Sun ZW et al. (1999) Genetics "A general requirement for the Sin3-Rpd3 histone deacetylase complex in regulating silencing in Saccharomyces cerevisiae."

Hampsey M, Sun ZW

[Genetics, 1999]

The Sin3-Rpd3 histone deacetylase complex, conserved between human and yeast, represses transcription when targeted by promoter-specific transcription factors. SIN3 and RPD3 also affect transcriptional silencing at the HM mating loci and at telomeres in yeast. Interestingly, however, deletion of the SIN3 and RPD3 genes enhances silencing, implying that the Sin3-Rpd3 complex functions to counteract, rather than to establish or maintain, silencing. Here we demonstrate that Sin3, Rpd3, and Sap30, a novel component of the Sin3-Rpd3 complex, affect silencing not only at the HMR and telomeric loci, but also at the rDNA locus. The effects on silencing at all three loci are dependent upon the histone deacetylase activity of Rpd3. Enhanced silencing associated with sin3Delta, rpd3Delta, and sap30Delta is differentially dependent upon Sir2 and Sir4 at the telomeric and rDNA loci and is also dependent upon the ubiquitin-conjugating enzyme Rad6 (Ubc2). We also show that the Cac3 subunit of the CAF-I chromatin assembly factor and Sin3-Rpd3 exert antagonistic effects on silencing. Strikingly, deletion of GCN5, which encodes a histone acetyltransferase, enhances silencing in a manner similar to deletion of RPD3. A model that integrates the effects of rpd3Delta, gcn5Delta, and cac3Delta on silencing is proposed.

Baudrimont A et al. (2010) PLoS Genet "Leptotene/zygotene chromosome movement via the SUN/KASH protein bridge in Caenorhabditis elegans."

Pasierbek P, Machacek T, Penkner A, Klein F, Fridkin A, Woglar A, Baudrimont A, Jantsch V, Gruenbaum Y, Gloggnitzer J, Wegrostek C

[PLoS Genet, 2010]

The Caenorhabditis elegans inner nuclear envelope protein matefin/SUN-1 plays a conserved, pivotal role in the process of genome haploidization. CHK-2-dependent phosphorylation of SUN-1 regulates homologous chromosome pairing and interhomolog recombination in Caenorhabditis elegans. Using time-lapse microscopy, we characterized the movement of matefin/SUN-1::GFP aggregates (the equivalent of chromosomal attachment plaques) and showed that the dynamics of matefin/SUN-1 aggregates remained unchanged throughout leptonene/zygotene, despite the progression of pairing. Movement of SUN-1 aggregates correlated with chromatin polarization. We also analyzed the requirements for the formation of movement-competent matefin/SUN-1 aggregates in the context of chromosome structure and found that chromosome axes were required to produce wild-type numbers of attachment plaques. Abrogation of synapsis led to a deceleration of SUN-1 aggregate movement. Analysis of matefin/SUN-1 in a double-strand break deficient mutant revealed that repair intermediates influenced matefin/SUN-1 aggregate dynamics. Investigation of movement in meiotic regulator mutants substantiated that proper orchestration of the meiotic program and effective repair of DNA double-strand breaks were necessary for the wild-type behavior of matefin/SUN-1 aggregates.

Minn I et al. (2009) Mol Biol Cell "SUN-1 and ZYG-12, mediators of centrosome-nucleus attachment, are a functional SUN/KASH pair in Caenorhabditis elegans."

Minn I, Hanna-Rose W, Rolls MM, Malone CJ

[Mol Biol Cell, 2009]

Klarsicht/ANC-1/Syne/homology (KASH)/Sad-1/UNC-84 (SUN) protein pairs can act as connectors between cytoplasmic organelles and the nucleoskeleton. Caenorhabditis elegans ZYG-12 and SUN-1 are essential for centrosome-nucleus attachment. Although SUN-1 has a canonical SUN domain, ZYG-12 has a divergent KASH domain. Here, we establish that the ZYG-12 mini KASH domain is functional and, in combination with a portion of coiled-coil domain, is sufficient for nuclear envelope localization. ZYG-12 and SUN-1 are hypothesized to be outer and inner nuclear membrane proteins, respectively, and to interact, but neither their topologies nor their physical interaction has been directly investigated. We show that ZYG-12 is a type II outer nuclear membrane (ONM) protein and that SUN-1 is a type II inner nuclear membrane protein. The proteins interact in the luminal space of the nuclear envelope via the ZYG-12 mini KASH domain and a region of SUN-1 that does not include the SUN domain. SUN-1 is hypothesized to restrict ZYG-12 to the ONM, preventing diffusion through the endoplasmic reticulum. We establish that ZYG-12 is indeed immobile at the ONM by using fluorescence recovery after photobleaching and show that SUN-1 is sufficient to localize ZYG-12 in cells. This work supports current models of KASH/SUN pairs and highlights the diversity in sequence elements defining KASH domains.

