STRUCTURAL INTERACTION BETWEEN THE NUCLEAR-PORE COMPLEX AND A SPECIFIC TRANSLOCATING RNP PARTICLE

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 R NP ribbon bent into a ringlike structure. Upon translocation through t he nuclear pore complex (NPC), the ribbon is straightened and enters t he 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 ma kes contact with the periphery of the NPC. To determine the nature of this latter interaction, we have now studied the connections between t he RNP particle and the border of the NPC during different stages of t ranslocation using electron microscope tomography. It was observed tha t the 3' terminal domain of the ribbon always touches the nuclear ring of the NPC, but the precise area of contact is variable. Sometimes al so a region on the opposite side of the ribbon ring reaches the nuclea r ring. The pattern of contacts could be correlated to the stage of tr anslocation, and it was concluded that the particle-nuclear ring inter actions 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 doma in of the ribbon and the entrance to the central channel is probably c rucial to accomplish the ordered translocation of the premessenger RNP particle through the NPC.