The B-type lamins remain associated with these vesicles, but lamins A and C dissociate from the nuclear membrane and are released as free dimers in the cytosol. In concert with dissolution of the nuclear lamina, the nuclear membrane fragments into vesicles ( Figure 8.32). At mitosis, Cdc2 and other protein kinases phosphorylate the lamins, causing the filaments to dissociate into free lamin dimers. The nuclear lamina consists of a meshwork of lamin filaments. When genes encoding these mutant lamins were introduced into cells, their expression was found to block normal breakdown of the nuclear lamina as the cells entered mitosis.ĭissolution of the nuclear lamina. Moreover, the requirement for lamin phosphorylation in the breakdown of the nuclear lamina has been demonstrated directly by the construction of mutant lamins that can no longer be phosphorylated. Cdc2 (as well as other protein kinases activated in mitotic cells) phosphorylates all the different types of lamins, and treatment of isolated nuclei with Cdc2 has been shown to be sufficient to induce depolymerization of the nuclear lamina. Phosphorylation of the lamins is catalyzed by the Cdc2 protein kinase, which was introduced in Chapter 7 (see Figure 7.40) and will be discussed in detail in Chapter 14 as a central regulator of mitosis. Disassembly of the nuclear lamina results from phosphorylation of the lamins, which causes the filaments to break down into individual lamin dimers ( Figure 8.31). The nuclear lamina is composed of fibrous proteins, lamins, which associate with each other to form filaments. The best understood of these events is depolymerization of the nuclear lamina-the meshwork of filaments underlying the nuclear membrane. In open mitosis, the nuclear envelope breaks down and then re-forms around the two sets of separated (more.)ĭisassembly of the nuclear envelope, which parallels a similar breakdown of the endoplasmic reticulum, involves changes in all three of its components: The nuclear membranes are fragmented into vesicles, the nuclear pore complexes dissociate, and the nuclear lamina depolymerizes. In closed mitosis, the nuclear envelope remains intact and chromosomes migrate to opposite poles of a spindle within the nucleus.
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