Lamin B from rat liver nuclei exists both as a lamina protein and as an intrinsic membrane protein.

Rat liver nuclear matrix structures were isolated while preserving the integrity of the nuclear envelope, i.e. in the absence of any detergent extraction. In order to determine the relationships between the nuclear membranes and peripheral lamina, nuclear matrix-envelope preparations were submitted to sodium carbonate extraction (0.1 M, pH 11.5), a solvent which solubilizes both peripheral membrane proteins and membrane-enclosed contents. One-dimensional and two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of material insoluble in sodium carbonate confirmed that intrinsic membrane proteins were indeed retained in the membrane structures. Approximately 50 to 60% of the lamin B present in matrix-envelope preparations was found in these insoluble membranes while a smaller amount of lamin A and even less of lamin C resisted complete extraction. The identity of the lamins was confirmed by their migration on two-dimensional gels and by comparison of one-dimensional peptide maps. The same results were obtained using nuclear membranes prepared by a milder heparin procedure. The location of lamin B as an intrinsic membrane protein was also established by photoaffinity labeling with the membrane-penetrating reagent azidopyrene. A small but reproducible amount of labeling occurred as well on lamin A polypeptides. These results support the hypothesis that the peripheral lamina is attached to the nuclear envelope and anchored there via the presence of lamin B molecules within the bilayer of the inner nuclear membrane.