Src is expressed ubiquitously in vertebrate cells; however, brain, osteoclasts, and platelets express 5–200 fold higher levels of this protein than most other cells. In fibroblasts, Src is bound to endosomes, perinuclear membranes, secretory vesicles, and the cytoplasmic face of the plasma membrane where it can interact with a variety of growth factor, integrin, and G-protein-coupled receptors and serve as an essential intermediary in signaltransduction. The expression of high levels of Src in platelets (anucleate cells) and in neurons (which are postmitotic) indicates that Src participates in processes other than cell division. From the N- to C-terminus, Src contains an N-terminal 14-carbon myristoyl group, a unique domain, an SH3 domain, an SH2 domain, an SH2-kinase linker, a protein-tyrosine kinase domain (SH1), and a C-terminal regulatory segment. Src is a non-receptor protein tyrosine kinase that participates in numerous signaling pathways. Src interacts with several protein-tyrosine kinase receptors at the plasma membrane. 
The flow of information is bi-directional with the receptors affecting Src activity and vice versa. Src interacts with EGFR (ErbB1) and ErbB2, two key protein-tyrosine kinase receptors. EGFR mutations occur in NSCLC and ErbB2 overexpression is associated with breast cancer. The ErbB family is also implicated in colorectal, stomach, head and neck, and pancreatic carcinomas as well as glioblastoma. c-MET, or hepatocyte growth factor receptor (HGFR), is a protein-tyrosine kinase receptor that participates in embryonic development, wound healing, cell migration, and angiogenesis. Src also interacts with Platelet-derived growth factor (PDGF), Insulin-like growth factors (IGF-1 and -2) and fibroblast growth factor (FGF) family. Besides protein-tyrosine kinases, Src-family kinases are controlled by integrin receptors, G-protein coupled receptors, antigenand Fc-coupled receptors, cytokine receptors, and steroid hormone receptors. Src participates in pathways regulating cell survival, proliferation, and regulation of gene expression. The enzyme also plays an essential role in bone formation and remodeling and may play a role in breast, prostate, and lung cancer metastasis to the skeleton.


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