This section explains how the protein produced by the K-ras gene is a tumor “activator.”

K-ras The protein produced by the K-ras gene is a tumor “activator.” K-ras is analogous to a car accelerator, because its overactivity contributes to tumor development. The K-ras protein resides on the inner side of the cell membrane, where it conducts growth signals from cell-surface receptors to the nucleus. This process is called signal transduction. Signal transduction begins with the arrival of a growth factor at the cell surface, where it recognizes a specific receptor anchored in the cell membrane. The binding of the growth factor to its receptor conducts a growth signal into the cell interior. The K-ras protein accepts the growth signal and, in turn, relays it to other molecules in the cytoplasm. Raf and other signal transducers are protein kinases, which activate other molecules by adding phosphate groups. This signaling cascade culminates in the nucleus with the activation of Fos and Jun, two transcription factors that join together to initiate transcription of genes involved in cell replication. Mutations in the K-ras gene result in a K-ras protein that is essentially stuck in an “on” position – perpetuating a signaling cascade in the absence of any real signal from a growth factor.