tokens: Cbl is a signaling molecule with multiple functional domains : an SH2 domain which binds phosphotyrosine residues , a RING finger domain which acts as a ubiquitin ligase , a proline - rich region which serves as a docking site for SH3 - containing proteins , phosphotyrosine residues which serve as docking sites for SH2 - containing proteins such a CrkL and the p85 subunit of PI 3 - kinase , and a nuclear localization signal .