Preparation of synchronized human cell extracts to study ubiquitination and degradation

Ubiquitination and protein degradation regulate cell cycle progression in all eukaryotes. During mitosis, ubiquitination by the Anaphase-Promoting Complex/Cyclosome (APC/C) triggers sister chromatid separation and mitotic exit. The APC/C is tightly regulated by phosphorylation, ubiquitination, association of activators or inhibitors, and competitive binding of substrates. Much of our understanding of the mechanism of APC/C-dependent ubiquitination has been obtained from studies using extracts of Xenopus laevis eggs or synchronized human tissue culture cells. Here, we describe protocols to prepare extracts of synchronized human cells, and discuss experiments to use extracts for the biochemical analysis of APC/C-dependent ubiquitination.