Post-translational modification by ubiquitin is an essential regulatory process. My goal is to understand how ubiquitin is ligated to target proteins, and how this process is regulated. RING-between-RING (RBR) E3 ligases are a class of ubiquitin ligases that are distinct from RING or HECT E3 ligases. RBRs harbour a catalytic cysteine that forms an intermediate with ubiquitin, usually assisted by an E2 conjugating enzyme. An important RBR ligase is Parkin, mutations in which lead to early-onset hereditary Parkinsonism. Parkin and other RBR ligases share a catalytic RBR module but are usually autoinhibited and activated via distinct mechanisms. Recent insights into Parkin regulation predict large, unknown conformational changes during Parkin activation. However, current data on active RBR ligases reflect the absence of regulatory domains. Therefore, it remains unclear how individual RBR ligases are activated, and whether they share a common mechanism. I will present our latest findings on the mechanisms of Parkin function, including structural data to show how effector binding influences Parkin activity, drives conformational changes, and finally, how ubiquitin binding promotes cooperation between Parkin molecules, which suggests a role for interdomain association in the RBR ligase mechanism.