Cytokinesis plasma membrane

Cytokinesis begins some time in anaphase. A cleavage furrow begins to form in the plasma membrane in the same plane as the metaphase plate. It is not known how the furrow is positioned. The furrow is created and deepens due to the actions of the contractile ring which is an actomyosin network assembled specifically for this purpose and dispersed afterward. Thus, the stability of this network is quite different from its analog in muscle. The dynamics of these networks are also different (see Prob. 5.16). 

In telophase, nuclei for each daughter cell form at the two poles, and the mitotic spindle apparatus disappears. Furthermore, nuclear membranes, nuclear lamina, nuclear pores, and nucleoli are reformed. The cell is now ready for cytokinesis, which is physical division of the cytoplasm. The cytoplasm divides as actin/myosin filaments contract and pinch off the plasma membrane, which results in two daughter cells that enter into Go or Gi for another round of division. The main checkpoint that exists during M phase in mammalian cells is the spindle checkpoint it is in place to ensure proper microtubule assembly, proper cell division, and that each daughter cell receives one copy of DNA. 

Figure 5 Force production by the contractile ring in cytokinesis, (a) A ring of actin filaments forms at the plasma membrane and contracts to divide the cell in half, (b) Structure of cytochalasin B, a small molecule that targets actin.

Lgl binds to myosin II, which functions in cytokinesis (see Figure 19-20). Lgl Itself is uniformly localized around the cortex. Lgl is phosphorylated by the apical complex and may be inactivated on the apical side to allow basal Miranda accumulation. Two yeast proteins related to Lgl also bind to myosin II they have been implicated in exocytosis and secretion, specifically in the docking of post-Golgi vesicles with the plasma membrane (Chapter 17). Mutations in the two yeast proteins suppress mutations in the myosin II gene. This observation is interpreted to mean that the function of myosin II in spindle orientation is opposite to that of the Lgl-like proteins reduction of one protein s function is ameliorated by reduction of the other, restoring a semblance of the normal balance. In this case, therefore, myosin and Lgl probably act in opposite directions myosin II moving Miranda or other materials to control spindle orientation and Lgl restraining It. 

Further support for cell wall biosynthesis as a target site was provided by Umetsu et al, who reported abnormal swelling of cultured soybean cells treated with dichlobenil. Dichlobenil was also shown to be an effective and reversible inhibitor of cell wall formation in tobacco protoplasts. It was considerably more potent than other cell wall inhibitors, such as coumarin, and it caused the formation of multinucleate protoplasts. Inhibition of cellulose biosynthesis prevented cell wall formation and cytokinesis but nuclear division that is, mitosis was unaffected. Further studies with tobacco protoplasts confirmed the specific and potent inhibition associated with the cellulose-synthesizing complex at the plasma membrane. 

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