Substrate binding on the APC/C occurs between the co-activator CDH1 and the processivity factor DOC1

The anaphase–promoting complex/cyclosome (APC/C) is a 22 S ubiquitin ligase complex that initiates chromosome segregation and mitotic exit. We have used biochemical and electron microscopic analyses of Saccharomyces cerevisiae and human APC/C to address how the APC/C subunit Doc1 contributes to recruitment and processive ubiquitylation of APC/C substrates, and to understand how APC/C monomers interact to form a 36 S dimeric form. We show that Doc1 interacts with Cdc27, Cdc16 and Apc1, and is located in vicinity of the cullin–RING module Apc2–Apc11 in the inner cavity of the APC/C. Substrate proteins also bind in the inner cavity, in close proximity to DOC1 and the co–activator CDH1, and induce conformational changes in APC2–APC11. Our results suggest that substrates are recruited to the APC/C by binding to a bipartite substrate receptor composed of a co–activator protein and Doc1.