Distinct ‘safe zones’ at the nuclear envelope ensure robust replication of heterochromatic chromosome regions

Abstract

Chromosome replication and transcription occur within a complex nuclear milieu whose functional subdomains are beginning to be mapped out. Here we delineate distinct domains of the fission yeast nuclear envelope (NE), focusing on regions enriched for the inner NE protein, Bqt4, or the lamin interacting domain protein, Lem2. Bqt4 is relatively mobile around the NE and acts in two capacities. First, Bqt4 tethers chromosome termini and the mat locus to the NE specifically while these regions are replicating. This positioning is required for accurate heterochromatin replication. Second, Bqt4 mobilizes a subset of Lem2 molecules around the NE to promote pericentric heterochromatin maintenance. Opposing Bqt4-dependent Lem2 mobility are factors that stabilize Lem2 beneath the centrosome, where Lem2 plays a crucial role in kinetochore maintenance. Our data prompt a model in which Bqt4-rich nuclear subdomains are ‘safe zones’ in which collisions between transcription and replication are averted and heterochromatin is reassembled faithfully.

Integrity of a heterochromatic domain ensured by its boundary elements

“…Recently, two studies have suggested that the localization of the mating-type region at the nuclear periphery might be part of its silencing mechanism. First, it was reported that H3K9me is considerably reduced in the mating-type region in mutants lacking the nuclear membrane protein Bqt4 (35). Second, mutants in the nuclear rim protein Amo1 were also reported to reduce H3K9me in the mat2-mat3 region, in particular outside cenH, attributed to defective maintenance (17).…”

“…In the case of Amo1, reduced H3K9me was accompanied by partial delocalization from the nuclear periphery (17). In the case of Bqt4, reduced H3K9me was accompanied by partial delocalization from the nuclear periphery specifically during replication (35). Because anchoring at the nuclear periphery by the boundary elements could be part of their mechanism of action, we conducted an epistasis analysis to investigate the effects of combining mutations at IR-R with mutations in potential anchors at the nuclear envelope.…”

Nuclear Envelope Proteins Modulating the Heterochromatin Formation and Functions in Fission Yeast

“…Lem2 localizes at the INM and beneath the SPB, but biased to the SPB [128,129]. These localizations depend on Csi1 for the SPB and Bqt4 for the NE, respectively.…”

“…Lem2 also interacts with chromatin regions close to the sub-telomeric heterochromatin region [124]. ChIP-seq and ChIP-chip analyses show that Lem2 binds to the telomere side of sub-telomeric heterochromatin at tel3L and tel3R, but no significant enrichment at tel1L and tel2L regions [33,124,129]. Loss of Lem2 shows de-repression of tlh1/2 genes located within the sub-telomeric region but does not affect H3K9me2 levels in this region [33].…”

“…This domain binds to DNA and also interacts with the Bqt4-binding motif shared in Lem2, Sad1, and Rap1 in a competitive manner [133]. In cells with deleted Csi1 and Bqt4, Lem2 shows a biased localization to the SPB, suggesting that other factors are also involved in the Lem2 localization to the SPB [129]. One of those factors appears to be Nur1 because Lem2 and Nur1 depend on each other for SPB localization in fission yeast Schizosaccharomyces japonicus [135].…”