page_title: Type three secretion system
section_title: T3SS proteins
hierarchical_section_title: Type three secretion system / T3SS proteins
caption_attribution_description: English: Cut-away view and description of individual substructures of the needle complex from Salmonella typhimurium. (OR = outer ring; IR = inner ring). The black line indicates 10 nanometers.
context_page_description: Type three secretion system is a protein appendage found in several Gram-negative bacteria.
  In pathogenic bacteria, the needle-like structure is used as a sensory probe to detect the presence of eukaryotic organisms and secrete proteins that help the bacteria infect them. The secreted effector proteins are secreted directly from the bacterial cell into the eukaryotic cell, where they exert a number of effects that help the pathogen to survive and to escape an immune response.
context_section_description: The T3SS proteins can be grouped into three categories:
  Structural proteins: build the base, the inner rod and the needle.
  Effector proteins: get secreted into the host cell and promote infection / suppress host cell defences.
  Chaperones: bind effectors in the bacterial cytoplasm, protect them from aggregation and degradation and direct them towards the needle complex.
  Most T3SS genes are laid out in operons. These operons are located on the bacterial chromosome in some species and on a dedicated plasmid in other species. Salmonella, for instance, has a chromosomal region in which most T3SS genes are gathered, the so-called Salmonella pathogenicity island (SPI). Shigella, on the other hand, has a large virulence plasmid on which all T3SS genes reside. It is important to note that many pathogenicity islands and plasmids contain elements that allow for frequent horizontal gene transfer of the island/plasmid to a new species.
  Effector proteins that are to be secreted through the needle need to be recognized by the system, since they float in the cytoplasm together with thousands of other proteins. Recognition is done through a secretion signal—a short sequence of amino acids located at the beginning (the N-terminus) of the protein (usually within the first 20 amino acids), that the needle complex is able to recognize. Unlike other secretion systems, the secretion signal of T3SS proteins is never cleaved off the protein.
caption: Diagram of individual substructures of the needle complex from Salmonella typhimurium
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