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Cytobalancing: Coming soon to a cell near you

Joe Maciag

Joe Maciag
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IDPs (intrinsically disordered polypeptides), these naturally occurring regions with no specific structural motifs, can cause quite a bit of trouble. They also help to maintain your life throughout the day, so they should not be considered completely horrible. IDPs are all part of a balancing act, a cytobalancing act in which thousands of proteins and substrates are constantly interacting. It all blends perfectly until one of them gets out of order. When IDPs get out of hand, people die.

These polypeptides are used by our cells to elicit signals, maintain regulatory functions and control gene expression. One of their most important characteristics is the ability to be highly specific binders. Some IDPs maintain their unfolded structures to carry out processes, while others fold upon binding. One specific IDP is known as PAGE4 (cancer/testis antigen prostate-associated gene 4). It is a protein whose function is not well understood, yet the affect of its presence can be felt.

Increased expression of this IDP has been shown to alter the ability of a cell to recognize stress. With the addition of cytotoxic drugs, normal cells sense stress and initiate apoptotic signaling cascades. PAGE4 somehow counters this signaling [1]. So, when does an IDP become over-expressed?  Often in cancerous cells, but you probably could have guessed. Increased concentration of PAGE4 (or most proteins) can result in non-specific binding, but IDPs are especially susceptible to demonstrate non-specific binding at higher concentrations because of the abundance of exposed polar amino acids.

There are an unlimited number of potential candidates for IDPs to bind, meaning an unlimited potential for dysregulation of cellular processes [2], so protein concentrations are key to maintaining balance in regards to IDPs. Yu Zeng and colleagues [1] have realized this and have shown that down regulation of PAGE4 can restore the cellular ability to commit suicide. Using specific siRNAs they knocked down aberrant expression of PAGE4, thus creating cells capable of responding to stress. Cells ceased proliferating and began dismantling DNA as well as structural proteins. They have not, however, been able to define what PAGE4 specifically binds to elicit such a resistance [1].

Small molecule inhibitors of IDPs

Fig. 1: Small molecule inhibitors of the Myc/Max dimer

So now that we know IDPs can be involved in proliferation of cancerous cells, how do we regulate their activity? Scientists throughout the world are asking themselves this same question. Some are even finding that there are more possibilities for IDPs than for “normal” structured proteins. IDPs are special because of the abundance of polar regions. It is in these regions that scientists are generating specific small molecules to bind and inhibit. One target of interest is the Myc and Max dimer; two highly disordered proteins vital to regulation of cellular growth. Activity is achieved only through dimer formation. Many specific and non-specific molecules have been shown to inhibit dimer formation and folding of the disordered regions (Fig.1). They have the ability to bind three inhibitors simultaneously without affecting the affinity for small molecule binding. It is therefore clear that IDPs can potentially bind multiple targets [2].

Bullseye

IDPs potentially bind to multiple targets

So there it is, problem solved, right? Wrong. If one IDP can be successfully down-regulated with so many targets, what is the possibility of these small molecules binding other IDPs or even stably folded proteins? In fact, there is a pretty good chance of non-specific IDP binding. If this were to occur, many cellular processes would become unbalanced. The dilemma is specificity. We must find small molecules that have extremely specific binding (to IDPs) which do not affect any other processes. While this is not the only obstacle when designing molecules, it may be the most important. The potential for IDPs to be a target is promising, but I think this approach may come at a cost if this dilemma cannot be overcome. Continued efforts on the part of scientists will hopefully provide new ways of fighting disease, but until then there is work to be done.

1) Zeng, Y., He, Y., Yang, F., Mooney, S.M., Getzenberg, R.H., Orban, J. & Kulkarni, P. (2011). The Cancer/Testis Antigen Prostate-associated Gene 4 (PAGE4) Is a Highly Intrinsically Disordered Protein, Journal of Biological Chemistry, 286 (16) 13994. DOI: 10.1074/jbc.M110.210765

2) Metallo, S.J. (2010). Intrinsically disordered proteins are potential drug targets, Current Opinion in Chemical Biology, 14 (4) 488. DOI: 10.1016/j.cbpa.2010.06.169

Fig 2 credit:

http://us.123rf.com/400wm/400/400/maigi/maigi1007/maigi100700002/7282219-diagram-drawing-of-multiple-arrows-flying-into-a-bullseye-target-board.jpg