JAK-STAT pathway
The JAK-STAT pathway is a remarkably short, direct line from the cell surface to the genes. Imagine a phone that, when picked up at the front door, sends a runner straight to the cell's library to pull the right books — no long relay needed.
Some receptors have no enzyme activity of their own, so they borrow a partner kinase called JAK (Janus kinase) that sits just inside the membrane. When the hormone binds and brings receptor molecules together, the JAKs phosphorylate each other and then phosphorylate proteins called STATs. The activated STATs pair up, travel into the nucleus, and act directly as transcription factors, switching target genes on.
This pathway is how growth hormone, prolactin, leptin, and erythropoietin deliver many of their messages. Its directness makes it efficient, but the same simplicity is a vulnerability: mutations that keep JAK constantly active drive certain blood disorders and cancers, and JAK inhibitors are now used as medicines.
Growth hormone binds its receptor in the liver, activating JAK-STAT signaling that turns on the gene for IGF-1, the mediator of much of growth hormone's action.
JAK-STAT linking growth hormone at the surface to gene activation.
Receptors using JAK-STAT are sometimes called cytokine receptors because many immune signaling molecules use the same route; growth hormone and prolactin receptors belong to this family.