Dictyostelium Sun‐1 Connects the Centrosome to Chromatin and Ensures Genome Stability

Abstract

The centrosome-nucleus attachment is a prerequisite for faithful chromosome segregation during mitosis. We addressed the function of the nuclear envelope (NE) protein Sun-1 in centrosome-nucleus connection and the maintenance of genome stability in Dictyostelium discoideum. We provide evidence that Sun-1 requires direct chromatin binding for its inner nuclear membrane targeting. Truncation of the cryptic N-terminal chromatinbinding domain of Sun-1 induces dramatic separation of the inner from the outer nuclear membrane and deformations in nuclear morphology, which are also observed using a Sun-1 RNAi construct. Thus, chromatin binding of Sun-1 defines the integrity of the nuclear architecture. In addition to its role as a NE scaffold, we find that abrogation of the chromatin binding of Sun-1 dissociates the centrosome-nucleus connection, demonstrating that Sun-1 provides an essential link between the chromatin and the centrosome. Moreover, loss of the centrosomenucleus connection causes severe centrosome hyperamplification and defective spindle formation, which enhances aneuploidy and cell death significantly. We highlight an important new aspect for Sun-1 in coupling the centrosome and nuclear division during mitosis to ensure faithful chromosome segregation.Key words: aneuploidy, centrosome hyperamplification, nuclear envelope architecture, spindle formation defects, Unc-84 The nuclear envelope (NE) separates the nuclear compartment from the cytoplasm. It is composed of two membranes: the outer nuclear membrane (ONM) and the inner nuclear membrane (INM). The lumen between the two membranes is the perinuclear space (PNS). The ONM is continuous with the endoplasmic reticulum (ER), whereas the INM harbors a unique set of proteins. INM and ONM proteins can interact within the PNS. Underneath the INM, the nuclear lamina is located, which is formed by intermediate filament (IF) proteins and associated proteins. The lamina forms the nucleoskeleton and associates with the INM, chromatin and nuclear pore complexes. Proteins of the NE have important roles. They are involved in nuclear migration and positioning and are essential for many processes such as mitosis, meiosis, differentiation and cell migration. Furthermore, several of the NE proteins have been associated with inherited diseases (1,2). Research in mammalian cells and in Caenorhabditis eleganshas identified conserved components of the NE that link the nucleoskeleton to the cytoskeleton. In C. elegans, two putative INM proteins, matefin/SUN-1 and UNC-84, bind to the nuclear lamina and extend their C-terminus into the PNS where they interact with the C-termini of KASH domain proteins (Klarsicht/Anc-1/Syne homology, designated KASH domain). Matefin/SUN-1 and UNC-84 belong to the SUN family of proteins based on the presence of the conserved SUN (Sad1/UNC-84 homology) domain at their C-terminus. KASH domain proteins are type II transmembrane proteins of the NE and have been identified as molecular linkers connecting the nucleus to actin filaments [filamentous acti...

Structure of Sad1-UNC84 Homology (SUN) Domain Defines Features of Molecular Bridge in Nuclear Envelope

“…Homotrimeric Association of SUN Domain-Previous studies indicate that the SUN1 and SUN2 proteins can undergo homoor heteropolymerization, which involves both the SUN domain and the adjacent coiled coil regions (42,45,49,50). Our current crystallographic study has revealed that the SUN domain alone is sufficient to form a homotrimer (Fig.…”

“…Previous studies showed that SUN1 and SUN2 can form homo-or hetero-oligomers via two predicated coiled coil motifs located in the luminal region near the N-terminal end of the SUN domain (14,30,42,45,49,50). Oligomerization analysis of the potential coiled coil motifs within the luminal domain predicts that although the first coiled coil is highly likely to be dimeric, the second one has a modest probability to be trimeric (58) (supplemental Fig.…”

“…Deletion of the endogenous Sad1 protein is lethal to S. pombe (26). SUN proteins are an essential part of the molecular link coupling the centrosome and nuclear chromatin in mammals (45). SUN proteins are thought essential for telomere migration during homologous chromosome pairing (38, 39, 46 -48).…”

TheDrosophilaSUN protein Spag4 cooperates with the coiled-coil protein Yuri Gagarin to maintain association of the basal body and spermatid nucleus

“…Disruption of the MTOC-nucleus association frequently leads to cellular abnormalities, including defective nuclear positioning (Malone et al, 1999;Starr et al, 2001), abnormal morphology of cells and organelles (Fischer-Vize and Mosley, 1994;MosleyBishop et al, 1999;Patterson et al, 2004), altered cell migration (Gomes et al, 2005;Lüke et al, 2008) and defective chromosome segregation during mitosis (Malone et al, 2003;Xiong et al, 2008). These findings indicate that the nuclear-MTOC association is essential in many cellular processes.…”

The LINC and NPC relationship – it's complicated!

“…In yeast, which lack lamins, LINC complexes are anchored to chromosomes and promote chromosome movements (Morimoto et al, 2012). Furthermore, the nucleoplasmic domain of SUN proteins directly binds to chromatin in mammalian cells (Xiong et al, 2008).…”