Meta-Biol

• Pathways

Phosphorylation of TP53 (p53) at the N-terminal serine residues S15 and S20 plays a critical role in protein stabilization as phosphorylation at these sites interferes with binding of the ubiquitin ligase MDM2 to TP53. Several different kinases can phosphorylate TP53 at S15 and S20. In response to double strand DNA breaks, S15 is phosphorylated by ATM (Banin et al. 1998, Canman et al. 1998, Khanna et al. 1998), and S20 by CHEK2 (Chehab et al. 1999, Chehab et al. 2000, Hirao et al. 2000). DNA damage or other types of genotoxic stress, such as stalled replication forks, can trigger ATR-mediated phosphorylation of TP53 at S15 (Lakin et al. 1999, Tibbetts et al. 1999) and CHEK1-mediated phosphorylation of TP53 at S20 (Shieh et al. 2000). In response to various types of cell stress, NUAK1 (Hou et al. 2011), CDK5 (Zhang et al. 2002, Lee et al. 2007, Lee et al. 2008), AMPK (Jones et al. 2005) and TP53RK (Abe et al. 2001, Facchin et al. 2003) can phosphorylate TP53 at S15, while PLK3 (Xie, Wang et al. 2001, Xie, Wu et al. 2001) can phosphorylate TP53 at S20.

Phosphorylation of TP53 at serine residue S46 promotes transcription of TP53-regulated apoptotic genes rather than cell cycle arrest genes. Several kinases can phosphorylate S46 of TP53, including ATM-activated DYRK2, which, like TP53, is targeted for degradation by MDM2 (Taira et al. 2007, Taira et al. 2010). TP53 is also phosphorylated at S46 by HIPK2 in the presence of the TP53 transcriptional target TP53INP1 (D'Orazi et al. 2002, Hofmann et al. 2002, Tomasini et al. 2003). CDK5, in addition to phosphorylating TP53 at S15, also phosphorylates it at S33 and S46, which promotes neuronal cell death (Lee et al. 2007).

MAPKAPK5 (PRAK) phosphorylates TP53 at serine residue S37, promoting cell cycle arrest and cellular senescence in response to oncogenic RAS signaling (Sun et al. 2007).

NUAK1 phosphorylates TP53 at S15 and S392, and phosphorylation at S392 may contribute to TP53-mediated transcriptional activation of cell cycle arrest genes (Hou et al. 2011). S392 of TP53 is also phosphorylated by the complex of casein kinase II (CK2) bound to the FACT complex, enhancing transcriptional activity of TP53 in response to UV irradiation (Keller et al. 2001, Keller and Lu 2002).

The activity of TP53 is inhibited by phosphorylation at serine residue S315, which enhances MDM2 binding and degradation of TP53. S315 of TP53 is phosphorylated by Aurora kinase A (AURKA) (Katayama et al. 2004) and CDK2 (Luciani et al. 2000). Interaction with MDM2 and the consequent TP53 degradation is also increased by phosphorylation of TP53 threonine residue T55 by the transcription initiation factor complex TFIID (Li et al. 2004).

Aurora kinase B (AURKB) has been shown to phosphorylate TP53 at serine residue S269 and threonine residue T284, which is possibly facilitated by the binding of the NIR co-repressor. AURKB-mediated phosphorylation was reported to inhibit TP53 transcriptional activity through an unknown mechanism (Wu et al. 2011). A putative direct interaction between TP53 and AURKB has also been described and linked to TP53 phosphorylation and S183, T211 and S215 and TP53 degradation (Gully et al. 2012).

RNA polymerase II (Pol II) is the central enzyme that catalyses DNA- directed mRNA synthesis during the transcription of protein-coding genes. Pol II consists of a 10-subunit catalytic core, which alone is capable of elongating the RNA transcript, and a complex of two subunits, Rpb4/7, that is required for transcription initiation.
The transcription cycle is divided in three major phases: initiation, elongation, and termination. Transcription initiation include promoter DNA binding, DNA melting, and initial synthesis of short RNA transcripts. The transition from initiation to elongation, is referred to as promoter escape and leads to a stable elongation complex that is characterized by an open DNA region or transcription bubble. The bubble contains the DNA-RNA hybrid, a heteroduplex of eight to nine base pairs. The growing 3-end of the RNA is engaged with the polymerase complex active site. Ultimately transcription terminates and Pol II dissocitates from the template.

Gene expression encompasses transcription and translation and the regulation of these processes. RNA Polymerase I Transcription produces the large preribosomal RNA transcript (45S pre-rRNA) that is processed to yield 18S rRNA, 28S rRNA, and 5.8S rRNA, accounting for about half the RNA in a cell. RNA Polymerase II transcription produces messenger RNAs (mRNA) as well as a subset of non-coding RNAs including many small nucleolar RNAs (snRNA) and microRNAs (miRNA). RNA Polymerase III Transcription produces transfer RNAs (tRNA), 5S RNA, 7SL RNA, and U6 snRNA. Transcription from mitochondrial promoters is performed by the mitochondrial RNA polymerase, POLRMT, to yield long transcripts from each DNA strand that are processed to yield 12S rRNA, 16S rRNA, tRNAs, and a few RNAs encoding components of the electron transport chain. Regulation of gene expression can be divided into epigenetic regulation, transcriptional regulation, and post-transcription regulation (comprising translational efficiency and RNA stability). Epigenetic regulation of gene expression is the result of heritable chemical modifications to DNA and DNA-binding proteins such as histones. Epigenetic changes result in altered chromatin complexes that influence transcription. Gene Silencing by RNA mostly occurs post-transcriptionally but can also affect transcription. Small RNAs originating from the genome (miRNAs) or from exogenous RNA (siRNAs) are processed and transferred to the RNA-induced silencing complex (RISC), which interacts with complementary RNA to cause cleavage, translational inhibition, or transcriptional inhibition.