Helicobacter pylori virulence factor CagA (cytotoxin-associated gene A) is a 120–145kDa protein encoded on the 40kb cag pathogenicity island (PAI).[1] H. pylori strains can be divided into CagA positive or negative strains. Around 60% of H. pylori isolates in Western countries are positive, whereas the majority of East Asian isolates are positive.[1]
The cag PAI also encodes for a type 4 secretion system which is used to "inject" CagA into a target cell upon H. pylori attachment. After translocation, CagA localises to the inner surface of the cell membrane and undergoes tyrosine phosphorylation by Src family kinases (e.g. Fyn and Lyn)chemical intractability of the virulence factor has precluded significant mechanistic studies.