Nuclear ERM (ezrin, radixin, moesin) proteins: regulation by cell density and nuclear import.

The highly conserved ERM (ezrin-radixin-moesin) family of proteins function as molecular linkers between the actin cytoskeleton and transmembrane receptors. We now provide unequivocal evidence that full-length endogenous ezrin and moesin also localise to the nucleus in two independent mammalian cell lines. All three ERM family members can localise to the nucleus upon exogenous expression of their GFP-tagged counterparts, suggesting a common family trend. Furthermore, Dmoesin, the Drosophila ERM homologue, is present in the nucleus of an insect cell line and can localise to the nucleus when exogenously expressed in MDCK cells. The nuclear localisation of endogenous ezrin and moesin is regulated by cell density and is resistant to detergent extraction, suggesting tight association with nuclear structures. Furthermore, phosphorylation in the actin-binding domain is not a prerequisite for nuclear localisation. We have identified a specific nuclear localisation sequence, which is conserved and functional in all ERM family members, implying specific regulated nuclear import. Although the precise nuclear function of the ERM proteins is unknown, these data provide further evidence that an increasing number of cytoskeletal components directly link the plasma membrane with nuclear events.