InvB is a type III secretion chaperone specific for SspA

A wide variety of gram-negative bacteria utilize a specialized apparatus ca lled the type III secretion system (TTSS) to translocate virulence factors directly into the cytoplasm of eukaryotic cells. These translocated effecte rs contribute to the pathogen's ability to infect and replicate within plan t and animal hosts. The amino terminus of effector proteins contains sequen ces that are necessary and sufficient for both secretion and translocation by TTSS. Portions of these sequences contain binding sites for type III cha perones, which facilitate efficient secretion and translocation of specific effecters through TTSS. In this study, we have utilized the yeast two-hybr id assay to identify protein-protein interactions between effector and chap erone proteins encoded within Salmonella pathogenicity island 1 (SPI-1). Se veral interactions were identified including a novel interaction between th e effector protein, SspA (SipA), and a putative chaperone, InvB, InvB was d emonstrated to bind to the amino terminus of SspA in the bacterial cytoplas m. Furthermore, InvB acts as a type III chaperone for the efficient secreti on and translocation of SspA by SPI-1. InvB also permitted translocation of SspA through the SPI-2 TTSS, indicating that it is an important regulator in the recognition of SspA as a target of TTSS. Finally, it was determined that InvB does not alter the transcription of sspA but that its absence res ults in reduced SspA protein levels in Salmonella enterica serovar Typhimur ium.