In this section learn that the binding of growth factors outside the cell causes receptors ends to intertwine and activate each other, and once active, the modified receptor ends interact with messenger proteins.

The signal is conducted through the cell membrane, into the cytoplasm. The binding of the growth factor outside the cell has caused the ends of the receptor (in gray) to intertwine and activate each other (shown as yellow flashes of light). Once active, the modified receptor ends interact with messenger proteins that will carry the signal through the cytoplasm. Molecules identified: Ras: A protein loosely associated with the inner surface of the cell membrane. The Ras protein binds guanine nucleotides – guanosine diphosphate (GDP) and guanosine triphosphate (GTP). When a stimulatory signal arrives, Ras releases its GDP and acquires a GTP molecule, entering an active state and emitting a signal to another protein. After transmitting the signal, Ras deactivates itself by cleaving a phosphate molecule from the GTP to reduce it to GDP, or another protein called Ras-GAP comes in to break the GTP down. A single amino acid change can alter the function of Ras, causing it to bind GTP but making it unable to deactivate. Ras mutations are prevalent in many human cancers, including colon, skin, and lung. Ras GTPase activating protein (GAP): A regulator of the Ras protein's signaling activity. Ras-GAP stimulates Ras's own weak ability to reduce bound guanosine triphosphate (GTP) to guanosine-diphosphate (GDP), thereby rendering itself inactive. See Ras. Growth factor receptor-bound protein 2 (Grb2): A protein that acts as adaptor molecule between a growth factor receptor and other signaling proteins. In a key signaling pathway, Grb2 binds an active PDGF receptor and activates a guanine-nucleotide exchange factor for the Ras protein. Platelet derived growth factor (PDGF) receptors: PDGF receptors are large membrane-spanning proteins with an extracellular and an intracellular component. Two PDGF receptor proteins "dimerize" to bind a single platelet-derived growth factor. The cytoplasmic portion has kinase activity – able to add phosphate molecules to other molecules to activate them. This receptor can contribute to cancer if rendered active for an extended period of time.

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