Intranuclear membranes induced by lipidated proteins are derived from the nuclear envelope

Association of nuclear lamins with the inner nuclear membrane (INM) is mediated by lipid modifications: either by C-terminal isoprenylation or N-terminal myristoylation. Overexpression of lamins or other lipidated nuclear proteins induces the formation of intranuclear membrane-like arrays. Lamin-induced intranuclear array formation has been observed in Xenopus oocytes as well as in mammalian tissue culture cells. With the use of a membrane-specific fluorescence dye we show here that these arrays are made up of typical lipid membranes. While continuity between these intranuclear membranes and the INM has not been observed so far the presence of integral as well as luminal marker proteins of the endoplasmic reticulum (ER) indicates that these membranes are derived from the nuclear membrane/ER compartment. Earlier studies demonstrated that overexpression of integral membrane proteins of the INM can induce formation of intranuclear membranes, which bud from the INM. Integral membrane proteins reach the INM via the pore membranes while lipidated proteins are imported into the nucleoplasm via the classical NLS pathway where they interact with the INM via their lipid moieties. Together with the previously published data our results show that the formation of intranuclear membranes follows similar routes irrespective of whether the proteins triggering membrane formation are integral membrane or lipidated proteins.