MRC National Institute for Medical Research, London
Science for Health
Motif selectivity and mitotic signaling
05 July 2011
The initiation, execution and termination of mitosis are complex processes that, like so many other cellular activities, are regulated by specific classes of protein kinases. Although the major players have been known for some time, questions concerning the interplay between these enzymes, their specificity and identity of their target substrates have remained largely obscure.
Since the discovery of cyclin-dependent kinases (CDKs) as master regulators of the cell-cycle, a host of additional kinases that act in parallel with CDKs have been identified. At the 'business end' of the cell-cycle is mitosis where, following duplication of the genome in the preceding S-phase, cells divide accurately partitioning the correct DNA complement into each of the resulting daughter cells. These are complex processes but they are, in the main, regulated by members of just four kinase families: CDKs (CDK1/cyclinB), NIMA-related kinases (Nek2), Polo kinases (mainly Plk1) and Aurora kinases (A & B). The regulatory enzymes may have overlapping cellular localisations during the mitotic cycle.
Steve Smerdon, from NIMR’s Divison of Molecular Structure, has collaborated with laboratories from the US and across Europe, led by Michael Yaffe’s group at MIT, to look into this issue. Using a method called “positional scanning oriented peptide library screening” the specificity of these kinases was investigated and the overlap in preferred substrate motifs was compared to their pattern of sub-cellular distribution. This showed that localisation and kinase motif preference occupy orthogonal 'spaces', providing the net specificity required for correct mitotic progression. Thus, kinases with similar motifs are localised differently while those that co-localise have distinct specificities and, indeed, have evolved to negatively select against each other’s motifs. These new data potentially provide both a systems-level mechanism for regulating substrate phosphorylation and a coordinated evolutionary pressure to maintain discriminatory substrate motifs and localisations for major mitotic kinases and their substrates.
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Mitotic kinase functionality represented by Venn diagrams of localization space and motif space. In localisation space, each circle represents the total subcellular locales available to that kinase. In motif space, each circle represents the sequences that can be phosphorylated by the kinase. In this representation, a major mitotic kinase can overlap with every other kinase in at most one of these two spaces.
The human genome encodes over 500 kinases that, in turn, phosphorylate virtually every other protein in a human cell at some time or other. Although 500 is a large number, it is insufficient to provide enough specificity to allow a 'one functional site - one kinase' type model to operate. So, kinases are, at best, pseudo-specific at the level of substrate recognition. How this imprecision is accommodated to allow a rather more precise functional readout remains a largely unanswered problem. At least for one important biological process, that of cell division, we now have a clearer view of some of the elegantly simple molecular mechanisms that are at work.
Steve Smerdon
Original article
Spatial exclusivity combined with positive and negative selection of phosphorylation motifs is the basis for context-dependent mitotic signaling (2011)
Jes Alexander, Daniel Lim, Brian A. Joughin, Björn Hegemann, James R. A. Hutchins, Tobias Ehrenberger, Frank Ivins, Fabio Sessa, Otto Hudecz, Erich A. Nigg, Andrew M. Fry, Andrea Musacchio, P. Todd Stukenberg, Karl Mechtler, Jan-Michael Peters, Stephen J. Smerdon, and Michael B. Yaffe
Science Signaling, 4(179): ra42. Publisher abstract
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