br HMG proteins mobile modulators of chromatin structure and

HMG proteins: mobile modulators of chromatin structure and cellular phenotype The term “High Mobility Group” was originally coined for the HMG proteins because of their unusual solubility properties, their small size and their rapid mobility, relative to other chromatin proteins, during gel electrophoretic separations [1]. Fortuitously, recent real-time imaging experiments with living 4-Hydroxytamoxifen sale have demonstrated that, as their name implies, HMG proteins are indeed highly mobile within the nucleus where they are part of a dynamic network of architectural components that modulate the structure of chromatin and thereby affect multiple DNA-dependent activities [30,31,61,62,119]. All HMG proteins have similar biochemical and biophysical properties and each is characterized by the presence of a long, negatively charged carboxy terminal tail that serves a regulatory function(s) [27,61]. The HMG families also participate in many common biological processes such as involvement in embryonic development, regulation of transcription and modulation of DNA repair. Nevertheless, the HMGA, HMGB and HMGN families are distinguished from each other by their unique DNA-binding motifs, by their preferred binding substrates, by the changes they induce in their substrates and by the different subsets of cellular processes they influence. The distinctive constellation of features that characterize different HMG families determines how individual proteins differentially affect chromatin structure, nuclear metabolism and cellular phenotypes.
HMGA proteins and localized nucleosome structure The canonical DNA-binding domain of the HMGA family (formerly called HMG-I/Y) is a palindromic amino motif called the “AT-hook” that binds preferentially to the minor groove of short stretches of A/T-rich B-form DNA via recognition of structure rather than nucleotide sequence [138]. In mammals there are two subfamilies of HMGA proteins, HMGA1 and HMGA2 [132]. The HMGA1 subfamily consists of three proteins (HMGA1a, HMGA1b and HMGA1c) that are produced by translation of alternatively spliced transcripts derived from a single gene. The HMGA2 subfamily, on the other hand, contains only one protein that is coded for by a different gene. All HMGA proteins contain three AT-hooks except for the rare HMGA1c variant which only has two. A unique feature of the HMGA proteins is that, as free molecules, they are disordered random coils and only assume any type of defined secondary structure after binding to DNA or other substrates [73]. This intrinsic flexibility, combined with a disordered-to-ordered structural transition following substrate binding, is thought to play an important role in allowing the HMGA proteins to participate in a wide variety of biological processes [45,133]. One of the best characterized biological roles of the HMGA proteins is in coordinating the formation of multi-subunit, stereospecific protein–DNA complexes called enhanceosomes [99] on A/T-rich promoter regions of certain inducible genes during their transcriptional activation [48,157]. HMGA proteins can bend, straighten, unwind and supercoil DNA substrates in vitro without requiring energy (i.e., ATP) input, capabilities that likely contribute to their ability to participate in enhanceosome formation and the regulation of gene expression in vivo[135]. In addition to recognizing the structure of the narrow minor groove of A/T-rich DNA as a target for binding, HMGA proteins also recognize and bind to non-B-form DNAs with unusual structural features. These binding substrates include synthetic four-way and three-way junctions [66,68], bent and supercoiled DNAs [110], base-unpaired regions of A/T-rich DNA [91] and distorted or flexible regions of DNA on isolated nucleosome core particles [126,139,141]. In the case of nucleosomes, HMGA binding induces localized changes in the rotational setting of DNA on the surface of reconstituted core particles, a restricted form of chromatin remodeling that is not driven by ATP hydrolysis [141]. Peptide domain swap experiments employing hybrid recombinant proteins have demonstrated that the AT-hooks are the regions of HMGA proteins responsible for nucleosome binding [12].