= CDK1 phosphorylation state
{c}
A <CDK1 phosphorylation state> identifies modified <amino acid residues>. Different sites have opposite effects: in studied <Xenopus laevis> <Cdc2>, Thr14/Tyr15 are inhibitory, while Thr161 <phosphorylation> is activating. T denotes a <threonine residue>, Y a <tyrosine residue>, and numbers are <protein primary structure> positions. Do not assume this numbering for every <species>.<Role of phosphorylation in p34cdc2 activation: identification of an activating kinase by M. J. Solomon and others>[Solomon, Lee and Kirschner (1992), abstract: mapped Xenopus Thr14/Tyr15, loss of activation with Thr161→Ala, and cyclin binding by the triple mutant]{ref}
The 1992 study supplies a useful separation of mechanisms: replacing Thr14, Tyr15 and Thr161 still permitted <cyclin B> binding, but replacing Thr161 prevented activation of <histone H1> <phosphorylation>. Binding therefore did not suffice for catalytic activity in that preparation. Activating <phosphorylation> also occurred with catalytically inactive <Cdc2>, supporting a separate <CDK-activating kinase> rather than a requirement for intramolecular <autophosphorylation>.<Role of phosphorylation in p34cdc2 activation: identification of an activating kinase by M. J. Solomon and others>[Solomon, Lee and Kirschner (1992), abstract: cyclin-binding triple mutant and activating phosphorylation of the kinase-inactive K33R mutant]{ref}
\Image[/cdk1-cyclin-b-binding-phosphorylation-states.png]
{title=<Cyclin B> binding and site-specific <CDK1 phosphorylation states> answer different questions}
{description=Original qualitative schematic for the studied <Xenopus laevis> <Cdc2>. All four right-hand boxes contain bound <cyclin B>; only the green box has activating Thr161 <phosphorylation> with both inhibitory sites clear. “Inhibitory P present” groups Thr14 and/or Tyr15 modification. Arrows show changes of state, not an obligatory sequence, measured rates, or a complete <cell cycle> model. <Myt1> can add either inhibitory modification; <Cdc25> removes inhibitory <phosphate> groups.}
In this deliberately coarse classification, bound <cyclin B>, activating <phosphorylation>, and cleared inhibitory sites must coincide for the activity-permissive state. Removing inhibitory <phosphate> groups does not supply missing <cyclin B> or activating <phosphorylation>. Conversely, adding activating <phosphorylation> does not erase inhibitory modification. Actual preparations contain mixtures of states and additional regulators; this classification gives no numerical <reaction rate>.
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