Type III secretion system, low calcium response, chaperone LcrH/SycD, subgroup (IPR016379)

Short name: T3SS_Ca_resp_chp_LcrH/SycD_sub

Overlapping homologous superfamilies

Family relationships


The type III secretion system of Gram-negative bacteria is used to transport virulence factors from the pathogen directly into the host cell [PMID: 9618447] and is only triggered when the bacterium comes into close contact with the host. Effector proteins secreted by the type III system do not possess a secretion signal, and are considered unique because of this. Yersinia spp. secrete effector proteins called YopB and YopD that facilitate the spread of other translocated proteins through the type III needle and the host cell cytoplasm [PMID: 9440524]. Both are believed to act as pore translocases, forming apertures in the host cell membrane and allowing the bacterium easy access to its cytoplasm. YopD also acts as a negative regulator of the Yersinia low-calcium response, and in turn is controlled by a chaperone, SycD [PMID: 7937981]. This protein also regulates YopB secretion. SycD is located on the Yop pathogenicity island of Yersinia spp., and is speculated to prevent a premature interaction between YopB, YopD and the calcium-response LcrV protein [PMID: 10476031].

It has been speculated that a type III secretion mechanism also exists in Chlamydial species. With the sequencing of the Chlamydia trachomatis genome, several proteins similar to characterised type III proteins have emerged, including a SycD homologue [PMID: 11123678]. The Pseudomonas aeruginosa gene PcrH is also similar to the Yersinia chaperone, suggesting a comparable function.

Proteins in this entry are found in type III secretion operons. LcrH, from Yersinia is believed to have a regulatory function in the low-calcium response of the secretion system [PMID: 1705541,PMID: 11844757]. The same protein is also known as SycD (SYC = Specific Yop Chaperone) for its chaperone role [PMID: 9987117]. In Pseudomonas, where the homologue is known as PcrH, the chaperone role has been demonstrated and the regulatory role appears to be absent [PMID: 14673772]. ScyD/LcrH contains three central tetratricopeptide-like repeats that are predicted to fold into an all-alpha-helical array [PMID: 12799000].

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.