Heimo Riedel

A chimaeric receptor allows insulin to stimulate tyrosine kinase activity of epidermal growth factor receptor.

The cell surface receptors for insulin and epidermal growth factor (EGF) appear to share a common evolutionary origin, as suggested by structural similarity of cysteine-rich regions in their extracellular domains and a highly conserved tyrosine-specific protein kinase domain1-3. Only minor similarity is found outside this catalytic domain, as expected for receptors that have different ligand specificities and generate different biological signals4,5. The EGF receptor is a single polypeptide chain6 but the insulin receptor consists of distinct α and β summits7 that function as an α2β2 heterotetrameric receptor complex8-11. Provoked by this major structural difference in two receptors that carry out parallel functions, we have designed a chimaeric receptor molecule comprising the extracellular portion of the insulin receptor joined to the transmembrane and intracellular domains of the EGF receptor to investigate whether one ligand will activate the tyrosine kinase domain of the receptor for the other ligand. We show here that the EGF receptor kinase domain of the chimaeric protein, expressed transiently in simian cells, is activated by insulin binding. This strongly suggests that insulin and EGF receptors employ closely related or identical mechanisms for signal transduction across the plasma membrane.

Cytoplasmic domains determine signal specificity, cellular routing characteristics and influence ligand binding of epidermal growth factor and insulin receptors.

The cell surface receptors for insulin and epidermal growth factor (EGF) both employ a tyrosine‐specific protein kinase activity to fulfil their distinct biological roles. To identify the structural domains responsible for various receptor activities, we have generated chimeric receptor polypeptides consisting of major EGF and insulin receptor structural domains and examined their biochemical properties and cellular signalling activities. The EGF‐insulin receptor hybrids are properly synthesized and transported to the cell surface, where they form binding competent structures that are defined by the origin of their extracellular domains. While their ligand binding affinities are altered, we find that these chimeric receptors are fully functional in transmitting signals across the plasma membrane and into the cell. Thus, EGF receptor and insulin receptor cytoplasmic domain signalling capabilities are independent of their new heterotetrameric or monomeric environments respectively. Furthermore, the cytoplasmic domains carry the structural determinants that define kinase specificity, mitogenic and transforming potential, and receptor routing.

Structural and Functional Analysis of Cell Surface Receptors with Tyrosine Kinase Activity

Communication between specialized cells is an essential component of multicellular biological systems. Secreted polypeptide hormones and specific receptor molecules present on the surfaces of target cells represent key components in this communication network. The interaction of hormones such as insulin, insulin-like growth factor I (IGF-I), epidermal growth factor (EGF), platelet-derived growth factor (PDGF) and colony-stimulating factor I (CSF-I) with the extracellular portions of their cell surface receptors triggers a distinct cellular response, which may differ depending on the type and developmental stage of the target cell (Kahn 1985; Carpenter 1981). We have undertaken a molecular biological approach as a first step towards unravelling the cytoplasmic and nuclear events that are initiated by extracellular complex formation between ligands and their respective cell surface receptors. As a first step in this undertaking we characterized the primary sequences of several receptors, hoping to obtain clues regarding the overall domain structure of these cell surface molecules and their possible involvement in ligand-induced signal generation. Furthermore we are using these cloned receptor cDNA’s for the generation and testing of chimeric molecules in an attempt to define receptor domains and their functions.

Molecular Features Involved in Cell Surface Receptor Function and Their Role in Oncogenesis

Growth factors and their receptors are key components in the regulation of cell proliferation, and a variety of recent findings suggest that they may also play an important role in oncogenesis. Of approximately twenty identified oncogenes, the three that have been correlated with known cellular proteins have each been found to be related to either a growth factor or a growth factor receptor. The B chain of platelet-derived growth factor (PDGF) is encoded by the proto-oncogene c-sis (Waterfield et al. 1983; Doolittle et al. 1983), the erbB oncogene product appears to be a truncated form of the epidermal growth factor (EGF) receptor (Downward et al. 1984), and the proto-oncogene c-fms may be related or identical to the receptor for macrophage colony stimuTating factor (CSF-I) (Sherr et al. 1985).