Hao H et al. (2019) Nucleus "SUN/KASH interactions facilitate force transmission across the nuclear envelope."

Starr DA, Hao H

[Nucleus, 2019]

LINC complexes (Linker of Nucleoskeleton and Cytoskeleton), consisting of inner nuclear membrane SUN proteins and outer nuclear membrane KASH proteins, are essential for nuclear positioning, cell migration and chromosome dynamics. Crystal structures of the central SUN-KASH interaction domains revealed close interfaces predicted to mediate LINC complex formation and function. However, the in vivo significance of these conserved interactions were unclear. Cain et al (2018) used a combination of Caenorhabditis elegans genetics, imaging in cultured NIH 3T3 fibroblasts, and Molecular Dynamic simulations, to study SUN-KASH interactions in cells. The study found that a conserved aromatic residue at the -7 position of the C-terminal KASH domain and conserved cysteines in both the KASH and SUN domains play important roles in force transmission across the nuclear envelope. Other properties of LINC complexes may also play significant roles in transmitting mechanical forces generated by the cytoskeleton across the nuclear envelope. For example, the helicies preceding the SUN domain activate SUN proteins and may be important for activating KASH-SUN interactions or for association of SUN and/or KASH proteins with the membrane. The longer coiled-coils spanning the perinuclear space may also play a role in mediating force transmission. Finally, LINC complexes may be arranged in higher-order structures to facilitate high force-bearing processes.

Cain NE et al. (2014) J Cell Biol "The SUN protein UNC-84 is required only in force-bearing cells to maintain nuclear envelope architecture."

McDonald KL, Cain NE, Tapley EC, Starr DA, Cain BM

[J Cell Biol, 2014]

The nuclear envelope (NE) consists of two evenly spaced bilayers, the inner and outer nuclear membranes. The Sad1p and UNC-84 (SUN) proteins and Klarsicht, ANC-1, and Syne homology (KASH) proteins that interact to form LINC (linker of nucleoskeleton and cytoskeleton) complexes connecting the nucleoskeleton to the cytoskeleton have been implicated in maintaining NE spacing. Surprisingly, the NE morphology of most Caenorhabditis elegans nuclei was normal in the absence of functional SUN proteins. Distortions of the perinuclear space observed in unc-84 mutant muscle nuclei resembled those previously observed in HeLa cells, suggesting that SUN proteins are required to maintain NE architecture in cells under high mechanical strain. The UNC-84 protein with large deletions in its luminal domain was able to form functional NE bridges but had no observable effect on NE architecture. Therefore, SUN-KASH bridges are only required to maintain NE spacing in cells subjected to increased mechanical forces. Furthermore, SUN proteins do not dictate the width of the NE.

Penkner A et al. (2007) Dev Cell "The nuclear envelope protein Matefin/SUN-1 is required for homologous pairing in C. elegans meiosis."

Penkner A, Gruenbaum Y, Loidl J, Jantsch V, Tang L, Schweizer D, Novatchkova M, Fridkin A, Ladurner M

[Dev Cell, 2007]

We identify a highly specific mutation (jf18) in the Caenorhabditis elegans nuclear envelope protein matefin MTF-1/SUN-1 that provides direct evidence for active involvement of the nuclear envelope in homologous chromosome pairing in C. elegans meiosis. The reorganization of chromatin in early meiosis is disrupted in mtf-1/sun-1(jf18) gonads, concomitant with the absence of presynaptic homolog alignment. Synapsis is established precociously and nonhomologously. Wild-type leptotene/zygotene nuclei show patch-like aggregations of the ZYG-12 protein, which fail to develop in mtf-1/sun-1(jf18) mutants. These patches remarkably colocalize with a component of the cis-acting chromosomal pairing center (HIM-8) rather than the centrosome. Our data on this mtf-1/sun-1 allele challenge the previously postulated role of the centrosome/spindle organizing center in chromosome pairing, and clearly support a role for MTF-1/SUN-1 in meiotic chromosome reorganization and in homolog recognition, possibly by mediating local aggregation of the ZYG-12 protein in meiotic nuclei.

Penkner AM et al. (2009) Cell "Meiotic chromosome homology search involves modifications of the nuclear envelope protein Matefin/SUN-1."

Jantsch V, Ammerer G, Woglar A, Machacek T, Fridkin A, Pasierbek P, Gruenbaum Y, Penkner AM, Gloggnitzer J, Baudrimont A, Csaszar E

[Cell, 2009]

Genome haploidization during meiosis depends on recognition and association of parental homologous chromosomes. The C. elegans SUN/KASH domain proteins Matefin/SUN-1 and ZYG-12 have a conserved role in this process. They bridge the nuclear envelope, connecting the cytoplasm and the nucleoplasm to transmit forces that allow chromosome movement and homolog pairing and prevent nonhomologous synapsis. Here, we show that Matefin/SUN-1 forms rapidly moving aggregates at putative chromosomal attachment sites in the meiotic transition zone (TZ). We analyzed requirements for aggregate formation and identified multiple phosphotarget residues in the nucleoplasmic domain of Matefin/SUN-1. These CHK-2 dependent phosphorylations occur in leptotene/zygotene, diminish during pachytene and are involved in pairing. Mimicking phosphorylation causes an extended TZ and univalents at diakinesis. Our data suggest that the properties of the nuclear envelope are altered during the time window when homologs are sorted and Matefin/SUN-1 aggregates form, thereby controling the movement, homologous pairing and interhomolog recombination of chromosomes.

