In mitosis, the anaphase-promoting complex (APC) regulates the onset of sister-chromatid separation and exit from mitosis by mediating the ubiquitination and degradation of the securin protein and mitotic cyclins. With the use of a baculoviral expression system, we have reconstituted the ubiquitin ligase activity of human APC. In combination with Ubc4 or UbcH10, a heterodimeric complex of APC2 and APC11 is sufficient to catalyze the ubiquitination of human securin and cyclin B1. However, the minimal APC2/11 ubiquitin ligase module does not possess substrate specificity, because it also ubiquitinates the destruction box deletion mutants of securin and cyclin B1. Both APC11 and UbcH10 bind to the C-terminal cullin homology domain of APC2, whereas Ubc4 interacts with APC11 directly. Zn²⁺-binding and mutagenesis experiments indicate that APC11 binds Zn²⁺at a 1:3 M ratio. Unlike the two Zn²⁺ ions of the canonical RING-finger motif, the third Zn²⁺ ion of APC11 is not essential for its ligase activity. Surprisingly, with Ubc4 as the E2 enzyme, Zn²⁺ ions alone are sufficient to catalyze the ubiquitination of cyclin B1. Therefore, the Zn²⁺ ions of the RING finger family of ubiquitin ligases may be directly involved in catalysis.