The v‐cbl oncogene is the transforming gene of the murine Cas NS‐1 retrovirus which induces pre‐B cell lymphomas and myeloid leukaemias. Sequencing of c‐cbl has revealed that v‐cbl was generated by a large truncation that removed 60% of the C‐terminus of the corresponding protein. In this study we prepared antibodies to cbl and found that c‐cbl encodes a 120 kDa protein which is localized in the cytoplasm with a cytosolic and cytoskeletal distribution. Immunofluorescence studies show a striking pattern of brightly staining vesicles in mitotic cells similar to that observed with cytokeratin antibodies. In contrast to p120c‐cbl, which is exclusively cytoplasmic, the p100gag‐v‐cbl encoded by Cas NS‐1 is localized in both the cytoplasm and the nucleus. This redistribution to the nucleus correlates with the ability of cbl to induce acute transformation. Furthermore the truncated protein encoded by v‐cbl can bind DNA, unlike the full‐length protein. These results suggest that the C‐terminus of cbl is involved in the retention of p120c‐cbl in the cytoplasm and the inhibition of DNA binding. The findings also suggest that a truncated protein encoded by c‐cbl exists in the nucleus of normal cells.

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The peptide N-acetylaspartylglutamate (NAAG) is present in high concentrations in the mammalian central nervous system. Various mechanisms have been proposed for its action, including selective activation of the metabotropic glutamate receptor (mGluR) subtype 3, its action at the N-methyl-D-aspartate receptor, or the production of glutamate by its hydrolysis catalyzed by an extracellular protease. To re-examine its agonist activity at mGluR3, we coexpressed human or rat mGluR3 with G protein inward rectifying channels in Xenopus laevis oocytes. High-performance liquid chromatography analysis of commercial sources of NAAG showed 0.38 to 0.48% glutamate contamination. Although both human and rat mGluR3 were highly sensitive to glutamate, with EC 50 values of 58 and 28 nM, respectively, purified NAAG (100 M) had little activity (7.7% of full activation by glutamate). Only in the millimolar range did it show significant activity, possibly due to residual traces of glutamate remaining in the purified NAAG preparations. In contrast, the unpurified NAAG sample did produce a full agonist response with mGluR3 coexpressed with G␣ 15 , with an EC 50 of 120 M, as measured by a calcium release assay. This response can be explained by the 0.38 to 0.48% glutamate contamination. Our results suggest that NAAG may not have a direct agonist activity at the mGluR3 receptor. Thus, several in vivo and in vitro published results that did not address the issue of glutamate contamination of NAAG preparations may need to be re-evaluated.

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