Fractionation and partial characterization of proteins of the bileaflet nuclear membrane from rat liver.

The sodium dodecyl sulfate (SDS)-solubilized proteins of the bileaflet nuclear membrane from rat liver have been fractionated by gel Htration. More than 23 different molecular weight classes of proteins ranging from 160,000 to 16,000 can be distinguished by conventional SDS-polyacrylamide gel electrophoresis. Polypeptide chains of less than 16,000 were not detected. A triplet, the respective proteins of which have molecular weights of 64,000, 70,000, and 74,000, account for 25% of the total membrane protein, while a second group of proteins in the range of 53,000 to 60,000 represents an additional 21%. The amino acid composition of the individual fractions does not exhibit large deviations from the composition of the delipidated nuclear membrane. A trend was noted, however, in progressing from the high to the low molecular weight polypeptides; the ratio of acidic to basic residues decreased from 1.83 (Fraction A) to 0.92 (Fraction P). The major portion of the membrane phosphorus was separated into two discrete fractions by gel filtration. Ten per cent eluted near void volume and was predominantly RNA phosphorus, whereas 80% eluted with an apparent molecular weight of 15,000 and was entirely phospholipid phosphorus. The bulk of the neutral lipids was also found in this fraction; however, small amounts were also localized in other column fractions. In comparing the phosphorus elution profiles of the nuclear and microsomal membranes, the microsomal membrane did not yield the RNA component. The possibility that this RNA may represent some unique feature of the nuclear membrane, such as the nuclear pore complex, is suggested.