receptor tyrosine kinase
/ RTK; TIE-ro-seen KY-nase /
Some receptors do not just hand a message to a helper — they spring into action and start tagging things themselves. A receptor tyrosine kinase, or RTK, is a surface receptor that is also an enzyme. A kinase is a protein that attaches phosphate groups onto other proteins (a chemical tag that switches them on or off), and an RTK specifically tags the amino acid tyrosine. So an RTK is a receptor that, when it catches its ligand, becomes a working enzyme and starts stamping phosphate tags — first onto itself, then onto the relay proteins that gather around it.
The mechanism has a clever twist that distinguishes it from GPCRs. Each RTK sits alone in the membrane doing nothing. When a growth factor binds, it brings two receptor molecules together side by side — they pair up, or dimerize. Once paired, each receptor's inner kinase region reaches over and phosphorylates the other (called cross- or trans-phosphorylation), so the partners switch each other on. Those new phosphate-tyrosine tags become docking sites: relay proteins inside the cell recognize and grab onto them, assembling a signaling complex right at the membrane. From there the message fans out — most famously to the Ras protein and the MAP-kinase cascade that tells a cell to grow and divide, and to the PI3K-Akt pathway that promotes survival. So the logic is: ligand brings two receptors together, they activate each other, and the phosphate tags they create become a launchpad for the relay.
RTKs matter because they are the cell's growth and survival receptors — the insulin receptor, the epidermal growth factor receptor (EGFR), and many others are RTKs. That power is also their danger. Because RTKs tell cells to divide, mutations that jam an RTK in the on position (so it signals even with no ligand) are a frequent cause of cancer; EGFR and HER2 are famous examples. This makes RTKs prime drug targets, and several blockbuster cancer drugs are kinase inhibitors or antibodies that shut a hyperactive RTK down.
The drug trastuzumab (Herceptin) treats breast cancers driven by an over-abundant RTK called HER2. The antibody binds HER2 on the cancer-cell surface and helps shut down its relentless grow-and-divide signal — a textbook case of targeting an RTK that has gone into overdrive.
HER2 is an RTK; the cancer drug Herceptin works by blocking it — showing why these growth receptors are key drug targets.
Do not confuse the two phosphate-handling roles. A kinase adds phosphate (switching things on); a phosphatase removes it (switching them off). An RTK is a kinase, and its tags get erased later by phosphatases — without that erasing, the signal could never turn off.