InterPro domain: IPR012919

Sad1/UNC-84 (SUN)-domain proteins are inner nuclear membrane (INM) proteinsthat are part of bridging complexes linking cytoskeletal elements with thenucleoskeleton. Originaly identified based on an ~150-amino acid region ofhomology between the C terminus of the Schizosaccharomyces pombe Sad1 proteinand the Caenorhabditis elegans UNC-84 protein, SUN proteins are present in theproteomes of most eucaryotes. In addition to the SUN domain, these proteinscontain a transmembrane sequence and at least one coiled-coil domain andlocalise to the inner nuclear envelope. SUN proteins are anchored in the innernuclear envelope by their transmembrane segment and oriented in the membranesuch that the C-terminal SUN domain is located in the space between the innerand outer nuclear membrane. Here, the SUN domain can interact with the C-terminal tail of an outer nuclear envelope protein that binds to thecytoskeleton, 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 uncharacterised 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 organisation and assembly of outer dense fibres (ODFs), a specific structure of the sperm tail.
• Mammalian sperm-associated antigen 4-like protein (SPAG4L).
• Mammalian SUN1.
• Mammalian SUN2.
• Mammalian SUN3.

1. Comparative genomics, evolution and origins of the nuclear envelope and nuclear pore complex. Cell Cycle 3, 1612-37
2. The Sad1-UNC-84 homology domain in Mps3 interacts with Mps2 to connect the spindle pole body with the nuclear envelope. J. Cell Biol. 174, 665-75
3. Characterization of SUN-domain proteins at the higher plant nuclear envelope. Plant J. 61, 134-44