Nuclear matrix. Isolation and characterization of a framework structure from rat liver nuclei

A nuclear framework structure termed the nuclear matrix has been isolated and characterized. This matrix forms the major residual structure of isolated nuclei and consists largely of protein with smaller amounts of RNA, DNA, carbohydrate, and phospholipid. The nuclear matrix can be further resolved by combined treatment with DNase and RNase. The remaining nuclear protein structure, after extraction of 90% of the nuclear protein, 99% of the DNA, and 98% of the RNA and phospholipid, is termed the nuclear protein matrix. Electron microscopy of this final nuclear protein matrix reveals an anterior framework structure composed of residual nucleolar structures associated with a granular and fibrous internal matrix structure. The internal matrix framework is derived from the interchromatic structures of the nucleus, and is connected to a surrounding residual nuclear envelope layer containing residual nuclear pore complex structures. Sodium dodecyl sulfate-acrylamide gel electrophoresis of the nuclear matrix proteins demonstrates three major polypeptide fractions, P-1, P-2, and P-3, with average molecular weights of ∞69,000, 66,000 and 62,000, as well as several minor polypeptides which migrate at ∞50,000 and at higher molecular weights (>100,000). Polypeptides with molecular weights identical to those of P-1, P-2, and P-3 are also components of isolated nuclear envelopes and nucleoli, whereas isolated chromatin contains no detectable matrix polypeptides. This suggests that the major matrix polypeptides are localized in specific structural regions of the nucleus, i.e., nuclear envelope, nucleoli, and interchromatic structures. The presence of cytochrome oxidase activity in the isolated nuclear matrix indicates that at least some integral proteins of the nuclear membrane are associated with the matrix.