Intranuclear membrane structure formations by CaaX-containing nuclear proteins

The nuclear lamina is a protein meshwork lining the nucleoplasmic face of the nuclear envelope. Association of lamins; with the inner nuclear membrane is mediated by specific modifications in the CaaX motif at their C-termini. B-type lamins are permanently isoprenylated whereas lamin A loses its modification by a lamin A-specific processing step after incorporation into the lamina. Lamins are differentially expressed during development and tissue differentiation. Here we show that an increased synthesis of lamins B1 and B2 in amphibian oocytes induces the formation of intranuclear membrane structures that form extensive arrays of stacked cisternae. These 'lamin membrane arrays' are attached to the inner nuclear membrane but are not continuous with it. Induction of this membrane proliferation depends on CaaX-specific posttranslational modification. Moreover, in transfected HeLa cells, chimeric GFP containing a nuclear localization signal and a C-terminal CaaX motif of N-Ras induces intranuclear membrane stacks that resemble those induced by lamins and ER-like cisternae that are induced in the cytoplasm upon increased synthesis of integral ER membrane proteins. Implications for the synthesis of CaaX-containing proteins are discussed and the difference from intranuclear fibrous lamina annulate lamellae formations is emphasized.