Structural insights into non-covalent ubiquitin activation of the cIAP1-UbcH5B~ubiquitin complex — Activation of UbcH5B~Ub by cIAP1 and non-covalent ubiquitin — Structural insights into non-covalent ubiquitin activation of the cIAP1-UbcH5B∼ubiquitin complex — Activation of UbcH5B∼Ub by cIAP1 and non-covalent ubiquitin