The type III secretion system of Gram-negative bacteria is used to transport virulence factors from the pathogen directly into the host cell  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 . In turn, the transcription of these moieties is thought to be regulated by another gene, lcrV, found on the Yops virulon that encodes the entire type III system . The product of this gene, LcrV protein, also regulates the secretion of YopD through the type III translocon , and itself acts as a protective "V" antigen for Yersinia pestis, the causative agent of plague .
Recently, a homologue of the Y. pestis LcrV protein (PcrV) was found in Pseudomonas aeruginosa, an opportunistic pathogen. In vivo studies using mice found that immunisation with the protein protected burned animals from infection by P. aeruginosa, and enhanced survival. In addition, it is speculated that PcrV determines the size of the needle pore for type III secreted effectors. .