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 cargo es 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 w e have addressed this question in the Balbiani ring (BR) system of Chironom us tentans using immunoelectron microscopy. The RAE1 protein of C. tentans, Ct-RAE1, is 70% identical to human RAEl/mrnp41 (hRAE1) and is recognized b y antibodies raised against hRAE1. As in vertebrate cells, Ct-RAE1 is conce ntrated at the nuclear envelope and also dispersed throughout the nuclear i nterior. 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 loca lization of Ct-RAE1 at the NPC is correlated with the presence of an export ed RNP in the NPC. Finally, the anti-RAE1 antibody does not label the cytop lasmic side of BR particles in transit through the central channel, which i ndicates that Ct-RAE1 either remains anchored at the nuclear side of the NP C during translocation of the RNP through the central channel or becomes tr ansiently associated with the RNP but is rapidly released into the cytoplas m.