As a component of the LINC (LInker of Nucleoskeleton and Cytoskeleton) complex involved in the connection between the nuclear lamina and the cytoskeleton (PubMed:20711465, PubMed:16380439, PubMed:24062341, PubMed:25892231, PubMed:26842404). The nucleocytoplasmic interactions established by the LINC complex play an important role in the transmission of mechanical forces across the nuclear envelope and in nuclear movement and positioning (PubMed:19874786). Required for interkinetic nuclear migration (INM) and essential for nucleokinesis and centrosome-nucleus coupling during radial neuronal migration in the cerebral cortex and during glial migration (PubMed:19874786). Involved in telomere attachment to nuclear envelope in the prophase of meiosis implicating a SUN1/2:KASH5 LINC complex in which SUN1 and SUN2 seem to act at least partial redundantly (PubMed:17543860, PubMed:19211677, PubMed:19509342, PubMed:24062341, PubMed:25892231, PubMed:26842404). Required for gametogenesis and involved in selective gene expression of coding and non-coding RNAs needed for gametogenesis (PubMed:17543860). Helps to define the distribution of nuclear pore complexes (NPCs) (PubMed:17724119). Required for efficient localization of SYNE4 in the nuclear envelope (PubMed:23348741). May be involved in nuclear remodeling during sperm head formation in spermatogenesis (PubMed:20711465). May play a role in DNA repair by suppressing non-homologous end joining repair to facilitate the repair of DNA cross-links (By similarity). Isoform 5 may be involved in nuclear remodeling during sperm head formation in spermatogenesis. A probable SUN1 isoform 5:SYNE3 LINC complex may tether spermatid nuclei to anterior cytoskeletal structures such as actin filaments present at membraneous junctions of spermatids and Sertoli cells. Core component of the LINC complex which is composed of inner nuclear membrane SUN domain-containing proteins coupled to outer nuclear membrane KASH domain-containing nesprins. SUN and KASH domain-containing proteins seem to bind each other promiscuously; however, differentially expression of LINC complex constituents is giving rise to specific assemblies. At least SUN1/2-containing core LINC complexes are proposed to be hexameric composed of three protomers of each KASH and SUN domain-containing protein. Interacts with KASH5 (via the last 22 amino acids); this interaction mediates KASH5 telomere localization by forming a SUN1:KASH5 LINC complex. Isoform 5 is proposed to form a non-nuclear spermatogenesis-specific LINC complex with SYNE3 during sperm head formation. Interacts with SYNE2 and SYNE1; probably forming respective LINC complexes. Interacts with A-type lamin with a strong preference for unprocessed A-type lamin compared with the mature protein. Interaction with lamins B1 and C is hardly detectable. Interacts with NAT10. Interacts with EMD and TSNAX. Associates with the nuclear pore complex (NPC). Interacts with CCDC79/TERB1; promoting the accumulation of the LINC complex complexes at the telomere-nuclear envelope attachment sites. Interacts with IRAG2 (PubMed:29878215). Interacts (via KASH domain) with TMEM258 (By similarity). At oocyte MI stage localized around the spindle, at MII stage localized to the spindle poles. In round spermatids mainly localizes to the posterior pole of the nucleus. This localization is gradually disappearing during spermiogenesis. In elongated spermatids localizes to anterior regions outside the nucleus indicative for isoform 5. Localized to the anterior pole of spermatids. Seven isoforms have been found to be expressed. Widely expressed. Expressed in cochlear outer hair cells (at protein level). Seven isoforms are expressed in testis including testis-specific isoform 5. Isoform 5 is the only isoform expressed at the end of sperm differentiation. Six isoforms are expressed in muscle, heart and brain, four isoforms in kidney and three isoforms in liver. The coiled coil domains differentially mediate trimerization required for binding to nesprins and are proposed to dynamically regulate the oligomeric state by locking the SUN domain in an inactive confirmation. The coiled coil domains are proposed to be involved in load-bearing and force transmission from the cytoskeleton. The SUN domain may play a role in nuclear anchoring and/or migration. The disulfide bond with KASH domain-containing nesprins is required for stability of the respective LINC complexes under tensile forces. Mutant mice are viable, but display hearing loss at all frequencies. Extended N-terminus.