In the last post, I used the epidermal growth factor receptor as an example to illustrate how signals outside of cells can traverse the cell membrane and provide information that can be used inside. Recall that as a result of EGF binding two EGF receptors embrace and the close proximity of their internal kinase domains causes one partner to attach phosphate groups on to specific amino acids (tyrosine) to the other. The question I want to get to in this post is: What happens next? How is the addition of phosphates used to provide instructions to the other cell constituents, including the DNA in the nucleus, so that cells can begin to proliferate?
Before addressing these questions, allow me to devote a paragraph to the strategy that scientists took (and are taking) to solve these riddles. In essence, biologists have traditionally used two fundamentally different approaches. One, that associated with the discipline of biochemistry, is to take the individual components suspected of acting in some process out of the milieu in which they ordinarily act, purify them, and study their characteristics in a container at the laboratory bench. Biochemical analysis has the advantage that is can obtain detailed information about a particular protein and its operation. It has the disadvantage that it can miss the big picture. A second strategy, used by scientists who call themselves geneticists, is to induce errors in DNA (mutations) that perturb a process in an organism. By studying the effects of changes in individual genes on how a given process is disturbed, geneticists can often deduce the function of these genes and the proteins that they encode. This second approach requires that the process being studied occurs in an organism that can be genetically manipulated. For that reason, much of the internal signalling pathways that lead to cancer have been elucidated in what are called "model organisms", like yeast, fruit flies, and worms. As a former fruit fly geneticist, I'm proud of the central role that the little creature that I studied has played.
A major contributor to the cell signalling pathway that lies downstream of the growth factor receptor is a gene called "Ras". It was discovered more than 50 years ago in two different viruses that induced rat sarcomas, hence its name. Robert Weinberg, among others, showed that there are three cellular counterparts to the viral genes. And over the years, it has become apparent that the three Ras genes, these proto-oncogenes, play a pivotal role in growth promotion and in cancer progression. In fact, mutations in the cellular version of the Ras gene are found in about a quarter of all cancers.
The Ras protein sits inside the cell at the cell membrane (unlike the EGF receptor, it doesn't traverse the membrane) and exists in two states: on and off. When it binds GTP (guanine triphosphate) it becomes active; when the bound GTP loses a phosphate and become GDP (guanosine diphosphate), it turns inactive (see the figure above for the structure of these two nucleotides). Recall that GTP and GDP are small molecules (see figure above) that are omnipresent in cells, but GTP is usually 10 times more abundant than GDP. The active form of Ras has an affinity for another protein called Raf and when the two bind together, Raf becomes an active protein kinase. It in turn, transfers a phosphate group to a protein called MEK, which becomes an active protein kinase, transferring a phosphate group to yet another protein called Erk1. This Rube Golbergian parade is shown in the accompanying illustration.
But I haven't told you the whole story. Normally, this pathway is shut off when growth factors aren't present. How does Ras become activated? It normally is inactive because it and some other enzymes are continually removing one phosphate from any bound GTP that happens to stray into its grasp. The answer to this question requires a short detour that I'll take in the next post.