Chromosomes: Nonhistone Proteins


Chromatin is a dynamic structure alternating between condensed and decondensed states, depending on the stage of the cell cycle. Nonhistone proteins play essential roles in both states, including higher‐order chromosome organization, differential chromosome packaging and local chromatin architecture.

Keywords: scaffold; SMC; metaphase–anaphase switch; gene silencing; architectural transcription factor

Figure 1.

SMC proteins. (a) SMC proteins consist of five protein domains. There is an amino‐terminal Walker A motif and a carboxyl‐terminal Walker B motif, connected by two coiled coils, which are separated in the middle by a globular hinge (shown in blue). The Walker A and B motifs (coloured red) form composite ATP binding sites. (b) SMC proteins in eukaryotes are thought to form antiparallel heterodimers and it is postulated that the V‐shape may be modulated in relation to ATP (shown as green spheres) or nucleotide binding. In (c) the cohesin SMC dimers have a more ‘open’ V‐shape, allowing the dimers to link sister chromatids directly. Non‐SMC components (shown in yellow) usually associate with the globular termini of one arm of the dimer. In (d) the condensin SMC dimers have a more ‘closed’ V‐shape; again the non‐SMC components (in yellow) preferentially associate with the globular termini of one arm.

Figure 2.

Cohesin behaviour in sister chromatid and centromere cohesion in higher eukaryotes. As sister chromatids emerge from replication forks, cohesin (shown in yellow) binds and maintains sister chromatid cohesion. Coincident with condensin binding (not shown), cohesin dissociates and by prophase the majority of cohesin has dissociated from the sister chromatids and centromeres by a ubiquitin‐independent process. The metaphase/anaphase switch requires the activation of APC by polo‐like kinase. This allows APC to ubiquitinate securin, resulting in securin degradation, which in turn releases separase activity (shown as scissors). The metaphase/anaphase transition involves the proteolytic cleavage of the Scc1 subunit of the cohesin complex (shown in yellow) by separase. In mammals, however, it is unknown whether the residual cohesin remaining around the centromere is sufficient to maintain centromere cohesion, or whether other proteins (e.g. certain scaffold proteins) contribute to this function. Cohesin is shown in yellow, centromeres/kinetochores are coloured red, microtubules are denoted as brown converging lines and separase is shown as scissors.

Figure 3.

Schematic representation of HP1. HP1 consists of an amino‐terminal chromo domain (CHD) and a carboxyl terminal chromosome shadow domain (CSD). Both these domains are shown in red and are separated by a hinge shown in blue.

Figure 4.

(a) A cartoon of the ‘enhanceosome’. HMGA or HMGB (denoted by red arrows) bind to and bend DNA, bringing transcription factors (in various shades of blue) into contact with each other, resulting in cooperative interactions and recruitment of RNA polymerase II (green sphere), thereby allowing transcription to be initiated. (b) Schematic representation of HMGN. HMGN contains a bipartite nuclear localization signal (NLS shown in yellow), a nucleosome‐binding domain (NBD shown in red) and a carboxyl‐terminal chromatin unfolding domain (CHUD also in red).



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Further Reading

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Struhl K (2001) A paradigm for precision. Science 293: 1054–1055.

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Holland, Katalin A(May 2003) Chromosomes: Nonhistone Proteins. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0001158]