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Role for cyclin A in the dependence of mitosis on
HEADLINES
headlines Cyclin A is required for the onset of DNA replication in mammalian fibroblasts F. Girard, U. Strausfeld, A. Fernandez and N. ]. C. Lamb Cell 67, 1169-I 1 79
Role for cyclln A in the dependence of mitosis on completion of DNA replication D. H. Walker and J. L. Mailer Nature 354, 314-31 7 The amount of cyclin A protein in the cell oscillates during the cell cycle, peaking just before entry into mitosis. Like cyclin B, which is a subunit of the mitotic inducer MPF, cyclin A can induce meiotic maturation in oocytes, implying that it too is involved in the transition from the G2 to the M phase. However, cyclin A begins to accumulate earlier in the cell cycle than cyclin B. This suggests that cyclin A may perform distinct functions during the S or G2 phase. These two reports substantiate the idea that cyclin A is required during interphase, for two surprisingly different functions. Girard and colleagues microinjected antisense cyclin A expression plasmids into rat fibroblasts in the GI phase and found that the cells were unable to replicate their DNA. This block was specifically due to the absence of cyclin A because subsequent injection of cyclin A protein allowed the cells to progress through S phase. Thus, cyclin A is required for the GI-S transition or perhaps for ongoing DNA replication in S phase. By using Xenopus egg extracts that cycle in vitro between S and M phase states, Walker and Maller showed that these particular cell cycles were unaffected by removal of cyclin A unless the extracts were prevented from completing DNA replication, which normally delays mitosis. This delay did not occur in extracts lacking cyclin A. Thus, the authors suggest that cyclin A plays a role in the checkpoint mechanism that prevents entry into mitosis until DNA replication is complete. This is a very surprising finding because addition of large quantities of cyclin A to these extracts promotes entry into mitosis, rather than delaying it. The simplest explanation is that cyclin A performs several roles during the cell cycle, and that these different assay systems highlight distinct functions.
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Mitosis and inhibition of intracellular transport stimulate palmitoylation of a 62 kD protein D. I. Mundy and G. Warren J. Cell Biol. 116, 1 35-146 Fatty acid acylation of proteins is thought to be required during both the budding and fusion stages of vesicle-mediated intracellular protein transport. However, specific fatty acylated proteins involved in vesicle traffic have not been identified. Mundy and Warren now report that during mitosis, when vesicle-mediated intracellular traffic is inhibited, a protein of 62 kDa (p62) becomes palmitoylated. The appearance of p62 was independent of protein synthesis, and the protein and/or its palmitate group disappeared after cells returned to interphase. The p62 protein was shown to be tightly associated with the cytoplasmic face of membranes and cofractionated with the cis Golgi network protein p58. Interestingly, brefeldin A, carbonylcyanide m-chlorophenylhyd razone, monensin and aluminium fluoride, each of which inhibits vesicular transport at a distinct step, all induced the appearance of a similar 62 kDa palmitoylated protein. Thus, inhibition of membrane traffic appears to be the common factor responsible for the increase in palmitoylation of p62. The p62 protein could perform a common function in many steps of vesicle formation, for example in the assembly of coat proteins. Palmitoylation could regulate its function by modifying its binding to membranes or by influencing its interactions with the G proteins that regulate vesicular transport. Alternatively, p62 could function as an acylating enzyme or an acyl donor, its accumulation during inhibition of vesicular transport reflecting the fact that it is not being consumed. The exact role of p62 and its distribution during interphase and mitosis awaits its purification and characterization, and the production of specific antibodies against it. Nevertheless, this is the first demonstration that a specific protein is the target of fatty acylation during inhibition of intracellular traffic.
TRENDS IN CELL BIOLOGYVOL. 2 MARCH 1992
headlines Cyclin A is required for the onset of DNA replication in mammalian fibroblasts F. Girard, U. Strausfeld, A. Fernandez and N. ]. C. Lamb Cell 67, 1169-I 1 79
Role for cyclln A in the dependence of mitosis on completion of DNA replication D. H. Walker and J. L. Mailer Nature 354, 314-31 7 The amount of cyclin A protein in the cell oscillates during the cell cycle, peaking just before entry into mitosis. Like cyclin B, which is a subunit of the mitotic inducer MPF, cyclin A can induce meiotic maturation in oocytes, implying that it too is involved in the transition from the G2 to the M phase. However, cyclin A begins to accumulate earlier in the cell cycle than cyclin B. This suggests that cyclin A may perform distinct functions during the S or G2 phase. These two reports substantiate the idea that cyclin A is required during interphase, for two surprisingly different functions. Girard and colleagues microinjected antisense cyclin A expression plasmids into rat fibroblasts in the GI phase and found that the cells were unable to replicate their DNA. This block was specifically due to the absence of cyclin A because subsequent injection of cyclin A protein allowed the cells to progress through S phase. Thus, cyclin A is required for the GI-S transition or perhaps for ongoing DNA replication in S phase. By using Xenopus egg extracts that cycle in vitro between S and M phase states, Walker and Maller showed that these particular cell cycles were unaffected by removal of cyclin A unless the extracts were prevented from completing DNA replication, which normally delays mitosis. This delay did not occur in extracts lacking cyclin A. Thus, the authors suggest that cyclin A plays a role in the checkpoint mechanism that prevents entry into mitosis until DNA replication is complete. This is a very surprising finding because addition of large quantities of cyclin A to these extracts promotes entry into mitosis, rather than delaying it. The simplest explanation is that cyclin A performs several roles during the cell cycle, and that these different assay systems highlight distinct functions.
76
Mitosis and inhibition of intracellular transport stimulate palmitoylation of a 62 kD protein D. I. Mundy and G. Warren J. Cell Biol. 116, 1 35-146 Fatty acid acylation of proteins is thought to be required during both the budding and fusion stages of vesicle-mediated intracellular protein transport. However, specific fatty acylated proteins involved in vesicle traffic have not been identified. Mundy and Warren now report that during mitosis, when vesicle-mediated intracellular traffic is inhibited, a protein of 62 kDa (p62) becomes palmitoylated. The appearance of p62 was independent of protein synthesis, and the protein and/or its palmitate group disappeared after cells returned to interphase. The p62 protein was shown to be tightly associated with the cytoplasmic face of membranes and cofractionated with the cis Golgi network protein p58. Interestingly, brefeldin A, carbonylcyanide m-chlorophenylhyd razone, monensin and aluminium fluoride, each of which inhibits vesicular transport at a distinct step, all induced the appearance of a similar 62 kDa palmitoylated protein. Thus, inhibition of membrane traffic appears to be the common factor responsible for the increase in palmitoylation of p62. The p62 protein could perform a common function in many steps of vesicle formation, for example in the assembly of coat proteins. Palmitoylation could regulate its function by modifying its binding to membranes or by influencing its interactions with the G proteins that regulate vesicular transport. Alternatively, p62 could function as an acylating enzyme or an acyl donor, its accumulation during inhibition of vesicular transport reflecting the fact that it is not being consumed. The exact role of p62 and its distribution during interphase and mitosis awaits its purification and characterization, and the production of specific antibodies against it. Nevertheless, this is the first demonstration that a specific protein is the target of fatty acylation during inhibition of intracellular traffic.
TRENDS IN CELL BIOLOGYVOL. 2 MARCH 1992