1. ¹Division of Oncology, Department of Cancer Biology, The Institute of Medical Science, The University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan
2. ²Department of Molecular Oncology, The Tokyo Metropolitan Institute of Medical Science, 3-18-22 Honkomagome, Bunkyo-ku, Tokyo 113-8613, Japan

Correspondence: T Yamamoto, E-mail: tyamamot@ims.u-tokyo.ac.jp

Received 20 June 2003; Revised 8 September 2003; Accepted 14 October 2003; Published online 8 December 2003.

Abstract

The Cbl family proteins Cbl, Cbl-b, and Cbl-c/Cbl-3 are thought to regulate signaling through protein-tyrosine kinases, positively as scaffold proteins and negatively as ubiquitin ligases. However, the precise signaling pathways and target proteins for each Cbl family member are not well understood. Here we show that Src is a preferential target of Cbl-c for degradation. Although exogenous expression of all Cbl family proteins suppressed the anchorage-independent growth of v-Src-transformed NIH3T3 cells, only Cbl-c caused reversion of the refractile morphology. The level of v-Src protein was reduced by Cbl-c, possibly through a lysosome-dependent pathway. The TKB domain and RING finger of Cbl-c were important for its antioncogenic activity. Wild-type Cbl-c promoted ubiquitination of Src in 293T cells, whereas a RING finger mutant did not. Cbl-c bound specifically to Src phosphorylated at Tyr419. Furthermore, Cbl-c together with UbcH5 induced ubiquitination of Src in vitro. Importantly, the Tyr419 nonphosphorylated form of Src was not ubiquitinated by Cbl-c. Therefore, activated Src may be a direct target of Cbl-c in vivo. Our results suggest that Cbl and Cbl-b suppress v-Src-induced transformation through mechanisms distinct from that of Cbl-c.