ATP-Dependent Chromatin-Remodeling Complexes

The importance of histones and chromatin structure in the regulation of eukaryotic gene transcription has become much more widely accepted over the past few years. It has been clear for a decade that histones contribute to the regulation of transcription both in vitro and in vivo (reviewed in references 14, 34, 50, 64, and 120). More recent studies have led to the striking observation that several protein complexes involved in transcription regulation can function, at least in part, by modifying histones or altering chromatin structure (for recent reviews, see references 3, 44, 49, 51, 52, 87, 100, and 119). While it is clear that many of these protein complexes have functions in addition to chromatin modification, they illustrate the importance of chromatin structure as a part of transcription regulation mechanisms. The most widely characterized chromatin-modifying complexes studied to date can be classified into two major groups, based on their modes of action, as follows: (i) ATP-dependent complexes, which use the energy of ATP hydrolysis to locally disrupt or alter the association of histones with DNA, and (ii) histone acetyltransferase (HAT) and histone deacetylase (HDAC) complexes, which regulate the transcriptional activity of genes by determining the level of acetylation of the amino-terminal domains of nucleosomal histones associated with them. This review will focus primarily on the ATP-dependent remodeling complexes. For recent reviews of HAT and HDAC complexes, see references 5, 20, 31, and 55. Here we provide an organized listing of the ATP-dependent chromatin-remodeling complexes described to date and illustrate the relationships between their subunits. We also review the data available with regard to their mechanisms of action and promoter targeting as well as regulation of their activity. Finally, we examine the relationship between these complexes and the HAT complexes.