Abstract
Amassments of heterochromatin in somatic cells occur in close contact with the nuclear envelope (NE) but are gapped by channel- and cone-like zones that appear largely free of heterochromatin and associated with the nuclear pore complexes (NPCs). To identify proteins involved in forming such heterochromatin exclusion zones (HEZs), we used a cell culture model in which chromatin condensation induced by poliovirus (PV) infection revealed HEZs resembling those in normal tissue cells. HEZ occurrence depended on the NPC-associated protein Tpr and its large coiled coil-forming domain. RNAi-mediated loss of Tpr allowed condensing chromatin to occur all along the NE's nuclear surface, resulting in HEZs no longer being established and NPCs covered by heterochromatin. These results assign a central function to Tpr as a determinant of perinuclear organization, with a direct role in forming a morphologically distinct nuclear sub-compartment and delimiting heterochromatin distribution.
Highlights
The interphase nucleus of eukaryotic cells is compartmentalized into distinct territories, including areas occupied by transcriptionally active euchromatin and those with highly condensed, transcriptionally more inert heterochromatin
This results in the contouring of heterochromatin exclusion zones (HEZs) at essentially all nuclear pore complexes (NPCs), thereby revealing shapes similar to those in terminally differentiated somatic cells. We have exploited this possibility of visualizing HEZs by infecting HeLa cells with poliovirus (PV). Using this in combination with RNA interference (RNAi), we found Tpr to be an element essential for HEZ establishment and for delimiting perinuclear heterochromatin distribution
NPC-associated HEZs in PV-infected cells In cell lines such as HeLa, only small amounts of heterochromatin are aligned along the nuclear envelope (NE) between neighbouring
Summary
The interphase nucleus of eukaryotic cells is compartmentalized into distinct territories, including areas occupied by transcriptionally active euchromatin and those with highly condensed, transcriptionally more inert heterochromatin. Even though different models exist of how nuclear compartmentalization might be accomplished (e.g., Jackson, 2003; Misteli, 2005, 2007; Cremer et al, 2006; Branco and Pombo, 2007; Hancock, 2007; Lanctot et al, 2007; Rippe, 2007; Schneider and Grosschedl, 2007; Richter et al, 2008), the cellular factors or mechanisms that establish the perinuclear HEZs have remained unknown While these zones are sometimes referred to as euchromatic, chromatin-free, or interchromatin channels, or as part of a reticular inter-chromosomal compartment traversing the nucleus, the designation HEZ used in this study refers to the NPC-proximal parts of these zones
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