The AT-Hook Protein D1 Is Essential for Drosophila melanogaster Development and Is Implicated in Position-Effect Variegation

Abstract

We have analyzed the expression pattern of the D1 gene and the localization of its product, the AT hookbearing nonhistone chromosomal protein D1, during Drosophila melanogaster development. D1 mRNAs and protein are maternally contributed, and the protein localizes to discrete foci on the chromosomes of early embryos. These foci correspond to 1.672-and 1.688-g/cm 3 AT-rich satellite repeats found in the centromeric heterochromatin of the X and Y chromosomes and on chromosomes 3 and 4. D1 mRNA levels subsequently decrease throughout later development, followed by the accumulation of the D1 protein in adult gonads, where two distributions of D1 can be correlated to different states of gene activity. We show that the EP473 mutation, a P-element insertion upstream of D1 coding sequences, affects the expression of the D1 gene and results in an embryonic homozygous lethal phenotype correlated with the depletion of D1 protein during embryogenesis. Remarkably, decreased levels of D1 mRNA and protein in heterozygous flies lead to the suppression of position-effect variegation (PEV) of the white gene in the white-mottled (w m4h ) X-chromosome inversion. Our results identify D1 as a DNA-binding protein of known sequence specificity implicated in PEV. D1 is the primary factor that binds the centromeric 1.688-g/cm 3 satellite repeats which are likely involved in whitemottled variegation. We propose that the AT-hook D1 protein nucleates heterochromatin assembly by recruiting specialized transcriptional repressors and/or proteins involved in chromosome condensation.

The Multi-AT-Hook Chromosomal Protein of Drosophila melanogaster, D1, Is Dispensable for Viability

“…It also binds to the complex 1.688 g/cm 3 satellite, which is chiefly a 359-bp repeat and 69% AT in composition. In agreement with this work, a heterochromatic localization was observed for D1 in both mitotic and interphase diploid cells by immunostaining (Renner et al 2000; 1 Aulner et al 2002). As revealed by immunostaining to salivary gland polytene chromosomes, D1 shows a less predominant localization to euchromatic sites, which could reflect its binding to interspersed AT tracts (Alfageme et al 1976;Rodriguez Alfageme et al 1980).…”

“…To this end, I examined the lethality of the P-insertion lines, carried out genetic screens to isolate D1 null mutants, and performed genetic tests to study the effects of loss of D1 protein. Although other investigators reported that the D1 gene is essential (Aulner et al 2002), the studies described herein demonstrated that D1 is not required for viability or fertility. In addition, it is not a suppressor of position effect variegation.…”

Coordinate activation of inflammatory gene networks, alveolar destruction and neonatal death in AKNA deficient mice

“…Although homeobox proteins have been established as master gene transcription factors [46], DNA architecturemodulating AT-hook proteins constitute an evolutionarily conserved family with analogous global gene-regulation functions [1,2,4,6,9]. In accordance with this notion, AT-hook proteins bind a broad spectrum of AT-rich sequence-containing promoters [5] and regulate pathwayspecific gene expression networks [3,8,47,48].…”

“…Examples of such regulators include AThook proteins, a family of transcription factors that target genes with A/T-rich promoters [4][5][6] and orchestrate their activation or repression [7][8][9][10][11]. Notably, AT-hook proteins have been mainly recognized within the high mobility group (HMG) family [12,13].…”

Discovery of Cellular Regulation by Protein Degradation

“…Another 1982 study by Levinger and myself showed that the D1 protein preferentially binds to (A ϩ T)-rich DNA in vitro, including the 1.672 and 1.688 satellite DNA repeats, and is a component of isolated nucleosomes containing these centromeric satellite DNAs (106). Later studies (107,108) indicated that D1, which contains 10 copies of the "AT-hook" domain, a motif that interacts with the minor groove of AT-rich DNA, is a part of heterochromatin-associated complexes that mediate transcriptional repression.…”