The multi-adaptor proto-oncoprotein Cbl is a key regulator of Rac and actin assembly

The induction of protein tyrosine kinase signaling pathways is a principal mechanism for promoting cellular activation. Biochemical and genetic analyses have implicated the multi-adaptor proto-oncogene protein Chl as a key negative regulator of activated protein tyrosine kinases. By inhibiting the function of Chl as a multi-domain adaptor protein, through expression of a truncated form (480-Cbl), we demonstrate that Chl is a potent negative regulator of actin assembly in response to receptor tyrosine kinase (RTK) activation. Expression of 480-Cbl dramatically enhances RTK-dependent induction of actin dorsal ruffles, which correlates with a pronounced increase in Rac activation. By contrast, mitogenic signaling by RTK targets, such as PI 3-kinase and MAP kinases, as well as RTK-mediated tyrosine phosphorylation do not appear to be affected by 480-Cbl expression. Further, we determined that Chl undergoes a striking RTK-activation-dependent translocation to sites of active actin dorsal ruffle nucleation. Hence, the selective regulation of RTK signaling to the actin cytoskeleton appears to result from recruitment of signaling proteins on a Cbl template bound to the actin cytoskeleton.