An amino-terminal secretion signal is required for Yp1A export by the Ysa, Ysc, and flagellar type III secretion systems of Yersinia enterocolitica biovar 1B

Yersinia enterocolitica biovar 1B maintains three distinct type III secretion (TTS) systems, which independently operate to target proteins to extracellular sites. The Ysa and Ysc systems are prototypical contact-dependent TTS systems that translocate toxic effectors to the cytosols of targeted eukaryotic host cells during infection. The flagellar TTS system is utilized during the assembly of the flagellum and is required for secretion of the virulence-associated phospholipase YpIA to the bacterial milieu. When ectopically produced, YpIA is also a secretion substrate for the Ysa and Ysc TTS systems. In this study, we define elements that allow YpIA recognition and export by the Ysa, Ysc, and flagellar TTS systems. Fusion of various amino-terminal regions of YpIA to Escherichia coli alkaline phosphatase (PhoA) lacking its native secretion signal demonstrated that the first 20 amino acids or corresponding mRNA codons of YpIA were sufficient for export of YpIA-PhoA chimeras by each TTS system. Export of native YpIA by each of the three TTS systems was also found to depend on the integrity of its amino terminus. Introduction of a frameshift mutation or deletion of yplA sequences encoding the amino-terminal 20 residues negatively impacted YptA secretion. Deletion of other ypl4 regions was tolerated, including that resulting in the removal of amino acid residues 30 through 40 of the polypeptide and removal of the 5' untranslated region of the mRNA. This work supports a model in which independent and distantly related TTS systems of Y. enterocolitica recognize protein substrates by a similar mechanism.