LAMIN-A PRECURSOR IS LOCALIZED TO INTRANUCLEAR FOCI

Lamin A is synthesized in the cytoplasm as a precursor bearing a carbo xyl-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 remov al of the fast three amino acids, carboxymethylation of the cysteine r esidue and, finally, proteolytic removal of 15 amino acids from the ca rboxyl terminus. This last step gives rise to mature lamin A from whic h the isoprenylated terminus has been removed. Isoprenylation is a pre requisite 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 termi nus of the lamin A precursor that does not recognize mature lamin A. T his antibody detects intranuclear foci by immunofluorescence. Larger a mounts of lamin A precursor were accumulated by treating cells with me vinolin (MVN), an inhibitor of isoprenoid synthesis. In MVN-treated ce lls, the lamin A precursor accumulated most strikingly in the peripher al nuclear lamina where it was assembled, while intranuclear foci were maintained. The addition of an excess of mevalonate (MVA), which rest ores isoprenylation activity, to MVN-treated cells led to a progressiv e disappearance of the lamin A precursor from the peripheral lamina. T his 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 appea rs competent for assembly into the peripheral nuclear lamina, and that ail the processing steps leading to mature lamin A can occur within t he nuclear space.