APC/C activity during the cell cycle. Shifting gears in protein degradation

Abstract

For correct cell division to take place, many different mechanisms ensure genomic integrity and formation healthy daughter cells. One mechanism that has evolved to provide a safe passage from one cell cycle phase into the next, is protein degradation. With our work we provide new insights into activity and regulation of one of the major protein complexes responsible for protein degradation, the APC/C. It targets many different proteins at several points during the cell cycle for destruction. Degradation of these substrates is dependent on recognition motifs in their sequence, known as 'degrons', however they can also depend on targeting tails to directly bind the APC/C.

Interestingly, the APC/C requires one of two coactivators to become active and destroy proteins. These activators, known as Cdc20 and Cdh1, play a role during different phases of the cell cycle, and are also subject to different specific inhibitors, to prevent precocious APC/C activity. We find that Nek2A is the most sensitive APC/C substrate to date, which is degraded upon APC/C activation by Cdc20 upon entry into mitosis, even if Cdc20 is almost completely blocked by inhibitors. We further find that Nek2A is so sensitive to APC/C activity that even before mitosis, Cdh1 is responsible for degradation, again while other substrates are not targeted for destruction due to the inhibitory protein Emi1. Furthermore, we show that at the end of G1, two transcription factors are novel substrates, and must be degraded by the APC/C to start replication of the DNA in S-phase. Failure to degrade these proteins leads to cell cycle halt and eventually cell death. Based on Emi1, we have developed an inducible APC/C inhibitor. APC/C activity is most prominent during mitosis when it is required to progress cells from metaphase to anapahase, after mitotic checkpoint satisfaction. Using this novel tool we can arrest several different cellines for a prolonged time in metaphase, by directly blocking the APC/C, largely independent of the mitotic checkpoint. This offers new and exciting possibilities for further studying the APC/C.