Characterization of translocon proteins in the type III secretion system of Lawsonia intracellularis

Lawsonia intracellularis, the etiologic agent of proliferative enteropathy (PE), is an obligate intracellular Gram-negative bacterium possessing a type III secretion system (T3SS), which enables the pathogen to translocate effector proteins into targeted host cells to modulate their functions. T3SS is a syringe-like apparatus consisting of a base, an extracellular needle, a tip, and a translocon. The translocon proteins assembled by two hydrophobic membrane proteins can form pores in the host-cell membrane, and therefore play an essential role in the function of T3SS. To date, little is known about the T3SS and translocon proteins of L. intracellularis. In this study, we first analyzed the conservation of the T3S apparatus between L. intracellularis and Yersinia, and characterized the putative T3S hydrophobic major translocon protein LI1158 and minor translocon protein LI1159 in the L. intracellularis genome. Then, by using Yersinia pseudotuberculosis as a surrogate system, we found that the full-length LI1158 and LI1159 proteins, but not the putative class II chaperone LI1157, were secreted in a − Ca(2+) and T3SS-dependent manner and the secretion signal was located at the N terminus (aa 1–40). Furthermore, yeast-two hybrid experiments revealed that LI1158 and LI1159 could self-interact, and LI1159 could interact with LI1157. However, unlike CPn0809 and YopB, which are the major hydrophobic translocon proteins of the T3SS of C. pneumoniae and Yersinia, respectively, full-length LI1158 was non-toxic to both yeast and Escherichia coli cells, but full-length LI1159 showed certain toxicity to E. coli cells. Taken together, despite some differences from the findings in other bacteria, our results demonstrate that LI1158 and LI1159 may be the translocon proteins of L. intracellularis T3SS, and probably play important roles in the translocation of effector proteins at the early pathogen infection stage.