Intrinsic protein flexibility in regulation of cell proliferation: advantages for signaling and opportunities for novel therapeutics.

It is now widely recognized that intrinsically disordered (or unstructured) proteins (IDPs, or IUPs) are found in organisms from all kingdoms of life. In eukaryotes, IDPs are highly abundant and perform a wide range of biological functions, including regulation and signaling. Despite increased interest in understanding the structural biology of IDPs, questions remain regarding the mechanisms through which disordered proteins perform their biological function(s). In other words, what are the relationships between disorder and function for IDPs? Several excellent reviews have recently been published that discuss the structural properties of IDPs.1-3 Here, we discuss two IDP systems which illustrate features of dynamic complexes. In the first section, we discuss two IDPs, p21 and p27, which regulate the mammalian cell division cycle by inhibiting cyclin-dependent kinases (Cdks). In the second section, we discuss recent results from Follis, Hammoudeh, Metallo and coworkers demonstrating that the IDP Myc can be bound and inhibited by small molecules through formation of dynamic complexes. Previous studies have shown that polypeptide segments of p21 and p27 are partially folded in isolation and fold further upon binding their biological targets. Interestingly, some portions of p27 which bind to and inhibit Cdk2/cyclin A remain flexible in the bound complex. This residual flexibility allows otherwise buried tyrosine residues within p27 to be phosphorylated by nonreceptor tyrosine kinases (NRTKs). Tyrosine phosphorylation relieves kinase inhibition, triggering Cdk2-mediated phosphorylation of a threonine residue within the flexible C-terminus of p27. This, in turn, marks p27 for ubiquitination and proteasomal degradation, unleashing full Cdk2 activity which drives cell cycle progression. p27, thus, constitutes a conduit for transmission of proliferative signals via posttranslational modifications. Importantly, activation of the p27 signaling conduit by oncogenic NRTKs contributes to tumorigenesis in some human cancers, including chronic myelogenous leukemia (CML)9 and breast cancer.10 Another IDP with important roles in human cancer is the proto-oncoprotein, Myc. Myc is a DNA binding transcription factor which critically drives cell proliferation in many cell types and is often deregulated in cancer. Myc is intrinsically disordered in isolation and folds upon binding another IDP, Max and DNA. Follis, Hammoudeh, Metallo and coworkers identified small molecules which bind disordered regions of Myc and inhibit its heterodimerization with Max. Importantly, these small molecules- through formation of dynamic complexes with Myc-have been shown to inhibit Myc function in vitro and in cellular assays, opening the door to IDP-targeted therapeutics in the future. The p21/p27 and Myc systems illustrate, from different perspectives, the role of dynamics in IDP function. Dynamic fluctuations are critical for p21/p27 signaling while the dynamic free state of Myc may represent a therapeutically approachable anticancer target. Herein we review the current state of knowledge related to these two topics in IDP research.