EspC is an abundant type 5 secreted protein Bovine serum albumin

EspC is an abundant type 5 secreted protein. Bovine serum albumin (BSA) was added to collected secreted protein fractions as a carrier protein to assist in the precipitation of proteins. A molecular weight standard is in the left most lane. Right: immunoblot analyses of secreted protein and whole cell lysate fractions from bacterial strains used in panel A (as indicated). The respective secreted

protein fractions were diluted 20 fold prior to SDS-PAGE. (C) Left: secreted protein fractions derived from ΔescNΔescU double mutant strains with the indicated plasmids. Right: Immunoblot analysis of secreted protein fractions. DnaK, an abundant non-secreted cytoplasmic protein, was used as a gel loading control (when needed) or to assess cytoplasmic contamination of secreted fractions or non-specific bacterial lysis. All samples were diluted 20 fold as in panel B. All experiments within Angiogenesis inhibitor the panels were performed twice and representative images are shown. To further characterize these strains, the respective culture supernatant fractions were evaluated. Under these growth conditions, four predominant protein

species are routinely detected in secretion fractions and have been identified using protein micro-sequencing [36]. These include EspA (predicted molecular mass of 20.5 kDa, filamentous translocon protein [37], EspB (predicted molecular mass of 33 kDa, YopD orthologue), EspD (predicted molecular not mass of 39.5 kDa, YopB orthologue) and EspC (predicted molecular mass 140 kDa, secreted by the type V secretion pathway). In contrast, low amounts of Tir and other type III effectors are secreted under these conditions but can be detected using immunoblotting approaches. As expected, ΔescU expressing EscU-HIS restored EspA, EspB and Tir protein secretion back to wild type EPEC levels (Figure 1B). ΔescU expressing either EscU(N262A) or EscU(P263A) had visibly lower amounts of protein species in their respective secretory profiles, however,

a notable ~30kDa protein species was detected by Coomassie staining and could represent low levels of either EspB or EspD (predicted molecular masses of 33 and 39.6 kDa respectively). Immunoblotting with anti-EspA, anti-EspB and anti-Tir antibodies demonstrated reduced levels of EspA (~20%), EspB (~20%) and Tir (~70%) from ΔescU bacteria expressing either EscU(N262A) or EscU(P263A) relative to EscU (as determined by densitometric analyses). Immunoblotting the whole cell lysates of these strains demonstrated equal steady state amounts of EspA, EspB and Tir were present, ruling out the possibility of intracellular protein expression differences. Immunoblotting the same whole cell lysate samples with anti-EscC and anti-EscJ antibodies revealed equal amounts of the type III secretion apparatus ring forming proteins EscC and EscJ.

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