class I type III secretion system (T3SS) chaperones and similar proteinsThis family contains class I type III secretion system (T3SS) chaperones mainly found in Gram-negative bacteria such as Pseudomonas, Yersinia, Salmonella, Escherichia and Erwinia, among others. A wide variety of these bacterial pathogens and symbionts require a T3SS to inject eukaryotic host cells with effector proteins important for suppressing host defenses and establishing infection. Many of these effector proteins interact with specific type III secretion chaperones prior to secretion. These T3SS chaperones have been classified as class I type III secretion chaperones (T3SC), which are small structurally conserved dimers that interact specifically with T3SS effector proteins. Class I T3SC consists of two subclasses: IA and IB. Class IA T3SC binds a single effector, whereas class IB T3SC binds to several effectors. Class IA and Class IB T3SCs typically exhibit little sequence similarities, but share a common overall heart-shaped structure fold (alpha-beta-beta-beta-alpha-beta-beta-alpha) and features, such as a small size, an acidic pI and an amphipathic C-terminal alpha-helix. Chaperone protein CesT serves a chaperone function for the enteropathogenic Escherichia coli (EPEC) translocated intimin receptor (Tir) protein, which confers upon EPEC the ability to alter host cell morphology following intimate bacterial attachment. In Pseudomonas aeruginosa, chaperone ExsC binds small secreted protein ExsE as well as the non-secreted anti-activator protein ExsD; it relieves repression of the transcriptional activator ExsA (which activates expression of T3SS genes) by ExsD. P. aeruginosa SpcU binds the cytotoxin ExoU, which is a broad-specificity phospholipase A2 (PLA2) and lysophospholipase, and maintains the N-terminus of ExoU in an unfolded state which is required for secretion. Salmonella enterica chaperone SicP forms a complex with effector protein SptP at an early stage of its secretion process in order to avoid premature degradation, while chaperone SigE binds to effector SigD, which, upon translocation into the host cell, preferentially dephosphorylates specific inositol phospholipids that are thought to be crucial for subsequent activation of the host cell Ser-Thr kinase Akt. This family also includes Yersinia chaperone/escortee pairs SycE/YopE, SycH/YopH, SycT/YopT and SycN+YscB/YopN, all of which bind to specific Yersinia outer proteins (Yops). Also included are several DspF and related sequences from several plant pathogenic bacteria. The "disease-specific" (dsp) region next to the hrp gene cluster of Erwinia amylovora is required for pathogenicity but not for elicitation of the hypersensitive reaction. In addition, a group of proteins including Escherichia coli YbjN, Erwinia amylovora AmyR, and their homologs, are included in this family. They share a class I T3SC-like fold with T3SS chaperone proteins but appear to function independently of the T3SS.