Localization of a major nuclear envelope protein by differential solubilization.

The nuclear envelope (NE) is the interface of the two major compartments of the cell. We used differential solubilization in conjunction with ultrastructural visualization to localize components of the NE in the surf clam Spisula solidissima. The high salt-resistant NE fraction can be separated into a pore complex-containing supernatant (4 M urea extract) and a membrane pellet devoid of pore complexes or pore remnants. Urea extraction of the membrane pellet reveals two major proteins with an apparent molecular weight (MWapp) of 67 000 (clam lamin) and 200 000 that are also found in the high-salt and detergent-extracted NE containing pore complexes. Urea extraction of the clam NE under reducing conditions removes the clam lamin. The 200 000 D protein remaining in the NE after removal of the pore complex is not solubilized by detergent extraction and thus can be localized on the inner nuclear part of the NE.