Mutation in a heterochromatin-specific chromosomal protein is associated with suppression of position-effect variegation in Drosophila melanogaster.

Abstract

We report here that a point mutation in the gene which encodes the heterochromatin-specific nonhistone chromosomal protein HP-I in Drosophila melanogaster is associated with dominant suppression of position-effect variegation. The mutation, a G-to-A transition at the first nucleotide of the last intron, causes missplicing of the HP-1 mRNA. This suggests that heterochromatin-specific proteins play a central role in the gene suppression associated with heterochromatic position effects.The partitioning of eukaryotic chromosomes into regions which differ in their degrees of compaction has long been appreciated. Most of the transcriptionally active chromatin appears to decondense after mitotic telophase into euchromatin, but a substantial fraction of chromosomal material remains condensed as heterochromatin. Heterochromatin replicates relatively late in the cell cycle and, in tissues which undergo polytenization, the heterochromatin may be underreplicated.The potential of heterochromatin formation to result in transcriptional inactivation is inferred from two genetic phenomena: Barr-body formation (Lyonization) in mammalian females and position-effect variegation in a variety of organisms (reviewed in ref. 1). In both cases chromosomal regions which are euchromatic under some circumstances assume the morphology of heterochromatin. The condensed structure observed in these cases is strongly correlated with transcriptional inactivity.In Drosophila, the genetic dissection of heterochromatin is aided by the availability of numerous rearrangements which lead to variegated expression of euchromatic genes that have come to be relocated near the heterochromatic breakpoint. A number of loci have been identified which, when mutated, act as dominant modifiers of such variegating position effects (2-7). Many of these loci are believed to encode chromatin proteins or factors that modify chromatin structure (see refs. 8 and 9 for recent reviews).A heterochromatin-specific chromosomal protein called HP-1 has been identified and characterized in D. melanogaster (10, 11). A cDNA encoding this protein has been cloned (10), and the gene has been localized to cytological position 29A on the polytene chromosome map. In this report, we provide the sequence of the gene$, identifying exon and intron boundaries, and present molecular evidence that a point mutation at one boundary, causing missplicing of the HP-1 pre-mRNA, is associated with dominant suppression of heterochromatic position effect. This indicates a requirement for HP-1 protein in generating normal heterochromatin structure. MATERIALS AND METHODSDrosophila Stocks. Su(var)205/In(2LR)CyO and the iso-2nd line (marked with b It rl) were obtained from T. Grigliatti (University of British Columbia, Vancouver). Flies were cultured in half-pint plastic bottles at room temperature, using a cornmeal-based medium supplemented with dried bakers' yeast.Northern Blot Analysis. Total nucleic acids were purified from several flies essentially according to the method of Meyerowitz and H...

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Subcellular distribution of HP1 proteins is altered in ICF syndrome

“…In addition to methylated DNA, the heterochromatin protein 1 (HP1) is a major component of heterochromatin and plays a key role in its formation and maintenance in Drosophila 7,8 and in various mammalian species. 9 -11 In humans, three HP1 protein isoforms, HP1a, HP1b and HP1g, have been isolated, which all co-localize with constitutive heterochromatin, suggesting that they are involved in its organization.…”

Direct interaction with a nuclear protein and regulation of gene silencing by a variant of the Ca 2+ -channel β 4 subunit

“…Gene silencing is one mechanism for achieving an appropriate gene-expression pattern (1). Heterochromatin proteins 1 (HP1s) were first identified as responsible for position-effect variegation, a type of gene silencing (2). HP1s specifically recognize and directly bind a lysine residue near the carboxyl terminus of histone H3 that is methylated by a unique methyltransferase, SUV39H1 (3)(4)(5).…”