Sad1/UNC-84 (SUN)-domain proteins are inner nuclear membrane (INM) proteins that are part of bridging complexes linking cytoskeletal elements with the nucleoskeleton. Originaly identified based on an ~150-amino acid region of homology between the C terminus of the Schizosaccharomyces pombe Sad1 protein and the Caenorhabditis elegans UNC-84 protein, SUN proteins are present in the proteomes of most eucaryotes. In addition to the SUN domain, these proteins contain a transmembrane sequence and at least one coiled-coil domain and localize to the inner nuclear envelope. SUN proteins are anchored in the inner nuclear envelope by their transmembrane segment and oriented in the membrane such that the C-terminal SUN domain is located in the space between the inner and outer nuclear membrane. Here, the SUN domain can interact with the C-terminal tail of an outer nuclear envelope protein that binds to the cytoskeleton, including the centrosome [1,2,3].

Some proteins known to contain a SUN domain are listed below:

• Fission yeast spindle pole body-associated protein Sad1.
• Yeast spindle pole body assembly component MPS3, essential for nuclear division and fusion.
• Yeast uncharacterized protein SLP1.
• Caenorhabditis nuclear migration and anchoring protein UNC-84.
• Caenorhabditis SUN domain-containing protein 1 (sun-1), involved in centrosome attachment to the nucleus.
• Mammalian sperm-associated antigen 4 protein (SPAG4), may assist the organization and assembly of outer dense fibers (ODFs), a specific structure of the sperm tail.
• Mammalian sperm-associated antigen 4-like protein (SPAG4L).
• Mammalian SUN1.
• Mammalian SUN2.
• Mammalian SUN3.

The profile we developed covers the entire SUN domain.