The polo-like kinase (Plk) has been shown to be associated with the anaphas
e-promoting complex at the transition from metaphase to anaphase and to reg
ulate ubiquitination, the process that targets proteins for degradation by
proteasomes. In this study, we have identified proteasomal proteins interac
ting with Plk by mass spectrometry and found that Plk and 20S proteasome su
bunits could be reversibly immunoprecipitated from both human CA46 cells an
d HEK 293 cells transfected with HA-Plk. Furthermore, both coprecipitated P
lk and baculovirus-expressed Plk were able to phosphorylate proteasome subu
nits, and metabolic labeling studies indicate that Plk is partially respons
ible for the phosphorylation of 20S proteasome subunits C9 and C8 in vivo.
In addition, phosphorylation of proteasomes by Plk enhanced proteolytic act
ivity toward an artificial substrate Suc-L-L-V-Y-AMC in vitro and in vivo.
Finally, we were also able to detect Plk associated with 26S proteasomes un
der certain conditions. Together our results suggest that Plk is an importa
nt mitotic regulator of proteasome activity.