Encodes a member of the mid-SUN subfamily of SUN-domain proteins that is localized to both the nuclear envelope and the ER. It is involved in early seed development and nuclear morphology. [TAIR]. Forms homomers and heteromers with SUN4. Interacts with SUN1, SUN2, SUN5, TIK and WIP1. No visible phenotype. Embryo lethal when associated with disruption mutants SUN4 and SUN5.