An HP1-like protein is missing from transcriptionally silent micronuclei of Tetrahymena

Abstract

We report the identification and cloning of a 28-kDa polypeptide (p28) in Tetrahymena macronuclei that shares several features with the well studied heterochromatinassociated protein HP1 from Drosophila. Notably, like HP1, p28 contains both a chromodomain and a chromoshadow domain. p28 also shares features with linker histone H1, and like H1, p28 is multiply phosphorylated, at least in part, by a proline-directed, Cdc2-type kinase. As such, p28 is referred to as Hhp1p (for H1͞HP1-like protein). Hhp1p is missing from transcriptionally silent micronuclei but is enriched in heterochromatin-like chromatin bodies that presumably comprise repressed chromatin in macronuclei. These findings shed light on the evolutionary conserved nature of heterochromatin in organisms ranging from ciliates to humans and provide further evidence that HP1-like proteins are not exclusively associated with permanently silent chromosomal domains. Our data support a view that members of this family also associate with repressed states of euchromatin.

A Dicer-like protein in Tetrahymena has distinct functions in genome rearrangement, chromosome segregation, and meiotic prophase

“…Accumulation of HP1/Swi6 and methylated H3 Lys 9 on heterochromatin are dependent on an RNAi-related mechanism in S. pombe Volpe et al 2002), Arabidopsis (Zilberman et al 2003), and Drosophila (Pal-Bhadra et al 2004). However, none of the currently known Tetrahymena chromodomain proteins (Madireddi et al 1996;Smothers et al 1997;Huang et al 1998;Nikiforov et al 2000) or H3 Lys 9 methylation (Strahl et al 1999;Taverna et al 2002) has been detected in the Mic of vegetative cells. Again, it is possible that small amounts of centromere sequence per genome might have prevented detection of these heterochromatin-associated factors.…”

RNAi-dependent H3K27 methylation is required for heterochromatin formation and DNA elimination in Tetrahymena

“…H3K27me3 was present at much lower abundance in the transcriptionally active macronucleus and mostly decorated chromatin bodies (Fig. 1B, first panel, open arrowhead), which are regions of condensed chromatin marked by Hhp1p, a HP1 homolog, and associated with transcriptional silencing (Huang et al 1998(Huang et al , 1999.…”

Class I Histone Deacetylase Thd1p Affects Nuclear Integrity in Tetrahymena thermophila

“…A strain carrying a Neo r disruption cassette integrated at the HHP1 locus was used as a control for effects of pregrowth in paromomycin. Hhp1 is a protein that localizes to heterochromatin, and ⌬HHP1 cells were previously shown to have no vegetative growth phenotype (22). These cells were treated identically to ⌬THD1 cells and were referred to as control cells.…”

“…Control cells are resistant to paromomycin due to complete replacement of the heterochromatin-associated protein gene HHP1 with a disruption cassette containing the neomycin resistance gene (Neo r ). These cells were previously shown to have no cytological phenotypes during vegetative growth (22) and are thus used as a control for any effects due to pregrowth of ⌬THD1 cells in 300 g/ml of paromomycin. In frame iii of panel A, arrows point to macronuclear "pockets" that stain less intensely with DAPI.…”