Figure 4

EscU auto-cleavage is required for efficient and stable effector secretion in an EPEC Δ sepD genetic background. (A) Left: Trans-complementation of ΔsepDΔescU with pJLT21 restored secretion of effectors to ΔsepD levels while ΔsepDΔescU/pJLT22 did not restore normal effector secretion. ΔsepDΔescU/pJLT23 secreted a protein with an apparent molecular mass similar to Tir (asterisk). The dominant effector proteins are labelled and have been previously identified using mass spectrometry analyses [35]. Purified BSA was added to collected secreted fractions and served to aid in protein precipitation. Right: genomic integration of mutant escU alleles (cis-complementation, single copy) produces the same secretion phenotypes as the plasmid trans-complemented escU strains. Total secreted proteins were visualized by Coomassie G-250 staining. (B) Secreted protein preparations were analyzed by immunoblot with anti-Tir antibodies. Due to the abundance of secreted Tir in ΔsepD and ΔsepDΔescU/pJLT21, (see Coomassie stain in panel A), only these samples were diluted 20 fold for immunoblotting purposes while the others were undiluted. A ΔsepDΔtir strain (undiluted) was included to show the specificity of the anti-Tir antibodies. Lower molecular weight protein species are therefore Tir breakdown products that were consistently observed and recognized by the anti-Tir antibodies. A novel Tir polypeptide, indicated by an arrow, was exclusively detected in the lane containing secreted proteins derived from ΔsepDΔescU/pJLT22.