Demonstration of a DNase-sensitive network associated with the nuclear pore complexes in rat liver nuclei

Rat liver nuclei have been studied by transmission electron microscopy after resuspension in a phosphate-buffered salt solution containing SO2-4 as the quantitatively dominant anion. Owing to the high solubility of chromatin in the presence of SO2-4 instead of Cl- at isotonicity, nuclei are depleted for chromatin by DNase I digestion in this buffer, eliminating the need for high-salt extraction. This shows that at least 75% of the nuclear pore complexes are associated with fibrogranular structures, which ramify as a network throughout the nucleus, interconnecting the nuclear lamina, interchromatin granule clusters and nucleoli. Perichromatin granules are located in this material proximal to the nuclear pore complexes. Most of the chromatin is removed without major impact on the network, but below a level of 25% residual chromatin there is a considerable reduction of this material, and only about 15% of the connections to the nuclear pore complexes are resistant to digestion with DNase I or streptodornase A and B. The percentage of nuclear pore complexes connected to the network is further reduced by salt extraction and RNase treatment. These results suggest that DNA is an integral part of the network, which presumably plays a role in nucleo-cytoplasmic transport of RNA and protein.