McGee MD et al. (2009) J Cell Sci "KDP-1 is a nuclear envelope KASH protein required for cell-cycle progression."

Stagljar I, Starr DA, McGee MD

[J Cell Sci, 2009]

Klarsicht, ANC-1 and Syne homology (KASH) proteins localize to the outer nuclear membrane where they connect the nucleus to the cytoskeleton. KASH proteins interact with Sad1-UNC-84 (SUN) proteins to transfer forces across the nuclear envelope to position nuclei or move chromosomes. A new KASH protein, KDP-1, was identified in a membrane yeast two-hybrid screen of a Caenorhabditis elegans library using the SUN protein UNC-84 as bait. KDP-1 also interacted with SUN-1. KDP-1 was enriched at the nuclear envelope in a variety of tissues and required SUN-1 for nuclear envelope localization in the germline. Genetic analyses showed that kdp-1 was essential for embryonic viability, larval growth and germline development. kdp-1(RNAi) delayed the entry into mitosis in embryos, led to a small mitotic zone in the germline, and caused an endomitotic phenotype. Aspects of these phenotypes were similar to those seen in sun-1(RNAi), suggesting that KDP-1 functions with SUN-1 in the germline and early embryo. The data suggest that KDP-1 is a novel KASH protein that functions to ensure the timely progression of the cell cycle between the end of S phase and the entry into mitosis.

Zuela N et al. (2016) Cells "Matefin/SUN-1 Phosphorylation on Serine 43 Is Mediated by CDK-1 and Required for Its Localization to Centrosomes and Normal Mitosis in C. elegans Embryos."

Gruenbaum Y, Zuela N

[Cells, 2016]

Matefin/SUN-1 is an evolutionary conserved C. elegans inner nuclear membrane SUN-domain protein. By creating a bridge with the KASH-domain protein ZYG-12, it connects the nucleus to cytoplasmic filaments and organelles. Matefin/SUN-1 is expressed in the germline where it undergoes specific phosphorylation at its N-terminal domain, which is required for germline development and homologous chromosome pairing. The maternally deposited matefin/SUN-1 is then essential for embryonic development. Here, we show that in embryos, serine 43 of matefin/SUN-1 (S43) is phosphorylated in a CDK-1 dependent manner and is localized throughout the cell cycle mostly to centrosomes. By generating animals expressing phosphodead S43A and phosphomimetic S43E mutations, we show that phosphorylation of S43 is required to maintain centrosome integrity and function, as well as for the localization of ZYG-12 and lamin. Expression of S43E in early embryos also leads to an increase in chromatin structural changes, decreased progeny and to almost complete embryonic lethality. Down regulation of emerin further increases the occurrence of chromatin organization abnormalities, indicating possible collaborative roles for these proteins that is regulated by S43 phosphorylation. Taken together, these results support a role for phosphorylation of serine 43 in matefin/SUN-1 in mitosis.

Daryabeigi A et al. (2016) Genetics "Nuclear Envelope Retention of LINC Complexes Is Promoted by SUN-1 Oligomerization in the Caenorhabditis elegans Germ Line."

Penkner A, Silva N, Woglar A, Jantsch V, Vesely C, Daryabeigi A, Rauter M, Jantsch M, Paouneskou D, Baudrimont A

[Genetics, 2016]

SUN (Sad1 and UNC-84) and KASH (Klarsicht, ANC-1 and Syne homology) proteins are constituents of the inner and outer nuclear membranes. They interact in the perinuclear space via carboxy-terminal SUN-KASH domains to form the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex thereby bridging the nuclear envelope. LINC complexes mediate numerous biological processes by connecting chromatin with the cytoplasmic force generating machinery. Here we show that the coiled-coil domains of SUN-1 are required for oligomerization and retention of the protein in the nuclear envelope, especially at later stages of female gametogenesis. Consistently, deletion of the coiled-coil domain makes SUN-1 sensitive to unilateral force exposure across the nuclear membrane. Premature loss of SUN-1 from the nuclear envelope leads to embryonic death due to loss of centrosome-nuclear envelope attachment. However, in contrast to previous notions we can show that the coiled-coil domain is dispensable for functional LINC complex formation, exemplified by successful chromosome sorting and synapsis in meiotic prophase I in its absence.