The Nuclear Pore Complex (NPC) regulates nucleocytoplasmic transport by providing small channels for passive diffusion and multiple docking surfaces that lead to a central translocation channel for active transport. In this study we have investigated by high resolution scanning and transmission electron microscopy the dynamics of NPC structure in salivary gland nuclei from Chironomus during Balbiani ring (BR) mRNP translocation, and present evidence of rearrangement of the transporter related to mRNP export. Analysis of the individual NPC components verified a strong evolutionary conservation of NPC structure between vertebrates and invertebrates. The transporter is an integral part of the NPC and is composed of a central short double cylinder that is retained within the inner spoke ring, and two peripheral globular assemblies which are tethered to the cytoplasmic and nucleoplasmic coaxial rings by eight conserved internal ring filaments. Distinct stages of BR mRNP nuclear export through the individual NPC components were directly visualized and placed in a linear transport sequence. The BR mRNP first binds to the NPC basket, which forms an expanded distal basket ring. In this communication we present stages of BR mRNP transport through the nucleoplasmic, central and cytoplasmic transporter subunits, which change their conformation during mRNP translocation, and the emergence of mRNP into the cytoplasm. We propose that the reorganization of the basket may be driven, in part, by an active translocation process at the transporter. Furthermore, the images provide dramatic evidence that the transporter functions as a central translocation channel with transiently open discrete gates in its globular assemblies. A model of NPC transporter reorganization accompanied with mRNP translocation is discussed.

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