Possible mechanisms for mutually dependent degradation of Cdc20p and Ama1p. A : Generic APC/C model derived from genetic, biochemical and structural information (adapted from models presented in [19, 24, 58, 59]); both the activator (green) and the substrate (red) are located in the inner cavity of the multi-subunit complex. The substrate is represented as binding between the interface of the activator (via D-box or GxEN) and Doc1p (purple, via D-box ). The “platform” (Apc1p, Apc4p and Apc5p) and Apc2p are shown in blue and the “arc lamp” (Cdc16p, Cdc23p and Cdc27p) in light brown. The activator is connected to the arc lamp (via Cdc27p) and to the platform (via Apc2p) by its IR and C-box motifs respectively . Doc1p is also connected to Cdc27p via its IR motif and to Apc2p (reviewed in [3, 5]). E2 shuttles into the complex during the course of a polyubiquitylation reaction. B: Trans-model. Ama1p and Cdc20p are destroyed when they are released from the activator binding position and move into the substrate position. C: Cis-model. Ama1p remains in the activator position and is destroyed by auto-ubiquitylation. D: Cis-dimer model. Ama1p and Cdc20p remain in the activator position. They are destroyed when they come in contact with another APC/C subunit bearing the reciprocal activator.