The Myc proteins comprise a family of ubiquitous regulators of gene expression implicated in over half of all human cancers. They interact with a large number of other proteins, such as transcription factors, chromatin-modifying enzymes and kinases. Remarkably, few of these interactions have been characterized structurally. This is at least in part due to the intrinsically disordered nature of Myc proteins, which adopt a defined conformation only in the presence of binding partners. Owing to this behaviour, crystallographic studies on Myc proteins have been limited to short fragments in complex with other proteins. Most recently, we determined the crystal structure of Aurora-A kinase domain bound to a 28-amino acid fragment of the N-Myc transactivation domain. The structure reveals an α-helical segment within N-Myc capped by two tryptophan residues that recognize the surface of Aurora-A. The kinase domain acts as a molecular scaffold, independently of its catalytic activity, upon which this region of N-Myc becomes ordered. The binding site for N-Myc on Aurora-A is disrupted by certain ATP-competitive inhibitors, such as MLN8237 (alisertib) and CD532, and explains how these kinase inhibitors are able to disrupt the protein–protein interaction to affect Myc destabilization. Structural studies on this and other Myc complexes will lead to the design of protein–protein interaction inhibitors as chemical tools to dissect the complex pathways of Myc regulation and function, which may be developed into Myc inhibitors for the treatment of cancer.
The EMLs are a conserved family of microtubule-associated proteins (MAPs). The founding member was discovered in sea urchins as a 77-kDa polypeptide that co-purified with microtubules. This protein, termed EMAP for echinoderm MAP, was the major non-tubulin component present in purified microtubule preparations made from unfertilized sea urchin eggs [J. Cell Sci. (1993) 104 , 445–450; J. Cell Sci. (1987) 87 (Pt 1), 71–84]. Orthologues of EMAP were subsequently identified in other echinoderms, such as starfish and sand dollar, and then in more distant eukaryotes, including flies, worms and vertebrates, where the name of ELP or EML (both for EMAP-like protein) has been adopted [BMC Dev. Biol. (2008) 8 , 110; Dev. Genes Evol. (2000) 210 , 2–10]. The common property of these proteins is their ability to decorate microtubules. However, whether they are associated with particular microtubule populations or exercise specific functions in different microtubule-dependent processes remains unknown. Furthermore, although there is limited evidence that they regulate microtubule dynamics, the biochemical mechanisms of their molecular activity have yet to be explored. Nevertheless, interest in these proteins has grown substantially because of the identification of EML mutations in neuronal disorders and oncogenic fusions in human cancers. Here, we summarize our current knowledge of the expression, localization and structure of what is proving to be an interesting and important class of MAPs. We also speculate about their function in microtubule regulation and highlight how the studies of EMLs in human diseases may open up novel avenues for patient therapy.
In eukaryotic cells, the peak of protein phosphorylation occurs during mitosis, switching the activities of a significant proportion of proteins and orchestrating a wholesale reorganization of cell shape and internal architecture. Most mitotic protein phosphorylation events are catalysed by a small subset of serine/threonine protein kinases. These include members of the Cdk (cyclin-dependent kinase), Plk (Polo-like kinase), Aurora, Nek (NimA-related kinase) and Bub families, as well as Haspin, Greatwall and Mps1/TTK. There has been steady progress in resolving the structural mechanisms that regulate the catalytic activities of these mitotic kinases. From structural and biochemical perspectives, kinase activation appears not as a binary process (from inactive to active), but as a series of states that exhibit varying degrees of activity. In its lowest activity state, a mitotic kinase may exhibit diverse autoinhibited or inactive conformations. Kinase activation proceeds via phosphorylation and/or association with a binding partner. These remodel the structure into an active conformation that is common to almost all protein kinases. However, all mitotic kinases of known structure have divergent features, many of which are key to understanding their specific regulatory mechanisms. Finally, mitotic kinases are an important class of drug target, and their structural characterization has facilitated the rational design of chemical inhibitors.