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The Ct-RAE1 protein interacts with Balbiani ring RNP particles at the nuclear pore.

RAE1 is an evolutionarily conserved protein that associates with both mRNPs and nucleoporins, and may bridge the interaction between mRNP export cargoes and the nuclear pore complex (NPC). However, the mechanism by which RAE1 functions in mRNA export is still unknown and the time point at which RAE1 interacts with the exported RNP has not been directly investigated. Here we have addressed this question in the Balbiani ring (BR) system of Chironomus tentans using immunoelectron microscopy. The RAE1 protein of C. tentans, Ct-RAE1, is 70% identical to human RAE1/mrnp41 (hRAE1) and is recognized by antibodies raised against hRAE1. As in vertebrate cells, Ct-RAE1 is concentrated at the nuclear envelope and also dispersed throughout the nuclear interior. Here we show that Ct-RAE1 does not bind to the BR particle either cotranscriptionally or in the nucleoplasm. Instead, the interaction between Ct-RAE1 and the exported BR particle occurs at the NPC. Moreover, the localization of Ct-RAE1 at the NPC is correlated with the presence of an exported RNP in the NPC. Finally, the anti-RAE1 antibody does not label the cytoplasmic side of BR particles in transit through the central channel, which indicates that Ct-RAE1 either remains anchored at the nuclear side of the NPC during translocation of the RNP through the central channel or becomes transiently associated with the RNP but is rapidly released into the cytoplasm.