Type IV secretion systems are multiprotein complexes that mediate the translocation of macromolecules (proteins, DNA or DNA-protein complexes) across the bacterial cell envelope into the extracellular medium or directly into recipient cells. This strategy is exploited for the delivery of effector molecules that modulate host cell interactions by bacterial pathogens and symbionts. Type IV secretion systems also mediate the translocation of DNA molecules from bacteria and the uptake of DNA into bacteria and thereby contribute to horizontal gene transfer.

The term type IV secretion system (T4SS) was originally based on the significant sequence similarities between the protein components of macromolecular transporters used for plasmid transfer and for the delivery of virulence factors from bacterial pathogens to their hosts. The two first examples that prompted the proposal of T4SSs as a distinct family of macromolecular transporters were the conjugative plasmid RP4 transfer machinery (Trb) and the VirB/D4 machinery used by the plant pathogen Agrobacterium tumefaciens for the delivery of single-stranded DNA and of protein virulence factors to plants.

T4SSs carry out one of three functions. The first group of T4SSs translocates exclusively DNA and serves for the uptake or secretion of genetic information. The second group translocates DNA molecules as well as proteins from donor to recipient cells. The third group exclusively translocates proteins and those are either secreted into the exterior medium or directly into recipient cells.

Since the original proposal that T4SS constitute a distinct class of secretion systems was made, the number of known T4SS has continued to grow and both metagenomic as well as functional analyses continue to add new examples. Since many T4SSs localize on plasmids they can be transmitted by horizontal gene transfer and this has very interesting implications for their evolution.

from Christian Baron in Bacterial Secreted Proteins

Further reading:

1. Bacterial Secreted Proteins: Secretory Mechanisms and Role in Pathogenesis
2. Microbiology Books

Labels: bacteria, protein, protein secretion, proteins