The fate and origin of the nuclear envelope during and after mitosis in Amoeba proteus. I. Synthesis and behavior of phospholipids of the nuclear envelope during the cell life cycle.

Abstract

The synthesis and behavior of Amoeba proteus nuclear envelope (NE) phospholipids were studied. Most NE phospholipid synthesis occurs during G~ and little during mitosis or S (A. proteus has no G1 phase). Autoradiographic observations after implantation of [~H]choline nuclei into unlabeled ceils reveal little turnover of NE phospholipid during interphase but during mitosis all the label is dispersed through the cytoplasm. Beginning at telophase all the NE phospholipid label returns to the daughter NEs. This observation, along with the finding that no NE phospholipid synthesis occurs during mitosis or S, indicates that no de novo NE phospholipid production is required for newly forming NEs. Similarly emetine, at concentrations that inhibit 97% of protein synthesis, does not prevent the post mitotic formation of NEs, suggesting that previously manufactured proteins are used in making new NEs.If a nucleus containing labeled NE phospholipids is transplanted into an unlabeled nucleate cell and the cell is allowed to grow and divide, the resultant four nuclei are equally labeled. This finding supports, but does not prove (see next paragraph), the conclusion that there probably is no continuity of the A. proteus NE during mitosis.When a phospholipid-labeled nucleus is implanted into a cell in mitosis, the grafted nucleus is not induced to enter mitosis. There is, however, a marked increase in the turnover of that nucleus's NE phospholipids with no apparent breakdown of the NE; this indicates that the mitotic cytoplasm possesses a factor that stimulates NE phospholipid exchange with the cytoplasm. That enhanced turnover is not accompanied by visible structural alteration makes less certain the earlier conclusion that no NE continuity exists during mitosis.Perhaps the most important finding in this study is that there are present, at restricted times in the cell cycle, factors capable of inducing accelerated exchange of structural components without microscopically detectable disruptions of structure.

Cell Cycle Regulation of Membrane Phospholipid Metabolism

“…Metabolic labeling of Chinese hamster ovary cells indicated that at least 50% of the nuclear envelope phospholipid present in G 2 was used to resynthesize the nuclear envelopes of the daughter cells (45). Studies with Amoeba proteus are worthy of mention because of their unique approach (46). Autoradiographic observations following implantation of [ 3 H]choline-labeled nuclei into unlabeled cells revealed little turnover of nuclear membrane phospholipid during interphase; however, during mitosis the label was dispersed throughout the cytoplasm coincident with the degradation of the nuclear envelope.…”

Whole Organism Model to Study Molecular Mechanisms of Differentiation and Dedifferentiation

“…However, in the case of Acanthamoeba, it was reported that trophozoites are formed in the mitotic phase of cell division cycle. Moreover, the absence of the G1 phase has been observed in some protists including Amoeba proteus [59]. However, an extended G1 phase can be observed under certain conditions.…”

Gamete interactions and the fate of sperm organelles in fertilized echinoderm eggs

“…The function of those portions of the sperm nuclear envelope that remain intact is unknown; their persistence during pronuclear development suggests that the sperm nuclear envelope is not homogenous and possesses features specific to the morphogenesis of the sperm nucleus into a male pronucleus. Factors and mechanisms regulating the disappearance of the sperm nuclear envelope have not been determined, and how this process is related to nuclear envelope disruption in mitotic cells is unclear (Johnson and Rao, 1971;Maruta and Goldstein, 1975;Masui and Markert, 1971;Masui and Clarke, 1979). It is noteworthy that breakdown of the sperm nuclear envelope occurs in proximity to the female pronucleus but the pronuclear envelope of the female pronucleus remains intact (Longo, 1973(Longo, ,1981.…”