NUCLEOPLASMIC LOCALIZATION OF PRELAMIN-A - IMPLICATIONS FOR PRENYLATION-DEPENDENT LAMIN-A ASSEMBLY INTO THE NUCLEAR LAMINA

The synthesis of the nuclear lamina protein lamin A requires the preny lation-dependent processing of its precursor protein, prelamin A. Unli ke p21ras, which undergoes similar initial posttranslational modificat ions, maturation of lamin A results in the proteolytic removal of the prenylated portion of the molecule. We have used an in vitro prenylati on system to demonstrate the nature of the prenyl substituent on prela min A to be a farnesyl group. Further, the in vitro farnesylation of p relamin A requires an intact cysteine-aliphatic-aliphatic-other (CAAX) amino acid sequence motif at its carboxyl terminus. The effect of blo cking the prenylation of prelamin A on its localization and assembly i nto the nuclear lamina was investigated by indirect immunofluorescence . Expression of wild-type prelamin A in lovastatin-treated cells showe d that nonprenylated prelamin A accumulated as nucleoplasmic particles . Upon addition of mevalonate to lovastatin-treated cells, the wild-ty pe lamin A was incorporated into the lamina within 3 hr. Expression of a mutant lamin A in which the carboxyl-terminal 21 amino acids were d eleted resulted in a lamin molecule that was directly assembled into t he lamina. These results indicate that the carboxyl-terminal peptide o f prelamin A blocks its proper assembly into the nuclear lamina and th at the prenylation-initiated removal of this peptide can occur in the nucleus.