The dual mechanism of separase regulation by securin.
BACKGROUND
Sister chromatid separation and segregation at anaphase onset are triggered by cleavage of the chromosomal cohesin complex by the protease separase. Separase is regulated by its binding partner securin in two ways: securin is required to support separase activity in anaphase; and, at the same time, securin must be destroyed via ubiquitylation before separase becomes active. The molecular mechanisms underlying this dual regulation of separase by securin are unknown.


RESULTS
We show that, in budding yeast, securin supports separase localization. Separase enters the nucleus independently of securin, but securin is required and sufficient to cause accumulation of separase in the nucleus, where its known cleavage targets reside. Securin also ensures that separase gains full proteolytic activity in anaphase. We also show that securin, while present, directly inhibits the proteolytic activity of separase. Securin prevents the binding of separase to its substrates. It also hinders the separase N terminus from interacting with and possibly inducing an activating conformational change at the protease active site 150 kDa downstream at the protein's C terminus.


CONCLUSIONS
Securin inhibits the proteolytic activity of separase in a 2-fold manner. While inhibiting separase, securin is able to promote nuclear accumulation of separase and help separase to become fully activated after securin's own destruction at anaphase onset.
Chromosome Segregation Laboratory, WC2A 3PX, London, United Kingdom.
Nadine C D
NC
Hornig
Philip P
PP
Knowles
Neil Q
NQ
McDonald
Frank
F
Uhlmann
D001665:Binding Sites; D002384:Catalysis; D018797:Cell Cycle Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002467:Cell Nucleus / Q000201:enzymology; D010450:Endopeptidases; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000378:metabolism; D005656:Fungal Proteins / Q000502:physiology*; D009687:Nuclear Proteins / Q000502:physiology*; D010447:Peptide Hydrolases / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins; D004718:Saccharomycetales; D064249:Securin; D064247:Separase
D018797:Cell Cycle Proteins; D004791:Enzyme Inhibitors; D005656:Fungal Proteins; D009687:Nuclear Proteins; C578004:PDS1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D064249:Securin; D010450:Endopeptidases; D010447:Peptide Hydrolases; C577986:ESP1 protein, S cerevisiae; D064247:Separase