I've written about two proteins: the EGF receptor and Ras. The former reacts to growth factors by dimerizing and transferring phosphate groups to one another's tails. The latter becomes activated by binding GTP, thereby initiating a cascade of protein kinase reactions leading to phosphorylation of transcription factors in the nucleus. How are these two related? Or more specifically, how does the positioning of phosphate groups on the EGF receptor lead to activation of Ras?
The answer to these questions came, peculiarly enough, from a series of studies on eye development in fruit flies. The story is too long to relate here, but the upshot was the discovery of two proteins that bound to the tail of the EGF receptor. The binding changed their position in the cell and allowed them to interact with the Ras protein, specifically catalyzing the transfer of a phosphate group to GDP turning it to GTP, and thereby activating its kinase ability.
In the course of these and later investigations, an important and previously unknown property of proteins was discovered:There are domains in proteins whose function is to recognize and bind to specific sequences of amino acids and thereby position one protein near another. Two such domains are called "SH2" and "SH3" (SH stands for "Src homology" because the domains were first discovered in the protein encoded by the Src oncogene). Since their initial identification many similar protein binding domains have been found but I'll focus on SH2 and SH3.
SH2 is a sequence of about 100 amino acids. When present in a protein, it folds up into a structure that can bind to a section of a protein carrying one phosphorylated tyrosine followed by a sequence of three specific amino acids. There are over 100 such SH2 domains found in the human protein set, each capable of binding to a different sequence of amino acids (but all requiring a phospho-tyrosine at one end of its recognition site). A protein called "Grb2" possesses a single SH2 domain. It uses this domain to grab on to the tail of an EGF-receptor, recognizing a specific phospho-tyrosine containing sequence. It's important to know that Grb2 possess no enzymatic activity. But it does carry, in addition to its single SH2 domain, two SH3 domains. These are about half the size of SH2's. They recognize and bind to a short protein sequence that includes the amino acid proline.
The sequence of events that occur after the EGF-receptor binds the EGF growth factor should be clearer if you examine the figure above. First, one or more of the tyrosine amino acids in the receptor tail become phosphorylated via the kinase domain of the receptor. Next,using its SH2 domain, the Grb2 protein recognizes one of these phosphotyrosines and its nearby amino acids. Since the Grb2 protein also possesses two SH3 domains, it can serve as a bridge to bind the Sos protein, which, of course, has two proline rich sites capable of being bound. Finally, Sos, which is a guanine nucleotide exchange enzyme, is now in position to catalyze the removal of GDP from the Ras protein. GTP, present in much greater abundance than GDP, takes its place and Ras becomes active and promotes the kinase cascade described in the last post.
I don't know about you, but I find these complex cascades of reactions overwhelming. But the example that I illustrated is only one of a larger number of parallel signalling pathways operating to promote growth. Ras itself is involved in two other cascades, each of which triggers another series of reactions. One of these, called PI3K, instigates a series of reactions that increases cell proliferation while also lessening cell death via apoptosis.
And that's not all. We know about additional pathways with names such as "Jak-STAT", "Wnt-Beta-Catenin", and "Notch". And there undoubtedly others that are unknown. For example, most breast cancers grow in response to estrogen. There is evidence that estrogen activates the Ras-Raf-MEK-Eft1 pathway described here and in the last post, but no one seems to understand the molecular mechanism involved despite the fact that breast cancer has been intensively studied for many years.
The last two posts have been about oncogenes - genes that promote proliferation by stimulating growth and inhibiting cell death. In the next post, I'll begin a discussion of tumor suppressors, genes that act as brakes for cell growth.