20S CYCLOSOME COMPLEX-FORMATION AND PROTEOLYTIC ACTIVITY INHIBITED BYTHE CAMP PKA PATHWAY/

THE 20S cyclosome complex (also known as the anaphase-promoting comple x) has ubiquitin ligase activity and is required for mitotic cyclin de struction(1-3) and sister chromatid separation(4,5). The formation and activation of the 20S cyclosome complex is regulated by an unknown me chanism. Here we show that Cut4 (ref. 6) is an essential component of the cyclosome in fission yeast. Cut4 shares sequence similarity with B imE, a protein that regulates mitosis in Aspergillus nidulans(7-9). Mu tations in cut4 result In hypersensitivity to cyclic AMP and to stress -inducing heavy metals, inhibition of the onset of anaphase, disruptio n of the 20S complex, and inhibition of mitotic cyclin ubiquitination, These phenotypes are fully suppressed by cAMP phosphodiesterase and t he protein kinase A (PKA) regulatory subunit and weakly suppressed by Sti1 (an activator of the Hsp70 and Hsp90 chaperones(10,11)), Suppress ion correlates with the amount of 20S complex, indicating that cycloso me formation and activation is inhibited by the cAMP/PKA pathway.