Regulation of Type III Secretion of Translocon and Effector Proteins by the EsaB/EsaL/EsaM Complex in Edwardsiella tarda

Abstract

The type III secretion system (T3SS) plays a crucial role in the pathogenesis of many Gram-negative bacteria, including <i>Edwardsiella tarda</i>, an important fish pathogen. Within the <i>E. tarda</i> T3SS, there are three proteins (EsaB/EsaL/EsaM) that are homologous to proteins present in many other bacteria, including SpiC/SsaL/SsaM in <i>Salmonella</i>, SepD/SepL/CesL in enteropathogenic <i>Escherichia coli</i> (EPEC) and enterohemorrhagic <i>E. coli</i> (EHEC), and YscB/YopN/SycN in <i>Yersinia</i> EsaL was found to interact with both EsaB and EsaM within the bacterial cell, as revealed by a coimmunoprecipitation assay. Moreover, EsaM is required for EsaB stability, and the two proteins interact with each other. EsaB, EsaL, and EsaM are all indispensable for the secretion of the T3SS translocon protein EseC into supernatants under pH 5.5 and pH 7.2 conditions. Unlike EseC, EseG is a T3SS effector whose secretion is suppressed by EsaL at pH 7.2 while it is promoted at pH 5.5 condition. Despite this finding, mutant strains lacking EsaB, EsaL, or EsaM (i.e., the Δ<i>esaB</i>, Δ<i>esaL</i>, or Δ<i>esaM</i> strain, respectively) were all outcompeted by wild-type <i>E. tarda</i> during a coinfection model. These results demonstrate that EsaB/EsaL/EsaM form a ternary complex controlling the secretion of T3SS translocon and effector proteins and contributing to <i>E. tarda</i> pathogenesis.