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Catalytic domain of the Protein Tyrosine Kinase, HER2 PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. HER2 (ErbB2, HER2/neu) is a member of the EGFR (HER, ErbB) subfamily of proteins, which are receptor PTKs (RTKs) containing an extracellular EGF-related ligand-binding region, a transmembrane helix, and a cytoplasmic region with a tyr kinase domain and a regulatory C-terminal tail. Unlike other PTKs, phosphorylation of the activation loop of EGFR proteins is not critical to their activation. Instead, they are activated by ligand-induced dimerization, leading to the phosphorylation of tyr residues in the C-terminal tail, which serve as binding sites for downstream signaling molecules. HER2 does not bind to any known EGFR subfamily ligands, but contributes to the kinase activity of all possible heterodimers. It acts as the preferred partner of other ligand-bound EGFR proteins and functions as a signal amplifier, with the HER2-HER3 heterodimer being the most potent pair in mitogenic signaling. HER2 plays an important role in cell development, proliferation, survival and motility. Overexpression of HER2 results in its activation and downstream signaling, even in the absence of ligand. HER2 overexpression, mainly due to gene amplification, has been shown in a variety of human cancers. Its role in breast cancer is especially well-documented. HER2 is up-regulated in about 25% of breast tumors and is associated with increases in tumor aggressiveness, recurrence and mortality. HER2 is a target for monoclonal antibodies and small molecule inhibitors, which are being developed as treatments for cancer. The first humanized antibody approved for clinical use is Trastuzumab (Herceptin), which is being used in combination with other therapies to improve the survival rates of patients with HER2-overexpressing breast cancer. The HER2 subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase. |
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