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In the next phase of protein synthesis, elongation, ..
Because CSC contributed to the chemoresistance inaddition to the sphere, colony and tumor formation, we finallyexamined the function of ALDH1A2 in the resistance of neuroblastomacells to 13-cis-RA, which is currently used in the maintenancetherapy for high-risk neuroblastoma patients. As RA generallyinduces the growth arrest and differentiation of neuroblastomacells, we first analyzed the growth arrest induced by 13-cis-RAtreatment for 72 h in BE(2)-C cells expressing scramble shRNA,ALDH1A2 shRNA, control cDNA and ALDH1A2 cDNA. The 13-cis-RA-inducedgrowth arrest was significantly promoted by ALDH1A2 knockdown andinhibited by ALDH1A2 overexpression (). We then investigated theirdifferentiation by phase-contrast microscopy. In scramble shRNA andcontrol cDNA cells, the elongation of neurites started at ~48 h andbecame evident at ~72 h after 13-cis-RA treatment. Compared toscramble shRNA cells, ALDH1A2 shRNA cells showed more elongatedneurites at 48 h after 13-cis-RA treatment (). In contrast, ALDH1A2 cDNA cellsdid not elongate the neurites compared to control cDNA cells at 72h after 13-cis-RA treatment (). These results suggested that ALDH1A2 was involved in theresistance of neuroblastoma cells to 13-cis-RA.
Of course, upregulation of EF2 is predicted to come at a cost to a cycling cell. The G1/S phase cannot occur unless EF2 kinase is activated, which implies the requirement for controlled phosphorylation of EF2. Therefore, a rise in EF2 levels could require increased expression of EF2 kinase to cope with a larger substrate burden. Such an upregulation has been repeatedly demonstrated by Hait and co‐workers in multiple cancers (; ; ). The overexpression of survivin, EF2, and EF2 kinase seen in cancer, suggests that these proteins fill critical niches in the life of a cancer cell. This observation makes them potential targets for cancer treatments.
mTOR Signaling and the Control of Protein Synthesis.
DNA replication takes place in three steps, Initiation, elongation and termination in eukaryotes. Single eukaryotic chromosome has many replicons. In case of eukaryote DNA replication, formation of pre-replicative complex takes place in G1 stage of cell cycle. Entry from G1 stage to stage is mediated by regulatory checkpoint to ensure the presence of requirement of replication is completed like amount of RNA, protein, lipids, and carbohydrate and there should be not any kind of damage in DNA. Although individual replicons have characteristic time to get activated during S phase but replicons near one another are activated at the same time known by Regional activation patterns. Once replication get completed, no other activation of any origin takes place until next round of replication starts after cell division.
Now in S phase pre-replicative complex trigger in active state by phosphorylation of Cdc6 and Cdt1 through kinases name Cdk and Ddk. Phosphorylation of Cdc6 leads to its degradation and not available for further initiation the replication. Now the MCM recruit CdC-45 and GINS. This complex is called as CMG (CdC-45, MCM-GINS) complex. CDt-45 and GIN-45 enhance the helicase activity of MCM and MCM causes unwinding of Ds DNA. After that polymerase a get recruited by helicase and make RNA primer nucleotide. The polymerase a makes 10 nucleotide long RNA sequence which work as primer for DNA syubnthesis. A 20 base pair DNA also form by polymerase a called as initiator DNA, or “iDNA”. After forming the iDNA the polymerase a get dissociated from DNA template. Replication factor C (RFC), a clamp loader then binds to the iDNA and loads PCNA (clamp) on iDNA in ATP dependent manner with the same mechanism mention in prokaryotic replication, DNA polymerase ε for leading strand and DNA polymerase δ on lagging strand also comes there with help of its clamp. Further mechanism similar to prokaryote rather than primer removal which carried out when leading strand and lagging strand synthesis get completed. This marks the transition from initiation to elongation stage.
How does DNA control the synthesis of proteins? | …
Protein synthesis is a finely tuned and tightly controlled process. The elongation step of this process has attracted the attention of cancer researchers. In particular, elongation factor 2 (EF2), the critical enzyme governing elongation of nascent proteins, has been investigated as a target for new therapies and as a potential contributor to the success of conventional therapies.
Cell cycling and protein synthesis are both key physiological tasks for cancer cells. Here we present a model for how the elongation phase of protein synthesis, governed by elongation factor 2 and elongation factor 2 kinase, both modulates and responds to cell cycling. Within this framework we also discuss survivin, a protein with both pro‐mitotic and anti‐apoptotic roles whose persistence in the cell is tied to protein synthesis due to its short half‐life. Finally, we provide a brief overview of efforts of cancer researchers to target EF2 and EF2 kinase.
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Control of the elongation phase of protein synthesis
This model presents a puzzle for protein synthesis activity during the cell cycle. How does the cell provide for any protein‐related needs during the G2/M phase? For instance, proteins with short half‐lives generally must be replaced almost continually and some of these proteins, such as survivin, are critical for cell cycle progression during G2/M.
The mTOR Pathway in the Control of Protein Synthesis
During G2/M, the cellular pool of EF2 is phosphorylated by EF2 kinase and protein synthesis drops precipitously (). In addition to the activating effects on EF2 kinase seen at the G1/S transition, by G2/M the phosphorylation at Ser366 which inhibits EF2 kinase during the G1‐phase has been removed (; ).
the elongation phase of protein synthesis ..
The answer to this question is found in the cyclin complex which is active during G2/M: CyclinB and cdc2/CDK1. Cdc2 becomes an active kinase during this phase and makes two important contributions to the protein pool during G2/M. First, cdc2 phosphorylates survivin itself, which stabilizes the protein and lengthens its half‐life (). If cdc2 is unable to perform this stabilization, the result is a weakening of mitotic spindles due to insufficient survivin (). The second contribution to the protein pool has only recently been described by the Proud group and offers a potential loophole for the inhibition of protein synthesis during G2/M. The investigators report that cdc2 is able to deactivate EF2 kinase though phosphorylation at Ser359 (). Importantly, phosphorylation at this site inhibits EF2 kinase activity (freeing EF2 to elongate nascent proteins) even in the presence of other activating influences such as Ca+2. Therefore, the activity of cdc2 during G2/M can counteract the other cell cycle influences on EF2 kinase and thereby facilitate protein synthesis during the phase. It should be noted that this effect is only observed when sufficient amino acids are supplied to the cell (). It seems that even during cycling, a starvation response to halt protein synthesis will override any other influences on the elongation process (). Taken together, a complex picture of regulation over EF2 kinase and, subsequently, EF2 emerges (, ).
Initiation of Protein Synthesis; Elongation; ..
During the G1‐phase of the cell cycle protein synthesis is enabled, requiring EF2 activity. Due to the reciprocal relationship of EF2 and EF2 kinase, this capability requires the inactivation of EF2 kinase. The inactivation of EF2 kinase is accomplished through the phosphorylation of the Serine 359 or Serine 366 residues () (). The phosphorylation of Ser366 is the mechanism of control generally observed during G1, while the role of Ser359 is important during G2/M and will be discussed in connection with that phase of the cell cycle.
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