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SummaryGram-negative bacteria use type III secretion (TTS) systems to translocate proteins into the extracellular environment or directly into eukaryotic cells. These complex secretory systems are assembled from over 20 different structural proteins, including 10 that have counterparts in the flagellar export pathway. Secretion substrates are directed to the TTS machinery via mRNA and/or amino acid secretion signals. TTS chaperones bind to select secretion substrates and assist in the export process. Recent progress in the understanding of TTS is reviewed.

Human pathogenic yersiniae organisms export and translocate the Yop virulence proteins and V antigen upon contact with a eukaryotic cell. Yersinia pestis mutants defective for production of YscX or YscY were unable to export the Yops and V antigen. YscX and YscY were both present in the Y. pestis cell pellet fraction; however, YscX was also found in the culture supernatant. YscY showed structural and amino acid sequence similarities to the Syc family of proteins. YscY specifically recognized and bound to a region of YscX that included a predicted coiled-coil region. These data suggest that YscY may function as a chaperone for YscX in Y. pestis.Upon contact with the surface of a eukaryotic cell, Yersinia species pathogenic for humans export a distinct set of plasmidencoded virulence proteins called Yops (for review, see references 26 and 36). Yop proteins are translocated directly from the cytoplasm of the bacterium into the cytoplasm of the host cell (12,57,62). Once translocated into the eukaryotic cell, Yop proteins function to disrupt intracellular signaling pathways (YopH [7,14,48,49] (YopJ [38,39]). This capability enables the yersiniae to avoid phagocytosis and ensures survival of the bacteria within host tissues (14,15,17,54,55,65).Yops are secreted across the bacterial inner and outer membranes by a type III or "contact-dependent" secretion mechanism (36). Maximal expression and secretion of Yops in vitro occurs at 37°C in medium lacking calcium. Genes encoding the Yersinia type III secretion apparatus (11,25,47) are clustered within several large transcriptional units which include yscBC DEFGHIJKLM (21, 32, 35, 52), yscNOPQRSTU (6, 16), yscW (2, 32), and yopNtyeAsycNyscXYVlcrR (8,13,18,28,29,51). Mutational inactivation of any of the ysc genes (with the exception of yscB and yscH) prevents Yop secretion (2,3,6,28,35,36,51,52 [41,64]). The targeting of proteins for export through the type III secretion apparatus has been shown to involve one or both of two identified secretion targeting signals (4, 9). One secretion signal has been identified within the sequences encoding the initial 15 amino acid residues of YopE, YopN (4), and YopK (YopQ [5]). This signal appears to be encoded in the mRNA sequence rather than being part of the peptide sequence, suggesting a cotranslational mechanism of Yop secretion. A second secretion signal is dependent upon the interaction of the secreted protein with an accessory protein termed a specific Yop chaperone (Syc) (9, 68). The Syc proteins are small cytoplasmic proteins that specifically recognize an amino-terminal region of one or two specific Yops (69, 71). Syc proteins for five Yops have been identified: SycE (68) for YopE, SycH (69, 71) for YopH, SycT (27) for YopT, SycD (40, 69) (LcrH) for YopB and YopD, and SycN (13, 28) and YscB (30) for YopN. The YopE protein contains both an mRNA targeting signal and a Syc-dependent targeting signal (4, 9). Either secretion signal can apparently function independently, although somewhat inefficiently, to target YopE for secre...

YopN is a secreted protein that prior to secretion directly interacts with the cytosolic SycN/YscB chaperone complex and TyeA. This study identifies a secreted YopN-TyeA hybrid protein that is expressed by Yersinia pestis, but not by Yersinia enterocolitica. DNA sequence analysis and site-directed mutagenesis studies demonstrate that the hybrid protein is the result of a ؉1 translational frameshift event.