HER1 acts as a central mediator for multiple signaling pathways. A variety of extracellular ligands trigger the phosphorylation of HER1 on Tyr 845. These include the following: thrombin, endothelin, and LPA, which bind G-protein coupled receptors; growth hormone, which binds a cytokine receptor; and estrogen, which binds a steroid hormone receptor. Moreover, c-Src kinase activity is required for the ability of LPA, endothelin, growth hormone, and estrogen to induce phosphorylation of Tyr 845. We hypothesize that c-Src-mediated phosphorylation of Tyr 845 is a central signaling event and is required for mitogenesis to occur in response to a variety of external stimuli in addition to EGF. The signaling molecules that transmit mitogenic cues from phosphorylated Tyr 845 have yet to be delineated, but may include such effectors as STAT5b, PI-3K, or ERK5. ER, estrogen receptor.

Structure of c-Src. C-Src is the prototype of a large family of cytoplasmic tyrosine kinases that associate with cellular membranes through lipid modifications at their amino-termini. As a linear molecule, the relationship between the various domains can be seen: an amino-terminal membrane-association domain that contains the site of myristylation; a unique domain that exhibits the widest sequence divergence among family members of any of the domains; an SH3 domain that binds polyproline motifs on target molecules; a SH2 domain that binds phosphotyrosine residues on target molecules; a SH2/kinase linker; the catalytic domain; and the negative regulatory domain that contains the predominant site of tyrosine phosphorylation on the inactive molecule (Y527 in chicken, Y530 in human). Mutations that abrogate myristylation, the SH2 domain, and the catalytic activity were shown to reduce EGF-stimulated DNA synthesis in C3H10T½ murine fibroblasts, providing evidence for the involvement of c-Src in mitogenic pathways [18*].

Tyr 845 is located in the catalytic domain of HER1 in a highly conserved subdomain that has functional homologs in other tyrosine kinases. Tyr 845 resides in the activation loop of the HER1 tyrosine kinase catalytic domain (upper panel), a region that shares a high degree of homology among all tyrosine kinases (lower panel). Upon ligand-binding to the EGF receptor (EGFR), the activation loop containing phosphorylated Tyr 845 is modeled (upper panel [63]) to flip into a configuration that promotes access to ATP and substrate, as is the case with phosphorylation of the analogous tyrosines within other tyrosine kinases. Mutation of sites in other tyrosine kinases that are analogous to Tyr 845 renders the molecules catalytically inactive and abrogates downstream biologic signaling. Similar effects of mutagenizing Tyr 845 have been observed [24*,32**]. A unique characteristic of Tyr 845 appears to be that its phosphorylation is mediated by c-Src, whereas homologs in other receptor tyrosine kinases are phosphorylated by the receptors themselves. The phosphorylation of Tyr 845 by c-Src is proposed to facilitate cross-talk between HER1 and signaling pathways activated by other cellular receptors, such as G-protein-coupled receptors and estrogen receptor.