SLAP2 adaptor binding disrupts c-CBL autoinhibition to activate ubiquitin ligase function

Abstract

CBL is a RING type E3 ubiquitin ligase that functions as a negative regulator of tyrosine kinase signaling and loss of CBL function is implicated in several forms of leukemia. The Srclike adaptor proteins (SLAP/SLAP2) bind to CBL and are key components of CBL-dependent downregulation of antigen receptor, cytokine receptor, and receptor tyrosine kinase signaling. To understand the molecular basis of the interaction between SLAP/SLAP2 and CBL, we solved the crystal structure of CBL tyrosine kinase binding domain (TKBD) in complex with SLAP2. The carboxy-terminal region of SLAP2 adopts an α-helical structure which binds in a cleft between the 4H, EF-hand, and SH2 domains of the TKBD. This SLAP2 binding site is remote from the canonical TKBD phospho-tyrosine peptide binding site but overlaps with a region important for stabilizing CBL in its autoinhibited conformation. Addition of SLAP2 to autoinhibited CBL in vitro activates CBL autoubiquitination. As well, disruption of the CBL/SLAP2 interface through mutagenesis demonstrates a role for this protein-protein interaction in regulation of CBL E3 ligase activity in cells. Our results reveal that SLAP2 binding provides an alternative mechanism for activation of CBL ubiquitin ligase function.