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Order in the cell maintained by a disordered protein?

Annette Bodenheimer

Graduate student Annette BodenheimerI'm on ScienceSeeker-Microscope

Normally proteins have a globular shape in order to be enzymatically or structurally relevant. Intrinsically disordered proteins (IDPs) broke the protein norms by maintaining their functional roles with little to no overall structure. Most proteins have regions of disorder, such as loops or linker regions (these are the bane of a crystallographer’s life). One of these IDPs appears to be a Swiss Army knife of proteins, CBP [CREB {cAMP (cyclic adenosine monophosphate) response element binding protein} Binding Protein].

CBP has a paralogue, an ancestral sibling, known as p300. These two proteins are key players in many diverse roles within the cell, such as DNA damage response, cell cycle regulation, cellular differentiation, proliferation, and apoptosis. In other words, it has been implicated in every stage of a cell’s life. p300 and CBP are both coactivators of transcription by relaxing chromatin through acetylation of histones and serving as a scaffold for transcriptional machinery.


CBP binding partnersBoth proteins are able to accomplish a wide variety of tasks by binding a diverse population of proteins. CBP and p300 are capable of binding multiple proteins at one time due to their overall linear and disordered structure. Several domains have been discovered within their structure. At both ends are “activation domains”, which initially contain no secondary structure until interaction with other proteins occurs. The first cysteine/histidine rich region (CH1, but also known as TAZ1) has been implicated as an E4 ligase, responsible for adding on additional ubiquitin to already ubiquinated proteins (a red flag for protein degradation machinery to rip apart a no longer needed molecule). These cysteine/histidine rich domains are now characterized as zinc finger domains. KIX domain is the segment that was originally known to bind CREB; it is now known to bind a plethora or other macromolecules. Next down the line is the bromodomain, which enables CBP/p300 to bind to acetylated lysines on histones. The second cysteine/histidine rich region (CH2) has been characterized as a plant homeo domain (PHD), but its function has not been fully understood yet. The histone acetyl transferase (HAT) domain is responsible for transferring an acetyl group from acetyl coenzyme A (acetyl CoA) to a lysine on a histone to further relax chromatin; this will allow for recruitment of other transcription factors. CBP/p300 have been known to acetylate other proteins such as activator of thyroid and retinoid receptor (ACTR) and factor acetyl transferases (FAT). Beside the HAT domain is the third and final cysteine/histidine rich region (CH3, but also known as TAZ2), known to have a high number of binding partners. Finally, there is a polyglutamine (pQ) domain, which contains a subdomain, nuclear coactivator binding domain (NCBD).

Like the Married with Children theme song says, “You can’t have one without the other,” holds true for CBP and p300. Mice studies have shown that lacking CBP or p300 is fatal and having a null allele for either is detrimental to proper develop and ultimately led to death. While both are highly homologous, they are needed for different functions in the body. One example is their role on cell-cycle regulation. p21Cip, a cell-cycle inhibitor, transcription is dependent upon p300 levels, while p27Kip, another cell-cycle inhibitor, requires adequate levels of CBP. Without the proper regulation of these inhibitors, the cell may continue to grow in less than optimal conditions or may propagate problems that have not been fixed. Another instance of the complementarity of these two proteins is seen in haematopoietic stem cells (HSC), which give rise to mature blood cells. CBP is required for the self-renewal of HSCs, but p300 is needed for differentiation of HSCs. Both proteins have different sites of phosphorylation, which lead to downstream targeting or interactions with different proteins.

Interactions of CBP, p53, and HDM2Now that we have a general idea about the functions of CBP/p300, I want to discuss an interesting interaction they have with p53 (a transcription factor that responds to DNA damage and leads to cell cycle arrest or apoptosis) and HDM2 (Human homolog of mouse Double Minute 2) is responsible for maintaining low levels of p53 during “normal” cell life). p53 is a tetrameric transcription factor that responds to DNA damage and promotes cell cycle arrest. If things are beyond repair, p53 can also signal for apoptosis. HDM2’s role is to signal for ubiquitination of p53 through its binding of p53’s transactivation domain (TAD). TAD contains two subdomains, activation domains 1 and 2 (AD1 and AD2). The left side of the figure shows the normal cell interactions, where we can see that AD2 of p53 binds to different domains of a single CBP molecule and AD1 interacts with HDM2. HDM2 bound to p53’s AD1 allows for ubiquitination of the C-termini, signaling for proteosomal degradation. On the right side of the figure is when the cell is stressed. Phosphorylation of AD1 decreases HDM2’s affinity for p53 and then binds to the CBP domain its fellow AD2 is bound. Acetylation of the C-termini activates p53, allowing for transcription of stress response proteins.

So in short, the overall disorder of CBP and p300 are vital for optimal cooperative binding of multiple partners. Sometimes a little disorder is necessary for overall order. (This sounds like the perfect excuse for my messy house.)

Referemces

Kalkhoven E. (2004). CBP and p300: HATs for different occasions, Biochemical Pharmacology, 68 (6) 1145-1155. DOI:

Vo N. & Goodman R.H. (2001). CREB-binding protein and p300 in transcriptional regulation., The Journal of biological chemistry, PMID:

Avantaggiati M.L., Ogryzko V., Gardner K., Giordano A., Levine A.S. & Kelly K. (1997). Recruitment of p300/CBP in p53-dependent signal pathways., Cell, 89 1175-1184. PMID:

Ferreon J.C., Lee C.W., Arai M., Martinez-Yamout M.A., Dyson H.J. & Wright P.E. (2009). Cooperative regulation of p53 by modulation of ternary complex formation with CBP/p300 and HDM2., Proceedings of the National Academy of Sciences of the United States of America, PMID:

 

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