Src (gene)

Overview
Src is a family of proto-oncogenic tyrosine kinases originally discovered by J. Michael Bishop and Harold E. Varmus. The discovery of Src family proteins has been instrumental to the modern understanding of cancer as a disease where normally healthy cellular signalling has gone awry.

v-src
Francis Peyton Rous was credited with being the first to come up with the idea that viruses could cause cancer. In 1911 he performed an experiment where he removed a type of tumor called a fibrosarcoma from chickens, ground them up, and used centrifugation to remove cells and debris. He injected the remaining liquid into healthy chicks and found that the chicks developed sarcomas. The causative agent in the liquid was later found to be a virus that was called the Rous sarcoma virus (RSV).

Later work by others showed that RSV was a type of retrovirus. Non-cancer-forming retroviruses contain 3 genes, called gag, pol, and env. Some tumor-inducing retroviruses (such as RSV), however, contain a gene called v-src (viral-sarcoma). It was found that the v-src gene in RSV is required for the formation of cancer and that the other genes have no role in oncogenesis.

Src tyrosine kinases transmit integrin-dependent signals central to cell movement and proliferation. Hallmarks of v-src induced transformation are rounding of the cell and the formation of actin rich podosomes on the basal surface of the cell. These structures are correlated with increased invasiveness, a process thought to be essential for metastasis.

v-src lacks the C-terminal inhibitory phosphorylation site (tyrosine-527), and is therefore constitutively active as opposed to normal src (c-src) which is only activated under certain circumstances where it is required (e.g. growth factor signaling). v-src is therefore an instructive example of an oncogene whereas c-src is a proto-oncogene.

c-src
In 1979, J. Michael Bishop and Harold E. Varmus discovered that normal chickens contain a gene that is structurally closely-related to v-src. The normal cellular gene was called c-src (cellular-src). This discovery changed the current thinking about cancer from a model wherein cancer is caused by a foreign substance (a viral gene) to one where a gene that is normally present in the cell can cause cancer. It is believed that at one point an ancestral virus mistakenly incorporated the c-src gene of its cellular host. At some point, the normal gene became mutated into an abnormally-functioning oncogene, as is now observed in RSV. Once the oncogene is transfected back into a normal host, it can lead to cancer.

src: The transforming (sarcoma inducing) gene of Rous sarcoma virus. The protein product is pp60vsrc, a cytoplasmic protein with tyrosine-specific protein kinase activity, that associates with the cytoplasmic face of the plasma membrane. The protein consists of 3 domains, an N-terminal SH3 domain, a central SH2 domain and a Tyrosine kinase domain. The SH2 and SH3 domains cooperate in the auto-inhibition of the kinase domain. c-Src is phosphorylated on an inhibitory tyrosine near the c-terminus of the protein. This produces a binding site for the SH2 domain which, when bound, facilitates binding of the SH3 domain to a low affinity polyproline site within the linker between the SH2 domain and the Kinase domain. Binding of the SH3 domain results in misalignment of residues within the kinase domain's active site inactivating the enzyme. This allows for multiple mechanism for c-Src activation: dephosphorylation of the C-terminal tyrosine by a protein tyrosine phosphatase, binding of the SH2 domain by a competitive phospho-tyrosine residue, as seen in the case of c-Src binding to Focal Adhesion Kinase, or competitive binding of a polyproline binding site to the SH3 domain, as seen in the case of the HIV NEF protein.

Src Family Kinases
The Src family includes nine members: Src, Lck, Hck, Fyn, Blk, Lyn, Fgr, Yes, and Yrk.