Responses to extracellular stimuli are often transduced from cell-surface receptors to protein tyrosine kinases which, when activated, initiate the formation of protein complexes that transmit signals throughout the cell. A prominent component of these complexes is the product of the proto-oncogene c-Cbl, which specifically targets activated protein tyrosine kinases and regulates their signalling. How, then, does this multidomain protein shape the responses generated by these signalling complexes?

The c-Cbl protein is tyrosine phosphorylated and forms complexes with a wide range of signalling partners in response to various growth factors. How c-Cbl interacts with proteins, such as Grb2, phosphatidylinositol 3-kinase, and phosphorylated receptors, is well understood, but its role in these complexes is unclear. Recently, the Caenorhabditis elegans Cbl homolog, Sli

v‐cbl is the transforming gene of a murine retrovirus which induces pre‐B cell lymphomas and myelogenous leukaemias. It encodes 40 kDa of a gag fusion protein which is localized in the cytoplasm and nucleus of infected cells. The c‐cbl oncogene encodes a 120 kDa cytoplasmic protein and its overexpression is not associated with tumorigenesis. The c‐cbl sequence has shown that v‐cbl was generated by a truncation that removed 60% of the C‐terminus. In this study, we carried out experiments to identify the position within cbl where the transition occurs between non‐tumorigenic and tumorigenic forms. These experiments focused attention on a region of 17 amino acids which is deleted from cbl in the 70Z/3 pre‐B lymphoma due to a splice acceptor site mutation. This mutation activates cbl's tumorigenic potential and induces its tyrosine phosphorylation. We also show that the expression of the v‐abl and bcr‐abl oncogenes results in the induction of cbl tyrosine phosphorylation, and that abl and cbl associate in vivo. These findings demonstrate that tyrosine‐phosphorylated cbl promotes tumorigenesis and that cbl is a downstream target of the bcr‐abl and v‐abl kinases.

The activation of signalling pathways by ligand engagement with transmembrane receptors is responsible for determining many aspects of cellular function and fate. While these outcomes are initially determined by the nature of the ligand and its receptor, it is also essential that intracellular enzymes, adaptor proteins and transcription factors are correctly assembled to convey the intended response. In recent years, it has become evident that proteins that regulate the amplitude and duration of these signalling responses are also critical in determining the function and fate of cells. Of these, the Cbl family of E3 ubiquitin ligases and adaptor proteins has emerged as key negative regulators of signals from many types of cell-surface receptors. The array of receptors and downstream signalling proteins that are regulated by Cbl proteins is diverse; however, in most cases, the receptors have a common link in that they either possess a tyrosine kinase domain or they form associations with cytoplasmic PTKs (protein tyrosine kinases). Thus Cbl proteins become involved in signalling responses at a time when PTKs are first activated and therefore provide an initial line of defence to ensure that signalling responses proceed at the desired intensity and duration.

The c-Cbl protooncogene can function as a negative regulator of receptor protein tyrosine kinases (RPTKs) by targeting activated receptors for polyubiquitination and downregulation. This function requires its tyrosine kinase binding (TKB) domain for targeting RPTKs and RING finger domain to recruit E2 ubiquitin-conjugating enzymes. It has therefore been proposed that oncogenic Cbl proteins act in a dominant-negative manner to block this c-Cbl activity. In testing this hypothesis, we found that although mutations spanning the RING finger abolish c-Cbl-directed polyubiquitination and downregulation of RPTKs, they do not induce transformation. In contrast, it is mutations within a highly conserved alpha-helical structure linking the SH2 and RING finger domains that render Cbl proteins oncogenic. Thus, Cbl transformation involves effects additional to polyubiquitination of RPTKs that are independent of the RING finger and its ability to recruit E2-conjugating enzymes.