Scatter Factor (SF) is a fibroblast‐secreted protein which promotes motility and matrix invasion of epithelial cells. Hepatocyte Growth Factor (HGF) is a powerful mitogen for hepatocytes and other epithelial tissues. SF and HGF, purified according to their respective biological activities, were interchangeable and equally effective in assays for cell growth, motility and invasion. Both bound with identical affinities to the same sites in target cells. The receptor for SF and HGF was identified as the product of the MET oncogene by: (i) ligand binding and coprecipitation in immunocomplexes; (ii) chemical crosslinking to the Met beta subunit; (iii) transfer of binding activity in insect cells by a baculovirus carrying the MET cDNA; (iv) ligand‐induced tyrosine phosphorylation of the Met beta subunit. SF and HGF cDNA clones from human fibroblasts, placenta and liver had virtually identical sequences. We conclude that the same molecule (SF/HGF) acts as a growth or motility factor through a single receptor in different target cells.

The Tpl‐2 protein serine/threonine kinase was originally identified, in a C‐terminally deleted form, as the product of an oncogene associated with the progression of Moloney murine leukemia virus‐induced T cell lymphomas in rats. The kinase domain of Tpl‐2 is homologous to the Saccharomyces cerevisiae gene product, STE11, which encodes a MAP kinase kinase kinase. This suggested that Tpl‐2 might have a similar activity. Consistent with this hypothesis, immunoprecipitated Tpl‐2 and Tpl‐2deltaC (a C‐terminally truncated mutant) phosphorylated and activated recombinant fusion proteins of the mammalian MAP kinase kinases, MEK‐1 and SEK‐1, in vitro. Furthermore, transfection of Tpl‐2 into COS‐1 cells or Jurkat T cells. markedly activated the MAP kinases, ERK‐1 and SAP kinase (JNK), which are substrates for MEK‐1 and SEK‐1, respectively. Tpl‐2, therefore, is a MAP kinase kinase kinase which can activate two MAP kinase pathways. After Raf and Mos, Tpl‐2 is the third serine/threonine oncoprotein kinase that has been shown to function as a direct activator of MEK‐1.

The human c-MET oncogene encodes a transmembrane tyrosine kinase (p190c-met) with structural and functional features of a growth-factor receptor. Monoclonal antibodies (MAbs) have been used to investigate the distribution of the c-Met protein in human normal and neoplastic tissues. By immunofluorescence microscopy homogeneous expression was detected in normal hepatocytes as well as in epithelial cells lining the stomach, the small and the large intestine. Positive staining was also found in epithelial cells of the endometrium and ovary, and in basal keratinocytes of esophagus and skin. By Northern blot analysis, high levels of c-met messenger RNA were detected in specimens of liver, gastro-intestinal tract and kidney. c-met-specific mRNA was also found in thyroid, pancreas and placenta, in which organs c-Met protein was barely detectable by immunofluorescence. The antibodies revealed expression of c-MET protein in hepatomas (11/14), carcinomas of colon and rectum (19/21), stomach (11/22), kidney (16/19), ovary (9/17) and skin (7/17). Carcinomas of the lung (13/20), thyroid (11/13) and pancreas (5/7) were also positive. In these last cases (lung, thyroid and pancreas) tumor cells were homogeneously stained by the antibodies, whereas in their normal counterparts staining was barely detectable. These data suggest that the receptor encoded by c-MET plays a physiological role in epithelial cell growth and that its expression is altered in human carcinomas.

Members of a family of highly conserved proteins, termed 14-3-3 proteins, were found by several experimental approaches to associate with Raf-1, a central component of a key signal transduction pathway. Optimal complex formation required the amino-terminal regulatory domain of Raf-1. The association of 14-3-3 proteins and Raf-1 was not substantially affected by the activation state of Raf.