• CagA (Cytotoxin-Associated Gene A) is a protein encoded by the cagA gene, which is found in specific strains of Helicobacter pylori. These strains are referred to as CagA-positive Helicobacter pylori or Type I Helicobacter pylori strains.
• CagA-positive strains of H. pylori are linked to more severe clinical outcomes, including peptic ulcer disease, gastric adenocarcinoma (a type of stomach cancer) and gastric mucosa-associated lymphoid tissue (MALT) lymphoma.
• CagA is the first identified bacterial-derived oncoprotein.
• CagA was initially identified as a gene associated with the cytotoxic activity of Helicobacter pylori and was believed to mediate the vacuole-inducing phenotype observed in HeLa cells (Cover et al., 1990). However, subsequent studies revealed that the vacuolating cytotoxin activity persisted in H. pylori mutants lacking the cagA gene (Tummuru et al., 1994), thereby indicating that CagA is not responsible for vacuole formation. This activity was later attributed to a distinct virulence factor, VacA.
  
• The CagA gene was successfully cloned in 1993 (Tummuru et al., 1993).  It is located on a specific genomic region known as the cag Pathogenicity Island (CagPAI) (Censini et al., 1996, Akopyants et al., 1998), a ~40-kb DNA segment acquired via horizontal gene transfer that is strongly associated with increased virulence.
• The cagA gene, typically 3,200 to 3,800 base pairs long, encodes a protein with a molecular weight of approximately 120 to 140 kDa. This weight can vary slightly between strains, primarily due to differences in the gene’s 3′ region (Chattopadhyay et al., 2012). 
• A key feature of the C-terminal region of the CagA protein is the presence of repeated EPIYA motifs (Glu-Pro-Ile-Tyr-Ala), which contribute to variations in the protein’s molecular weight – ranging from 120 to 145 kDa.
• CagA is delivered into host gastric epithelial cells via the Type IV Secretion System (T4SS) encoded by genes in the cagPAI (Cover et al., 2020). Upon translocation into host cells, CagA localizes to the inner surface of the plasma membrane. 
• Inside the host cell, CagA undergoes tyrosine phosphorylation at specific EPIYA motifs by host cell non-receptor tyrosine kinases, particularly Src family kinases (e.g., Src, Fyn, Lyn) and Abl family kinases (e.g., Abl1) (Stein et al., 2002; Backert & Selbach, 2008).
• These motifs, classified into types EPIYA-A, -B, -C, and -D based on their flanking sequences, are differentially phosphorylated depending on the bacterial strain and influence CagA’s interaction with host signaling molecules. These phosphorylation sites are critical for many of CagA’s downstream effects. 
  
• Phosphorylated CagA acts as a molecular scaffold, with its EPIYA motifs serving as docking sites for SH2 domain-containing signaling proteins, thereby facilitating the assembly of aberrant signaling complexes.
• Phosphorylated CagA interacts with host factors such as SHP-2 (Src homology 2 domain-containing phosphatase 2), leading to its aberrant activation. This interaction promotes cytoskeletal rearrangements, cell elongation, and disruption of cell–cell junctions, contributing to cellular transformation and pro-oncogenic signaling (Higashi et al., 2002; Hatakeyama, 2004). 
• The interaction of phosphorylated CagA with the host phosphatase SHP-2 has been shown to be necessary for the induction of cell elongation, the so-called “hummingbird” phenotype, in the AGS gastric epithelial cell line, an in vitro model used to study CagA’s effects on cellular transformation.
• Importantly, CagA also exerts significant biological activity independent of its phosphorylation status. It can interact with tight junction proteins (e.g., ZO-1, JAM-A), E-cadherin, and the polarity regulator PAR1b/MARK2, leading to loss of epithelial polarity, barrier dysfunction, and dysregulation of cell adhesion and polarity pathways (Amieva et al., 2003; Saadat et al., 2007).
• CagA can deregulate several host signaling pathways including MAPK/ERK, PI3K/AKT, and Wnt/β-catenin signalling pathways. 
• Through these phosphorylation-dependent and -independent mechanisms, CagA acts as a multifunctional effector, profoundly altering host cell signaling and contributing to chronic inflammation, tissue damage, and ultimately, gastric carcinogenesis.
  
• Abbreviations 
  • CagA (Cytotoxin-Associated Gene A)
    

REFERENCES

• Amieva et al., 2003. Disruption of the epithelial apical-junctional complex by Helicobacter pylori CagA. Science. 300(5624), 1430-4. PMID-12775840; Full Text: Science, PMC  
• Chattopadhyay et al., 2012. Distinct repeat motifs at the C-terminal region of CagA of Helicobacter pylori strains isolated from diseased patients and asymptomatic individuals in West Bengal, India. Gut Pathog. 4(1): 4. PMID-22631862; Full Text: biomedcentral, PMC 
• Cover et al., 1990. Characterization of and human serologic response to proteins in Helicobacter pylori broth culture supernatants with vacuolizing cytotoxin activity. Infect Immun. 58(3), 603-610. PMID-2307514; Full-Text Links: asm, PMC 
• Cover et al., 2020. The Helicobacter pylori Cag Type IV Secretion System. Trends Microbiol. 28(8), 682-695. PMID-32451226; Full Text: Cell Press, PMC  
• Hatakeyama, 2004. Oncogenic mechanisms of the Helicobacter pylori CagA protein. Nat Rev Cancer. 4(9), 688-94. PMID-15343275; Full Text: Nature
• Higashi et al., 2002. Biological activity of the Helicobacter pylori virulence factor CagA is determined by variation in the tyrosine phosphorylation sites. Proc Natl Acad Sci U S A. 99(22), 14428-33. PMID-12391297; Full Text: PNAS, PMC
• Saadat et al., 2007. Helicobacter pylori CagA targets PAR1/MARK kinase to disrupt epithelial cell polarity. Nature. 447(7142), 330-3. PMID-17507984; Full Text: Nature
• Tummuru et al., 1993. Cloning and expression of a high-molecular-mass major antigen of Helicobacter pylori: evidence of linkage to cytotoxin production. Infect Immun. 61(5), 1799-809. PMID-8478069; Full-Text Links: asm, PMC280768
• Tummuru et al., 1994. Mutation of the cytotoxin-associated cagA gene does not affect the vacuolating cytotoxin activity of Helicobacter pylori. Infect Immun. 62(6), 2609-13. PMID-8188385; Full-Text Links: asm, PMC186552