The transport of Balbiani ring (BR) premessenger RNP particles in the larval salivary gland cells of the dipteran Chironomus tentans can be followed using electron microscopy. A BR RNP particle consists of an RNP ribbon bent into a ringlike structure. Upon translocation through the nuclear pore complex (NPC), the ribbon is straightened and enters the central channel of the NPC with the 5' end of the transcript in the lead. The translocating ribbon is likely to interact with the central channel but, in addition, the remaining portion of the ribbon ring makes contact with the periphery of the NPC. To determine the nature of this latter interaction, we have now studied the connections between the RNP particle and the border of the NPC during different stages of translocation using electron microscope tomography. It was observed that the 3' terminal domain of the ribbon always touches the nuclear ring of the NPC, but the precise area of contact is variable. Sometimes also a region on the opposite side of the ribbon ring reaches the nuclear ring. The pattern of contacts could be correlated to the stage of translocation, and it was concluded that the particle-nuclear ring interactions reflect a rotation of the ribbon ring in front of the central channel, the rotation being secondary to the successive translocation of the ribbon through the channel. The particle's mode of interaction with the NPC suggests that the initial contact between the 5' end domain of the ribbon and the entrance to the central channel is probably crucial to accomplish the ordered translocation of the premessenger RNP particle through the NPC.

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