Characterization and fractionation of rat liver nuclear matrix.

The rat liver nuclear matrix retains the shape of the nucleus and reveals a sponge-like structure in negative staining and scanning electron microscopy. A fibrous layer (dense lamina) with associated pore complexes is preserved on the surface of the nuclear matrix. In negative staining as well as in high resolution scanning electron microscopy this layer is perceived as a network consisting of alveoli of 10 to 30 nm in diameter with pore complexes differing in arrangement of the annular granules. In sections a fibrous layer 15 to 30 nm in width granules of 7 to 10 nm in diameter can be observed. Structures similar to the pore complexes are revealed not only at the fibrous layer but also inside the nucleus and in close proximity to the nucleoli. Four fractions of the nuclear matrix have been isolated by successive extraction with 0.05 M EDTA and 0.025 N NaOH. Almost 80% of the total matrix protein dissolves in dilute alkali. 4 to 5% of the total matrix protein is soluble in EDTA. From insoluble residue two fractions can be isolated: one rich in pore complexes and another one retaining the shape of the nucleus with spongy or alveolar structure. The latter fraction is regarded to constitute a proper framework or skeleton of the nucleus.