8). The intracellular replication of WT Salmonella and the complemented sseB strain was about 50- to 55-fold over a period of 14 h. The replication of the sseB strain without plasmid or with plasmids for the expression of any of the deletions alleles of sseB was reduced to an about 5-fold increase
of the intracellular bacteria and no significant difference between the various Selleckchem BLZ945 constructs was observed (Fig. 8A). Similar characteristics were observed for strains expressing deletion alleles of sseD and none of the mutant strains showed intracellular replication that was above the level of the sseD strain (Fig. 8B). Figure 8 Effect of mutations in SseB or SseD on intracellular replication of Salmonella. Macrophages were infected at a MOI of 1 with S. Typhimurium wild type (WT), sseB, sseB [psseB] or sseB harboring plasmids for expression of various sseB mutant alleles (sseB [psseBΔx]) (A), or WT, sseD, sseD [psseD], or various strains harboring chromosomal PF477736 deletion in sseD (B). Extracellular bacteria were killed by gentamicin treatment during 1 h post infection. Intracellular bacteria were quantified after host cell lysis with Triton X-100 at 2 h and 16 h post infection. The x-fold replication is the ratio of viable intracellular bacteria recovered at 16 h versus 2 h post infection. The replication rate
was assessed in triplicates and the standard deviation of the mean was calculated. Means and standard deviations of triplicate assays are shown and all experiments were performed at least twice. These data indicate that SseB and SseD do not tolerate major Selleckchem JNJ-26481585 alterations of the primary structure in order to fulfill their function as parts of the translocon selleck chemical of the SPI2-T3SS. The data also demonstrate that a fully functional translocon is required for the efficient intracellular replication. The residual ability of strains expressing sseBΔ2 or sseBΔ3 to translocate effector proteins appears to be insufficient to confer the ability of intracellular replication. Discussion In this
study we performed a structure-based functional dissection of the SPI2-T3SS translocon proteins SseB and SseD. Protein domains predicted as putative transmembrane regions or coiled-coil regions were deleted, as well as N- or C-terminal portions, and previously defined binding regions for the specific chaperone SseA [9, 10]. The deletional and functional analyses described here clearly demonstrate the sensitivity of SseB and SseD against structural alterations. Many of the deletion variants lost the ability to be secreted by the SPI2-T3SS. However, we also identified a subset of deletion variants that were synthesized in quantities similar to the WT proteins, secreted under in vitro conditions and bound to the bacterial surface. The lack of the chaperone binding site in SseB led to reduced amounts of protein. We found that some mutant forms of SseB were on surface structures on bacteria grown in vitro (Fig. 4B), but not on intracellular Salmonella (Fig. 5B).