Nuclear Membranes From Mammalian Liver

Abstract

Nuclear membranes were isolated from rat and pig liver by sonication of highly purified nuclear fractions and subsequent removal of adhering nucleoproteins in a high salt medium . The fractions were examined in the electron microscope by both negative staining and thin sectioning techniques and were found to consist of nuclear envelope fragments of widely varying sizes. Nuclear pore complex constituents still could frequently be recognized . The chemical composition of the nuclear membrane fractions was determined and compared with those of microsomal fractions prepared in parallel . For total nuclei as well as for nuclear membranes and microsomes, various enzyme activities were studied . The results indicate that a similarity exists between both fractions of cytomembranes, nuclear envelope, and endoplasmic reticulum, with respect to their RNA : protein ratio and their content of polar and nonpolar lipids . Both membranous fractions had many proteins in common including some membrane-bound enzymes . Activities in Mg-ATPase and the two examined cytochrome reductases were of the same order of magnitude . The content of cytochrome b 5 as well as of P-450 was markedly lower in the nuclear membranes . The nuclear membranes were found to have a higher buoyant density and to be richer in protein . The glucose-6-phosphatase and Na-K-ATPase activities in the nuclear membrane fraction were very low. In the gel electrophoresis, in addition to many common protein bands, some characteristic ones for either microsomal or nuclear membranous material were detected . Significant small amounts of DNA and RNA were found to remain closely associated with the nuclear envelope fragments . Our findings indicate that nuclear and endoplasmic reticulum membranes which are known to be in morphological continuity have, besides a far-reaching similarity, some characteristic differences .

Towards an integrated understanding of the structure and mechanics of the cell nucleus

“…Indeed, the chemical composition of the nuclear membrane is dominated by proteins: the ratio of phospholipids to proteins by weight ranges from 0.2 to 0.5. (80,81) While lamins and certain nuclear pore complexes exhibited behaviour of a contiguous two-dimensional supramolecular scaffold with material properties of a solid, nuclear membranes of isolated HeLa cell nuclei stained with a lipophilic probe deform and irreversibly vesiculate in response to micropipette aspiration. (34) This shows that nuclear membranes are fluid-like and deform at energies that are typically needed to induce out-of-plane deformations in lipid bilayers (i.e.…”

On the Attachment of the Nuclear Pore Complex

“…Some isolated nuclear envelopes appear to contain pore complexes (19)(20)(21)(22)(23)(24)(25)(26), implying that the bulk of the chromatin is also not necessary for structural integrity of the complex. These two lines of evidence, suggesting the nonessential nature of the association of the pore complex with the nuclear membrane and with the chromatin, lend encouragement to our attempts to isolate and characterize structurally intact, morphologically recognizable nuclear pore complexes.…”

“…Phospholipids represent 80-90% of the lipid found in isolated nuclear envelope fractions (19,21,53) and are therefore a good marker for the presence of nuclear membranes. Phospholipid determinations subsequent to detergent treatment of nuclei have been published.…”

“…Nuclear pore complexes have been described in nuclear envelope fractions (19)(20)(21)(22)(23)(24)(25)(26). Their presence suggests that the bulk of the peripheral chromatin is not necessary for maintaining the gross structural integrity of the complex.…”

“…The roles which have been postulated for the pore and the complex fall primarily into two mutually compatible classes: (a) involvement in nucleo-cytoplasmic communication (9-12), and (b) organization of the interphase chromatin (13 16). Little direct evidence has been obtained to support either role, and there is only fragmentary information concerning the chemical composition of the complex (17, 18).Before attempting the isolation and chemical characterization of the pore complex, it was necessary to determine the structural relationships between the pore complex and its surrounding nuclear membranes and between the pore complex and the underlying peripheral chromatin.Nuclear pore complexes have been described in nuclear envelope fractions (19)(20)(21)(22)(23)(24)(25)(26). Their presence suggests that the bulk of the peripheral chromatin is not necessary for maintaining the gross structural integrity of the complex.…”