HMG-D and Histone H1 Interplay during Chromatin Assembly and Early Embryogenesis

Abstract

HMG-D is an abundant chromosomal protein associated with condensed chromatin during the first nuclear cleavage cycles of the developing Drosophila embryo. We previously suggested that HMG-D might substitute for the linker histone H1 in the preblastoderm embryo and that this substitution might result in the characteristic less compacted chromatin. We have now studied the association of HMG-D with chromatin using a cellfree system for chromatin reconstitution derived from The vertebrate high mobility group proteins of the HMGB class (formerly termed HMG1/2) (1) are relatively abundant nuclear DNA-binding proteins that bend DNA substantially and appear to act primarily as architectural facilitators in the assembly of nucleoprotein complexes (recently reviewed in Refs. 2-4). There is direct evidence that these proteins facilitate the binding of transcription factors to their cognate sites both in vitro and in vivo, but any involvement in the assembly of histone-DNA complexes is less well documented. Circumstantial evidence indicates that certain functions of the vertebrate HMGB proteins (5-7) may parallel those of the linker histone H1 (8). Both have been shown to bind to linker DNA sequences (7, 9, 10), and both stabilize and bind to four-way junctions (11)(12)(13)(14). HMG-D is one of two Drosophila proteins closely related to the vertebrate HMGB proteins but in contrast to these proteins contains only a single HMG DNA-binding domain (6,15,16). The HMG domain is followed by a region that has basic sequences similar to the C-terminal domain of histone H1 and a short C-terminal acidic stretch also seen in HMGB proteins. The NMR structure of the HMG domain from HMG-D is very similar to the previously determined structure of the B domain of HMG1, which revealed the characteristic L-shaped fold formed by three ␣-helices (17-21).High mobility group proteins of the HMGB family bind DNA with little sequence specificity but recognize structural features in DNA (22-24), including cruciforms, kinks, DNA bulges, and bends (22,23,(25)(26)(27)(28). DNA footprint analysis and mutagenesis experiments suggest that HMG domain proteins bind in the minor groove (29). In addition, both crystallization and NMR studies show that HMG-D bending is achieved by the intercalation of hydrophobic residues at two different base steps (21, 30).The addition of histone H1 protein to an extended array of nucleosome core particles promotes its folding into a fiber with an approximate width of 30 nm (31-33). During early Drosophila development, histone H1 is not detectable (34 -36) until nuclear cycles 7/8 (37). Therefore, H1 is not involved in chromatin condensation in these earliest phases of Drosophila development characterized by rapid condensation-decondensation cycles. We have previously suggested that HMG-D could function as a linker protein in the absence of histone H1 in Drosophila (37). In support of this hypothesis, Wolffe and colleagues (10, 38) have postulated a similar role for the Xenopus HMG B1 protein. Thus, as the maternal pool of H...

Developmental role for ACF1-containing nucleosome remodellers in chromatin organisation

“…Second, nuclei of the early cleavage phase, which contain high levels of CHRAC/ACF, do not contain H2Av, but do display a uniform distribution of HP1a and are larger than nuclei after cellularisation and zygotic activation (Ner et al, 2001). This is similar to our observations of ectopic expression of ACF1 in salivary glands nuclei.…”

The TAGteam DNA motif controls the timing ofDrosophilapre-blastoderm transcription

“…Analysis of the mechanism by which the TAGteam sites act to influence pre-CB transcription may provide unique insights into transcriptional regulation, since the nuclear environment during this early period appears to differ significantly from that later when general transcription begins. For example, during this early period in Drosophila, the high mobility group protein D (HMG-D) stands in for histone H1 (Ner et al, 2001;Ner and Travers, 1994). Moreover, it has been shown that Xenopus embryos have a uniquely large excess of core histones prior to the midblastula transition (MBT) that may play an important part in pre-MBT transcriptional quiescence (Prioleau et al, 1994).…”

Changes in gene expression associated with acclimation to constant temperatures and fluctuating daily temperatures in an annual killifishAustrofundulus limnaeus

“…For example, seasonal shifts in thermal tolerance may be associated with changes in post-translational modification of the HMGB1 protein. The replacement of the linker histone H1 with HMGB1 during early development (Nightingale et al, 1996;Ner et al, 2001) may also explain the extreme temperature sensitivity of early embryos due to a loss of chromatin architecture, as HMGB1 proteins are easily denatured at biologically relevant temperatures. We believe that further investigations into the role of HMGB1 in temperature acclimation are likely to lead to a new understanding of how eukaryotic cells maintain homeostasis in the face of an ever-changing thermal environment.…”