Regulation of ISWI chromatin remodelling activity

The packaging of the eukaryotic genome into chromatin facilitates the storage of the genetic information within the nucleus, but prevents the access to the underlying DNA sequences. Structural changes in chromatin are mediated by several mechanisms. Among them, ATP-dependent remodelling complexes belonging to ISWI family provides one of the best examples that eukaryotic cells evolved to finely regulate these changes. ISWI-containing complexes use the energy derived from ATP hydrolysis to rearrange nucleosomes on chromatin in order to favour specific nuclear reactions. The combination of regulatory nuclear factors associated with the ATPase subunit as well as its modulation by specific histone modifications, specializes the nuclear function of each ISWI-containing complex. Here we review the different ways by which ISWI enzymatic activity can be modulated and regulated in the nucleus of eukaryotic cells.     

Histone modifications influence the action of Snf2 family remodelling enzymes by different mechanisms

Alteration of chromatin structure by chromatin modifying and remodelling activities is a key stage in the regulation of many nuclear processes. These activities are frequently interlinked, and many chromatin remodelling enzymes contain motifs that recognise modified histones. Here we adopt a peptide ligation strategy to generate specifically modified chromatin templates and used these to study the interaction of the Chd1, Isw2 and RSC remodelling complexes with differentially acetylated nucleosomes. Specific patterns of histone acetylation are found to alter the rate of chromatin remodelling in different ways. For example, histone H3 lysine 14 acetylation acts to increase recruitment of the RSC complex to nucleosomes. However, histone H4 tetra-acetylation alters the spectrum of remodelled products generated by increasing octamer transfer in trans. In contrast, histone H4 tetra-acetylation was also found to reduce the activity of the Chd1 and Isw2 remodelling enzymes by reducing catalytic turnover without affecting recruitment. These observations illustrate a range of different means by which modifications to histones can influence the action of remodelling enzymes.     

Dynamic chromatin: concerted nucleosome remodelling and acetylation

The flexibility of chromatin that enables translation of environmental cues into changes in genome utilisation, relies on a battery of enzymes able to modulate chromatin structure in a highly targeted and regulated manner. The most dynamic structural changes are brought about by two kinds of enzymes with different functional principles. Changes in the acetylation status of histones modulate the folding of the nucleosomal fibre. The histone-DNA interactions that define the nucleosome itself can be disrupted by ATP-dependent remodelling factors. This review focuses on recent developments that illustrate various strategies for integrating these disparate activities into complex regulatory schemes. Synergies may be brought about by consecutive or parallel action during the stepwise process of chromatin opening or closing. Tight co-ordination may be achieved by direct interaction of (de-)acetylation enzymes and remodelling ATPases or even permanent residence within the same multi-enzyme complex. The fact that remodelling ATPases can be acetylated by histone acetyltransferases themselves suggests exciting possibilities for the co-ordinate modulation of chromatin structure and remodelling enzymes.     

Acetylation increases access of remodelling complexes to their nucleosome targets to enhance initiation of V(D)J recombination

Targeted chromatin remodelling is essential for many nuclear processes, including the regulation of V(D)J recombination. ATP-dependent nucleosome remodelling complexes are important players in this process whose activity must be tightly regulated. We show here that histone acetylation regulates nucleosome remodelling complex activity to boost RAG cutting during the initiation of V(D)J recombination. RAG cutting requires nucleosome mobilization from recombination signal sequences. Histone acetylation does not stimulate nucleosome mobilization per se by CHRAC, ACF or their catalytic subunit, ISWI. Instead, we find the more open structure of acetylated chromatin regulates the ability of nucleosome remodelling complexes to access their nucleosome templates. We also find that bromodomain/acetylated histone tail interactions can contribute to this targeting at limited concentrations of remodelling complex. We therefore propose that the changes in higher order chromatin structure associated with histone acetylation contribute to the correct targeting of nucleosome remodelling complexes and this is a novel way in which histone acetylation can modulate remodelling complex activity.