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BCLAF1 was originally identified as a protein that interacts with anti-apoptotic members of the Bcl2 family. Although linked to (and named for its) interaction with BCL2 family members, BCLAF1 does not share structural similarities with these proteins. The presence of an arginine-serine (RS)-rich region near the N-terminus is the most prominent feature of the BCLAF1 open reading frame. Proteins containing the RS domain are typically associated with biogenesis and processing events of the pre-mRNA, such as splicing of the pre-mRNA.
BCLAF1 as a component of ribonucleoprotein (RNP) complexes
Eukaryotic conversion of pre-mRNA into mature mRNA requires the careful coordination of various post-transcriptional events, such as pre-mRNA 5’ capping, splicing, polyadenylation, and mRNA export from the nucleus to the cytoplasm. The step-by-step completion of each stage consists of a dedicated library of molecular factors that are subsequently recruited to the RNA substrate. These factors interact with RNA in RNP complexes that are initially formed during transcription. The protein composition of RNPs is remodeled during each phase of its existence in a dynamic fashion in order to dictate the fate of the contained RNA molecule. Study that examined the composition of human mRNPs using LC-MS/MS discovered the presence of BCLAF1 among newly identified mRNP proteins. The association of BCLAF1 with mRNPs occurred independently of splicing, but was found to be dependent on the presence of CBP80/CBP20, proteins that form the 5’-m7G cap binding complex. A subsequent study identified BCLAF1 in a complex that mediated cyclin D1 message stability together with SNIP1, SkIP, TAP150, and Pinin. SNIP1 was found to be required for the recruitment of the RNA processing factor U2AF65 to cyclin D1 transcripts. Studies also found that BCLAF1 co-precipitates with hnRNP A1, an RNP protein that is bound to cellular RNAs from transcription to translation. Using a prototypical substrate for measuring alternative pre-mRNA splicing, discovered that BCLAF1-deficient fibroblasts have altered levels of spliced transcripts derived from adenovirus E1A pre-mRNA compared to wild-type cells. Although BCLAF1 is bound to general splicing factor U2AF65 in both the absence and presence of DNA damage, only in the presence of DNA damage it associated with phosphorylated BRCA1. These results indicate that the interaction between BCLAF1 and phosphorylated BRCA1 occurs in the DNA damage response and resulted in recruitment of splicing factor U2AF65 to the HPV16 DNA.
Fig. 1. BCLAF1 as a component of ribonucleoprotein (RNP) complexes. (Nilsson K et al.International Journal of Molecular Sciences. 2018.).
BCLAF1 is an important NF-κB signaling transducer and C/EBPβ regulator
BCLAF1 responds to NF-κB activation by upregulation. BCLAF1 contains a putative NF-κB-binding element within its promoter. Both ChIP and reporter assays indicate that NF-κB binds to this region and activates BCLAF1transcription. Kong et al. have also demonstrated that RelA (p65) binds to the BCLAF1 promoter. However, BCLAF1 upregulation by NF-κB may require its activity to reach a certain threshold. Compared with severe and acute DNA damage treatment, mild DNA damage induces senescence. NF-κB activation is low on day 3 of drug treatment, and a relatively full NF-κB activation was achieved on day 7 as indicated by efficient p65 nuclear translocation and dramatic p65 upregulation. Thus, NF-κB signaling during DNA damage-induced senescence appears to be a gradual process and subject to further amplification through upregulation. As reported, Nemo can transmit DDR to NF-κB activation; however, it did not mediate NF-κB upregulation.
Recent studies have linked BCLAF1 to DDR. Lee et al. reported that BCLAF1 interacted with γH2AX and may regulate the Ku70/DNA-PKcs complex in response to DNA damage. Savage et al. demonstrated that BCLAF1 complexes with BRCA1 and influences the radio-sensitivity of cells. Shao et al. have demonstrated that BCLAF1 is involved in persistent and mild DNA damage-induced senescence downstream of NF-κB activation. Depending on the extent of DNA damage, BCLAF1 probably reacts differently. In the acute DNA damage response, BCLAF1 may be recruited to DNA damage foci and facilitates DNA repair. However, under chronic DNA damage conditions, BCLAF1 expression is upregulated in response to NF-κB activation and subsequently amplifies NF-κB downstream signaling to induce cellular senescence.