JCB_201606043 2827..2842

The nuclear envelope (NE) consists of two lipid bilayer membranes that separate the nucleoplasm from the cytoplasm. The outer nuclear membrane (ONM) is continuous with the ER. The inner nuclear membrane (INM) has an underlying filamentous meshwork of lamin proteins called the nuclear lamina. The nuclear membranes and the lamina contain NE proteins that have important functions in regulating NE rigidity, gene expression, and chromosome organization. Dysfunctions in NE proteins impair NE architecture and cause human diseases such as rapid aging and cancers (Burke and Stewart, 2014). The linker of nucleoskeleton and cytoskeleton (LINC) complexes, which are highly conserved throughout evolution, consist of Klarsicht (Klar)/ANC-1/SYNE homology (KASH) and Sad1/UNC-84 (SUN) domain proteins (hereafter referred to as KASH and SUN proteins; Chang et al., 2015). KASH proteins span the ONM by the KASH domain, which bears a carboxyl tail that binds to the SUN domain of INM-resident SUN proteins in the perinuclear space (PNS). This KASH– SUN interaction forms a stable structure bridging the ONM and INM (Sosa et al., 2012). Cytoplasmic extensions of KASH proteins bind to cytoskeletal filaments, and SUN proteins interact with INM proteins and with the nuclear lamina. Therefore, the LINC complex controls nucleocytoskeletal force transduction and thereby contributes to nuclear migration and cytoskeletal organization (Chang et al., 2015). Mutations of the genes encoding LINC complexes lead to nuclear dysmorphology and defective nuclear positioning in mouse skeletal muscle (Zhang et al., 2007; Lüke et al., 2008; Lei et al., 2009; Puckelwartz et al., 2009). Mutations of the human LINC complex genes cause human genetic disorders such as arthrogryposis, cerebellar ataxia, deafness, and Emery–Dreifuss muscular dystrophy (Gros-Louis et al., 2007; Attali et al., 2009; Puckelwartz et al., 2009; Horn et al., 2013; Wang et al., 2015). Aberrant expressions of KASH and SUN proteins are causative in lung and breast cancers (Lv et al., 2015; Matsumoto et al., 2015). KASH and SUN proteins anchor at the NE through the “diffusion retention” model (Boni et al., 2015; Ungricht et al., 2015). SUN proteins retain KASH proteins through a physical interaction. Upon depletion of the Drosophila melanogaster SUN protein Klaroid (Koi), the two KASH proteins Klar and muscle-specific protein 300 (Msp300) are no longer localized at the NE (Kracklauer et al., 2007; Technau and Roth, 2008). Moreover, anchoring of SUN proteins at the INM can depend on the nuclear lamina proteins’ lamins and their associated proteins at the INM (Chang et al., 2015). In Drosophila and Caenorhabditis elegans carrying mutations of the lamin genes, SUN proteins are not localized at the NE (Lee et al., 2002; Linker of nucleoskeleton and cytoskeleton (LINC) complexes spanning the nuclear envelope (NE) contribute to nucleocytoskeletal force transduction. A few NE proteins have been found to regulate the LINC complex. In this study, we identify one, Kuduk (Kud), which can reside at the outer nuclear membrane and is required for the development of Drosophila melanogaster ovarian follicles and NE morphology of myonuclei. Kud associates with LINC complex components in an evolutionarily conserved manner. Loss of Kud increases the level but impairs functioning of the LINC complex. Overexpression of Kud suppresses NE targeting of cytoskeleton-free LINC complexes. Thus, Kud acts as a quality control mechanism for LINC-mediated nucleocytoskeletal connections. Genetic data indicate that Kud also functions independently of the LINC complex. Overexpression of the human orthologue TMEM258 in Drosophila proved functional conservation. These findings expand our understanding of the regulation of LINC complexes and NE architecture. Outer nuclear membrane protein Kuduk modulates the LINC complex and nuclear envelope architecture