Lamin A precursor is localized to intranuclear foci.

Lamin A is synthesized in the cytoplasm as a precursor bearing a carboxyl-terminal CaaX box or isoprenylation signal. This precursor is post-translationally processed through multiple steps: isoprenylation with a farnesyl residue on the cysteine of the CaaX box, proteolytic removal of the last three amino acids, carboxymethylation of the cysteine residue and, finally, proteolytic removal of 15 amino acids from the carboxyl terminus. This last step gives rise to mature lamin A from which the isoprenylated terminus has been removed. Isoprenylation is a prerequisite for all other steps of processing. The subcellular location of these processing steps for lamin A is still a matter of debate. We have produced an antibody specific to the 18 amino acid carboxyl terminus of the lamin A precursor that does not recognize mature lamin A. This antibody detects intranuclear foci by immunofluorescence. Larger amounts of lamin A precursor were accumulated by treating cells with mevinolin (MVN), an inhibitor of isoprenoid synthesis. In MVN-treated cells, the lamin A precursor accumulated most strikingly in the peripheral nuclear lamina where it was assembled, while intranuclear foci were maintained. The addition of an excess of mevalonate (MVA), which restores isoprenylation activity, to MVN-treated cells led to a progressive disappearance of the lamin A precursor from the peripheral lamina. This process was completed after 4 hours of MVA treatment, after which the lamin A precursor was restricted to intranuclear foci. We conclude from these results that the non-isoprenylated lamin A precursor appears competent for assembly into the peripheral nuclear lamina, and that all the processing steps leading to mature lamin A can occur within the nuclear space.