The Drosophila Su(var)2-10 locus regulates chromosome structure and function and encodes a member of the PIAS protein family

Abstract

The conserved heterochromatic location of centromeres in higher eukaryotes suggests that intrinsic properties of heterochromatin are important for chromosome inheritance. Based on this hypothesis, mutations in Drosophila melanogaster that alter heterochromatin-induced gene silencing were tested for effects on chromosome inheritance. Here we describe the characterization of the Su(var)2-10 locus, initially identified as a Suppressor of Position-Effect Variegation. Su(var)2-10 is required for viability, and mutations cause both minichromosome and endogenous chromosome inheritance defects. Mitotic chromosomes are improperly condensed in mutants, and polytene chromosomes are structurally abnormal and disorganized in the nucleus. Su(var)2-10 encodes a member of the PIAS protein family, a group of highly conserved proteins that control diverse functions. SU(VAR)2-10 proteins colocalize with nuclear lamin in interphase, and little to no SU(VAR)2-10 is found on condensed mitotic chromosomes. SU(VAR)2-10 is present at some polytene chromosome telomeres, and FISH analyses in mutant polytene nuclei revealed defects in telomere clustering and telomere-nuclear-lamina associations. We propose that Su(var2-10 controls multiple aspects of chromosome structure and function by establishing/maintaining chromosome organization in interphase nuclei. Heterochromatin is an enigmatic component of higher eukaryotic genomes. The paucity of genes and the abundance of repetitive sequences in heterochromatin contribute to it being described as functionally inert. However, heterochromatin houses essential single copy genes (Liu et al. 1998) and the rDNA loci, the most highly transcribed genes in the genome (for review, see Williams and Robbins 1992). In addition, the centromerethe site of kinetochore formation, spindle attachment, and checkpoint control during mitosis and meiosis-is usually buried deep within heterochromatin. Elegant studies from a variety of organisms (Goday and Pimpinelli 1989;Bass et al. 1997) indicate that heterochromatin plays other important roles in chromosome inheritance. For example, heterochromatic homology is required for faithful homolog pairing and chromosome segregation during male and female meiosis in Drosophila (McKee and Karpen 1990; Dernburg et al. 1996b; Karpen et al. 1996).Heterochromatin can silence the expression of genes that are normally found in euchromatin, resulting in a phenomenon known as position-effect variegation (PEV). PEV manifests as the mosaic or variegated expression of an affected locus owing to its abnormal juxtaposition near centric heterochromatin or telomeres (for review, see Wallrath 1998). Changes in the chromatin structure surrounding a variegating gene motivated the hypothesis that PEV is caused by spreading of the heterochromatic state into neighboring regions of euchromatin (for review, see Wakimoto 1998). However, PEV also occurs when heterochromatic associations produce large-scale alterations in the nuclear organization of chromosomes. For example, a large block of heteroc...

PIASy mediates SUMO-2 conjugation of Topoisomerase-II on mitotic chromosomes

“…Su(var)2-10 was initially identified as a Suppressor of Position-Effect Variegation (Wustmann et al, 1989). Mutations in this gene cause both minichromosome and endogenous chromosome inheritance defects (Hari et al, 2001): Su(var)2-10 mutants show aberrant condensation of mitotic chromosomes, as well as faulty chromosome organization within interphase nuclei, leading to the proposal that Su(var)2-10 controls multiple aspects of chromosome structure and function throughout the cell cycle.…”

“…Collectively, these phenotypes suggest that SUMO modification has an important role in centromeric function in S. pombe, probably through maintenance of heterochromatic structures, but that cells compensate for the loss of Pli1p through alternative mechanisms. Most strikingly, the closest homologue of PIASy in Drosophila, Su(var)2-10, encodes an essential protein (Hari et al, 2001). Su(var)2-10 was initially identified as a Suppressor of Position-Effect Variegation (Wustmann et al, 1989).…”

Nse1, Nse2, and a Novel Subunit of the Smc5-Smc6 Complex, Nse3, Play a Crucial Role in Meiosis

“…Interestingly, the Smc5ϩ6 non-SMC subunits Nse1 and Nse2 contain zinc finger domains related to the RING and Miztype domains, respectively (Fujioka et al, 2002;. This suggests that they are not only structural components of the complex, but likely act as E3-ligases to modify target proteins with ubiquitin or SUMO, thereby modulating their functions (Wu et al, 1997;Freemont, 2000;Joazeiro and Weissman, 2000;Hari et al, 2001;Takahashi et al, 2001).…”

Biological functions of the ISWI chromatin remodeling complex NURF

“…However, STAT activity is also influenced by a number of inhibitory, STAT-binding proteins. Among these are the suppressor of cytokine signaling (SOCS; Callus and Mathey-Prevot 2002) and protein inhibitor of activated STAT (PIAS) family of inhibitors (Betz et al 2001;Hari et al 2001). It is possible that NURF is required for expression of one such inhibitor, and that loss of NURF activates the HOP/STAT92E cascade by removing a STAT inhibitor.…”