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The morphological changes in early mitosis are mainly due to the phosphorylation of related proteins catalyzed by MPF (mitosis promoting factor), which is actually the Cyclin B/Cdk1 complex. The activated MPF may phosphorylate histone protein H1 to condense chromatin. It can also phosphorylate nuclear lamins to break nuclear envelope. The microtubular structure is closely related to MAPs (microtubule-associated proteins) which can also be phosphorylated by MPF, resulting in the formation of spindle. The activity of MPF depends on its phosphorylation state, as shown in the following figure.
Figure 8-B-2. The phosphorylation state of MPF (CycB-Cdc2 complex) and its activity. Cdc2 is a synonym of Cdk1. Y and T represent the specific Tyrosine and Threonine residue, respectively. MPF is active only when it is monophosphorylated at the specific Threonine residue.
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