The nuclear pore complex protein ALADIN is anchored via NDC1 but not via POM121 and GP210 in the nuclear envelope.

The nuclear pore complex (NPC) consists of approximately 30 different proteins and provides the only sites for macromolecular transport between cytoplasm and nucleus. ALADIN was discovered as a new member of the NPC. Mutations in ALADIN are known to cause triple A syndrome, a rare autosomal recessive disorder characterized by adrenal insufficiency, alacrima, and achalasia. The function and exact location of the nucleoporin ALADIN within the NPC multiprotein complex is still unclear. Using a siRNA-based approach we downregulated the three known membrane integrated nucleoporins NDC1, GP210, and POM121 in stably expressing GFP-ALADIN HeLa cells. We identified NDC1 but not GP210 and POM121 as the main anchor of ALADIN within the NPC. Solely the depletion of NDC1 caused mislocalization of ALADIN. Vice versa, the depletion of ALADIN led also to disappearance of NDC1 at the NPC. However, the downregulation of two further membrane-integral nucleoporins GP210 and POM121 had no effect on ALADIN localization. Furthermore, we could show a direct association of NDC1 and ALADIN in NPCs by fluorescence resonance energy transfer (FRET) measurements. Based on our findings we conclude that ALADIN is anchored in the nuclear envelope via NDC1 and that this interaction gets lost, if ALADIN is mutated. The loss of integration of ALADIN in the NPC is a main pathogenetic aspect for the development of the triple A syndrome and suggests that the interaction between ALADIN and NDC1 may be involved in the pathogenesis of the disease.