (TEV) protease site is added after the tag so that it can be removed after export.

Variants of the pCASP plasmid were constructed by removing the tag and chaperone parts and tested for the loss of secretion. When expressed without the N-terminal SptP tag, there is a dramatic decline in the amount of DH protein that appears in the supernatent (Figure 3A). The deletion of the tag from the silk monomers results in 100-fold less protein in the supernatant (Table II). Secretion is also reduced when the SicP chaperone is only expressed at wild-type levels and not co-transcribed on the plasmid (-SicP). We tested the effect of expressing the chaperone in conjunction with a (−) tag protein in the pCASP system. The construct was run using the secretion assay and analyzed by western blot (Figure 3B).

Secretion was also tested for Salmonella strains where components of the SPI-1 T3SS and flagellum have been knocked out (Figure 3C). The sicA promoter differs in activity for these knockouts, so a plasmid was constructed where the chaperone and tagged protein are under the control of an IPTG-inducible promoter. There was little effect when the flagella master regulators (ΔflhDC) are knocked out. However, when critical structural components of the SPI-1 T3SS are knocked out (ΔprgHIJKorgABC), secretion is eliminated. These results were confirmed using a set of earlier constructed knockout strains (ΔinvA and ΔflhGHI) (Supplementary information) (Lee and Galan, 2004). Together, these data indicate that the observed protein secretion is SPI-1 dependent.

Here, we describe a control system that contains all of the necessary genetic parts for heterologous proteins to be secreted to the extracellular environment through the Salmonella SPI-1 T3SS (Box 1). At the core of this system is a feedback circuit that responds to the capacity of the bacterium to export protein. This circuit activates the transcription of an operon containing the protein to be secreted, which is fused to the SptP N-terminal secretion signal. The SicP chaperone is also encoded, which binds and directs the heterologous protein to the SPI-1 T3SS.

The yields of secreted protein using type III secretion (Table II) are as good or better than other methods of extracellular protein production (Choi and Lee, 2004). Earlier reported values range from 0.5 to 10 mg l⁻¹ h⁻¹ in shake flasks using a range of protein export and recovery methods (Lucic et al, 1998; Tong et al, 2000; Fu et al, 2003). Unlike the Sec and Tat pathways, the T3SS translocates proteins through both membranes to the extracellular environment.

Natural Yersinia, Shigella, and Salmonella effector proteins have been observed to accumulate up to grams per liter in the growth media. The secretion rate of the SPI-1 T3SS has been measured in individual cells to be 7–60 proteins per cell per second when secreting a natural effector (Enninga et al, 2005; Schlumberger et al, 2005). This corresponds to an approximate theoretical yield of 10 mg l⁻¹ h⁻¹ OD for a protein the size of ADF-2. Our system is able to achieve ~10% of this yield for the spider silk ADF-2 in a shake flask culture.

Whole gene DNA synthesis is a useful tool to make large-scale changes to genes. In this work, three spider silk genes were computationally designed for expression in eubacteria and constructed using automated DNA synthesis and assembly. Only the amino-acid sequence information is required for gene construction, rather than physical genetic material from the source organism. The degeneracy of the genetic code allows many alternative nucleotide sequences to encode the same protein. This enables synthetic genes to be designed that simultaneously match the recombinant host's codon usage, while reducing the DNA repetitiveness. As of their highly repetitive amino-acid sequences, silk monomers represent one of the most challenging targets of automated DNA synthesis and assembly. The three synthetic ADF genes built here represent all of the known sequence information for these silks, but they are still fragments of the complete silk genes.

Supplementary Information