Rat liver nuclei deprived of chromatin and nucleoplasm show a spongelike network which preserves its connection with nucleoli, the inner membrane of the nuclear envelope, and nuclear pore complexes. It contains all of the HnRNA, provided the endogenous proteolytic activity is inhibited by a proteolytic inhibitor such as phenylmethyl sulfonyl chloride (PMSC) or the fluoride form (PMSF). In the absence of these proteolytic inhibitors, HnRNA is dissociated from the spongelike network and sediments in a sucrose gradient as polydispersed ribonucleoprotein complexes. Furthermore, purified HnRNA as well as rRNA do not bind to the spongelike network when added to these nuclei. These observations demonstrate that the association of HnRNA to the nuclear skeleton is not an artifact. RNase treatment of the spongelike network digests the majority of the rapidly labeled RNA but does not alter the morphological aspect nor the architecture of this network. EDTA and heparin treatments affect neither the attachment of HnRNA nor the structural organization of this network. Electron microscope studies of the network reveal a characteristic flexuous configuration. Its relationship with diffused and condensed chromatin is discussed.

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