CBL (gene)

Discovery

In 1989 a virally encoded portion of the chromosomal mouse Cbl gene was the first member of the Cbl family to be discovered and was named v-Cbl to distinguish it from normal mouse c-Cbl. The virus used in the experiment was a mouse-tropic strain of Murine leukemia virus isolated from the brain of a mouse captured at Lake Casitas, California known as Cas-Br-M, and was found to have excised approximately a third of the original c-Cbl gene from a mouse into which it was injected. Sequencing revealed that the portion carried by the retrovirus encoded a tyrosine kinase binding domain, and that this was the oncogenic form as retroviruses carrying full-length c-Cbl did not induce tumor formation. The resultant transformed retrovirus was found to consistently induce a type of pre-B lymphoma, known as Casitas B-lineage lymphoma, in infected mice.

Structure

Full length c-Cbl has been found to consist of several regions encoding for functionally distinct protein domains:

• N-terminal tyrosine kinase binding domain (TKB domain): determines the protein which it can bind to
• RING finger domain motif: recruits enzymes involved in ubiquitination
• Proline-rich region: the site of interaction between Cbl and cytosolic proteins involved in Cbl's adaptor functions
• C-terminal ubiquitin-associated domain (UBA domain): the site of ubiquitin binding

This domain structure and the tyrosine and serine-rich content of the protein product is typical of an "adaptor molecule" used in cell signalling pathways.

Homologues

Three mammalian homologues have been characterized, which all differ in their ability to function as adaptor proteins due to the differing lengths of their C-terminal UBA domains:

1. c-Cbl: ubiquitously expressed, 906 and 913 amino acids in length in humans and mice respectively
2. Cbl-b: ubiquitously expressed, 982 amino acids long.
3. Cbl-c: lacks the UBA domain and is therefore only 474 amino acids in length. It is primarily expressed in epithelial cells however its function is poorly understood.

Both c-Cbl and Cbl-b have orthologues in D. melanogaster (D-Cbl) and C. elegans (Sli-1), hinting at a long evolutionary path for these proteins.

Function

Ubiquitin ligase

Ubiquitination is the process of chemically attaching ubiquitin monomers to a protein, thereby targeting it for degradation. As this is a multi-step process, several different enzymes are involved, the final one being a member of the E3 family of ligases. Cbl functions as an E3 ligase, and therefore is able to catalyse the formation of a covalent bond between ubiquitin and Cbl's protein substrate - typically a receptor tyrosine kinase. The RING-finger domain mediates this transfer, however like other E3 ligases of the RING type no intermediate covalent bond is formed between ubiquitin and the RING-finger domain. The stepwise attachment of ubiquitin to the substrate receptor tyrosine kinase can lead to its removal from the plasma membrane and subsequent trafficking to the lysosome for degradation.

Interactions

Cbl gene has been shown to interact with:

• Abl gene,
• ARHGEF7,
• C-Met,
• CD2AP,
• CSF1R.
• CRK,
• CRKL,
• EGFR,
• FRS2,
• FYN,
• Grb2,
• HCK,
• IGF1R,
• LCP2,
• NCK1,
• PDGFRA,
• PIK3R1,
• PIK3R2,
• PLCG1,
• PTK2B,
• PTPN11,
• SH2B2,
• SH3KBP1
• SHC1,
• SLA2,
• SORBS1,
• SORBS2,
• SPRY2,
• Syk,
• UBE2L3,
• VAV1,
• YWHAB,
• YWHAQ, and
• ZAP-70,

References

References

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16. (April 1997). "Steel factor induces tyrosine phosphorylation of CRKL and binding of CRKL to a complex containing c-kit, phosphatidylinositol 3-kinase, and p120(CBL)". Journal of Biological Chemistry.
17. (May 1997). "Differential signaling after beta1 integrin ligation is mediated through binding of CRKL to p120(CBL) and p110(HEF1)". Journal of Biological Chemistry.
18. (October 1997). "Erythropoietin and IL-3 induce tyrosine phosphorylation of CrkL and its association with Shc, SHP-2, and Cbl in hematopoietic cells". Biochemical and Biophysical Research Communications.
19. (July 2002). "EGF-dependent association of phospholipase C-gamma1 with c-Cbl". Experimental Cell Research.
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21. (May 2002). "FRS2 alpha attenuates FGF receptor signaling by Grb2-mediated recruitment of the ubiquitin ligase Cbl". Proceedings of the National Academy of Sciences of the United States of America.
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37. (February 1998). "Interleukin-2 stimulation induces tyrosine phosphorylation of p120-Cbl and CrkL and formation of multimolecular signaling complexes in T lymphocytes and natural killer cells". Journal of Biological Chemistry.
38. (January 1999). "p85 subunit of PI3 kinase does not bind to human Flt3 receptor, but associates with SHP2, SHIP, and a tyrosine-phosphorylated 100-kDa protein in Flt3 ligand-stimulated hematopoietic cells". Biochemical and Biophysical Research Communications.
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46. (May 1999). "APS, an adaptor protein containing Pleckstrin homology (PH) and Src homology-2 (SH2) domains inhibits the JAK-STAT pathway in collaboration with c-Cbl". Leukemia.
47. (November 2000). "Characterization of the CIN85 adaptor protein and identification of components involved in CIN85 complexes". Biochemical and Biophysical Research Communications.
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50. (December 2000). "SETA is a multifunctional adapter protein with three SH3 domains that binds Grb2, Cbl, and the novel SB1 proteins". Cellular Signalling.
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52. (May 2002). "A novel Src homology 2 domain-containing molecule, Src-like adapter protein-2 (SLAP-2), which negatively regulates T cell receptor signaling". Journal of Biological Chemistry.
53. (January 2003). "The c-Cbl-associated protein and c-Cbl are two new partners of the SH2-containing inositol polyphosphate 5-phosphatase SHIP2". Biochemical and Biophysical Research Communications.
54. (February 2001). "Evidence for direct interaction between Sprouty and Cbl". Journal of Biological Chemistry.
55. (December 1998). "Cbl-mediated negative regulation of the Syk tyrosine kinase. A critical role for Cbl phosphotyrosine-binding domain binding to Syk phosphotyrosine 323". Journal of Biological Chemistry.
56. (April 2001). "Requirement of tyrosine-phosphorylated Vav for morphological differentiation of all-trans-retinoic acid-treated HL-60 cells". Cell Growth & Differentiation.
57. (March 2003). "Fgr but not Syk tyrosine kinase is a target for beta 2 integrin-induced c-Cbl-mediated ubiquitination in adherent human neutrophils". The Biochemical Journal.
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59. (October 1999). "Ligand-induced ubiquitination of the epidermal growth factor receptor involves the interaction of the c-Cbl RING finger and UbcH7". Journal of Biological Chemistry.
60. (August 2000). "Structure of a c-Cbl-UbcH7 complex: RING domain function in ubiquitin-protein ligases". Cell.
61. (July 1997). "Proto-oncoprotein Vav interacts with c-Cbl in activated thymocytes and peripheral T cells". Journal of Immunology.
62. (January 2001). "Regulation of Cbl molecular interactions by the co-receptor molecule CD43 in human T cells". Journal of Biological Chemistry.
63. (June 1996). "Activation-modulated association of 14-3-3 proteins with Cbl in T cells". Journal of Biological Chemistry.
64. (September 1996). "A novel phosphotyrosine-binding domain in the N-terminal transforming region of Cbl interacts directly and selectively with ZAP-70 in T cells". Journal of Biological Chemistry.
65. (March 1999). "Structure of the amino-terminal domain of Cbl complexed to its binding site on ZAP-70 kinase". Nature.