Association of gold-labelled nucleoplasmin with the centres of ring components of Xenopus oocyte nuclear pore complexes.

We have used heavy-metal shadowing to study the interaction of morphological components of Xenopus oocyte nuclear pore complexes with nucleoplasmin conjugated to colloidal gold. When microinjected into Xenopus oocytes, gold-labelled nucleoplasmin accumulated on the axis of the pores. Envelopes partially disrupted by treatment with low ionic strength buffer produced isolated islands of pores together with substantial quantities of rings deriving from the cytoplasmic and nucleoplasmic faces of the pores. In preparations from oocytes in which nucleoplasmin-gold had been microinjected, most (238/288) of the rings examined had also been labelled and, in the majority of these (60%), the label was located centrally within isolated rings. The central positioning of the nucleoplasmin-gold in isolated rings indicated that these morphological components of the pores were probably involved in the transport of nucleoplasmin into the nucleus, either by way of the initial binding of the molecule or by way of its translocation across the nuclear envelope. Although more work is required to resolve the precise stage at which the rings are involved, a number of lines of evidence suggested that they were more likely to be involved in the translocation step rather than in initial binding of nucleoplasmin.