In 1911 Peyton Rous, at the Rockefeller Institute, discovered the Rous sarcoma virus; the first virus known to cause solid tumors (1). Although Rous’ eponymous virus also would be known as the prototype retrovirus, his discovery generated only scant interest at the time, and would not be recognized by the Nobel Committed until 65 years later! [Nobel prizes are not awarded posthumously. Fortunately, Rous had longevity on his side. He died 4 years after receiving the prize, at age 87.]
In 1976 Harold Varmus and J. Michael Bishop, then at the University of California San Francisco, discovered that the Rous sarcoma virus oncogene, v-src, as well as the oncogenes of several other tumorgenic retroviruses, actually were derived from cellular genes that normally play an important role in controlling cell division and differentiation (2). Moreover, Varmus and Bishop showed that these cellular “proto-oncogenes” can be altered by mutation, to become “oncogenes” that contribute to cancer. [Varmus and Bishop received the 1989 Nobel Prize in Physiology or Medicine for their discovery of proto-oncogenes.]
But what is the actual activity of the protein coded for by the normal cellular c-src, and by v-src as well? The story of that discovery is rather delightful and begins as follows.
In 1978, Raymond Erikson and coworker Marc Collette, then at the University of Colorado Medical Center, were the first researchers to isolate the Src protein. They accomplished this by first preparing lysates from avian and mammalian cells, which had been transformed in culture into tumor cells by Rous sarcoma virus. Next, they precipitated those lysates with antisera from rabbits that bore Rous sarcoma virus-induced tumors. The premise of their strategy was that antibodies from the tumor-bearing rabbits would recognize and precipitate proteins that were specific to cells transformed by the virus .
With the Src protein now in hand, Ericson and Collette next sought its function. They initially asked whether Src might have protein kinase activity (i.e., an activity that adds a phosphate group to a protein.). This was a reasonable possibility because protein phosphorylation was already known to play a role in regulating various cellular processes, including cell growth and differentiation.
Ericson and Collette tested their premise by incubating their Src immunoprecipitates with [γ-32P] ATP (i.e. 32P-labelled adenosine triphosphate). In agreement with their proposal, they found that the antibody molecules in the Src immunoprecipitates had been phosphorylated. [Note that Src’s protein kinase activity was simultaneously and independently discovered by Varmus and Bishop.]
Ericson and Collette also carried out control experiments that were particularly revealing. When the same rabbit antisera was used to immunoprecipitate extracts from normal cells, or extracts from cells infected with a transformation-defective mutant of Rous sarcoma virus, no signs of protein kinase activity were seen in those immunoprecipitates. What’s more, the protein kinase activity was found to be temperature sensitive in immunoprecipitates from cells infected with a mutant Rous sarcoma virus that was temperature-sensitive for transformation.
These control experiments confirmed that the protein kinase activity in the immunoprecipitates was coded for by the virus. What’s more, they confirmed that the kinase activity of the retroviral Src protein plays an essential role in transformation. Furthermore, when taken with the earlier findings of Varmus and Bishop, they implied that the kinase activity of the cellular Src protein plays a key role in the control of normal cell proliferation.
While Erickson and coworkers were carrying out the above experiments in Denver, Walter Eckhart and Tony Hunter, at the Salk Institute, were looking into the basis for the transforming activity of the mouse polyomavirus middle T (MT) protein. [Unlike Rous sarcoma virus, which is a retrovirus, the mouse polyomavirus is a member of the Polyomavirus family of small DNA tumor viruses. SV40 is the prototype Polyomavirus.]
Since Erickson’s group was finding that Src expresses protein kinase activity, Eckhart and Hunter asked whether the polyomavirus MT protein might likewise be a protein kinase. Thus, as Erickson and Collette had done in the case of Src, Eckhart and Hunter examined immunoprecipitates of MT to see if they too might express a protein kinase activity, and found that indeed they did.
Interestingly, it was not known at the time of these experiments that MT actually does not express any intrinsic enzymatic activity of its own. Instead, MT interacts with the cellular Src protein to activate its protein kinase activity. See Aside 1.
[Aside 1: For aficionados, MT is a membrane-associated protein that interacts with several cellular proteins. Importantly, the phosphorylation events carried out by MT-activated Src cause a variety of signal adaptor molecules [e.g., Shc, Grb2, and Sos] and other signal mediators [e.g., PI3K and PLCγ] to bind to the complex, thereby triggering a variety of mitogenic signaling pathways. These facts were not yet known when Eckhart and Hunter were doing their experiments.]
At the time of these experiments, serine and threonine were the only amino acids known to be phosphorylated by protein kinases. In fact, Erikson and Collette, as well as Varmus and Bishop, believed that threonine was the amino acid phosphorylated by the Src kinase (see below). Consequently, Hunter asked whether the polyomavirus MT protein likewise would phosphorylate threonine. [Recall that MT actually does not express any intrinsic enzymatic activity of its own.]
Hunter’s experimental procedure was relatively straightforward and reminiscent of Erikson’s and Collette’s. It involved incubating immunoprecipitates of MT with [γ-32P]ATP, hydrolyzing the immunoglobulin, and then separating the amino acids in the hydrolysate by electrophoresis. But, to Hunter’s surprise, the position of the labeled amino acid in his electropherogram did not correspond to that of either threonine or serine.
Hunter was well aware that tyrosine is the only other amino acid with a free hydroxyl group that might be a target for the MT kinase activity. And, while there was no precedent for a tyrosine-specific protein kinase, Hunter proceeded to ask whether the polyomavirus MT protein indeed might phosphorylate tyrosine.
Hunter began by synthesizing a phosphotyrosine molecule that could be used as a standard marker against which to compare the labeled amino acid in a repeat of his earlier experiment. And, to his pleasure, Hunter found that the amino acid that was phosphorylated by the MT kinase activity ran precisely with the phosphotyrosine standard marker in his new electropherograms.
But why had other researchers not detected tyrosine phosphorylation earlier? It was partly because phosphotyrosine accounts for only about 0.03% of phosphorylated amino acids in normal cells. The remaining 99.97% are phosphoserine and phosphothreonine. But, again, that is not the entire explanation. The rest is truly precious.
In Hunter’s own words, he was “too lazy to make up fresh buffer” before doing his experiments. Had the buffer been fresh, its pH would have been the usual 1.9; a pH that, unbeknownst to all at the time, does not separate phosphotyrosine from phosphothreonine during the electrophoresis procedure. The pH of the old buffer that Hunter used in his experiment had inadvertently dropped to 1.7; a pH at which phosphotyrosine is resolved from phosphothreonine. That fact enabled Hunter to discriminate phosphotyrosine from phosphothreonine for the first time. Thus, Hunter attributes his hugely important discovery to his laziness.
The finding that tyrosine is the amino acid phosophorylated by the polyomavirus MT protein kinase activity led Hunter and his Salk Institute-colleague Bart Sefton to ask whether Src too might phosphorylate tyrosine, rather than serine or threonine (4). Indeed, they found that the retroviral Src protein, as well the normal cellular Src protein, function as tyrosine-specific protein kinases. [Recall that it became clear only later that MT actually has no intrinsic enzyme activity of its own and that it acts through Src.] Moreover, the levels of phosphotyrosine were 10-fold higher in cells infected with wild-type Rous sarcoma virus than in control cells, consistent with the premise that Src’s protein tyrosine kinase activity accounts for the altered growth potential of those cells.
Subsequently, Stanley Cohen, at Vanderbilt University, discovered that the epidermal growth factor (EGF) receptor contains an intrinsic protein-tyrosine kinase activity, further underscoring the importance of protein-tyrosine kinases in the normal control of cell proliferation. [Cohen shared the 1986 Nobel Prize in Physiology or Medicine with Rita Levi-Montalcini for their discoveries of growth factors, including EGF.] Subsequent studies identified additional receptor protein-tyrosine kinases, such as the fetal growth factor (FGF) receptor, and non-receptor protein-tyrosine kinases, such as Abl, each of which activates a mitogenic intracellular signaling pathway.
Tony Hunter and coworkers went on to demonstrate that protein-tyrosine kinases play key roles in additional crucial cellular processes, including cellular adhesion, vesicle trafficking, cell communication, the control of gene expression, protein degradation, and immune responses. Moreover, discoveries regarding the role of protein-tyrosine kinases in cell transformation and cancer gave rise to a promising new rational approach to cancer therapy; i.e., the targeting of protein-tyrosine kinases. For example, the drug Gleevec, which inhibits activation of the Abl and platelet-derived growth factor (PDGF) tyrosine kinases, was approved by the U.S. Food and Drug Administration for the treatment of chronic myelogenous leukemia and several types of gastrointestinal tumors.
References:
- Howard Temin: “In from the Cold,” Posted on the blog December 14, 2013.
- Harold Varmus: From English Literature Major to Nobel Prize-Winning Cancer Researcher, Posted on the blog January 5, 2016.
- Collett, M. S. and R. L. Erikson, 1978. Protein kinase activity associated with the avian sarcoma virus src gene product. Proc. Natl. Acad. Sci. USA 75: 2021-2024.
- Hunter, T., and B. M. Sefton. 1980. Transforming gene product of Rous sarcoma virus phosphorylates tyrosine. Proc. Natl. Acad. Sci. USA 77:1311–1315.
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