THE PROCESSING PATHWAY OF PRELAMIN-A

The conversion of mammalian prelamin A to mature lamin A proceeds thro ugh the removal of 18 amino acids from the carboxyl terminus. The init ial step in this processing is the isoprenylation of a CAAX box cystei ne. This proteolytic event is distinctive for prelamin A among the kno wn prenylated mammalian proteins. Since the carboxyl terminus of prela min A is removed during maturation, it is not obvious that this protei n would undergo the two reactions subsequent to prenylation observed i n other CAAX box proteins - the endoproteolytic removal of the carboxy l-terminal 3 amino acids and the subsequent methylation of the now car boxyl-terminal cysteine. To characterize the maturation of prelamin A further, we have developed a CHO-K1 cell line that possesses a dexamet hasone-inducible human prelamin A against a genetic background of high mevalonate uptake. Utilizing this cell line in association with antib odies specific to the transgenic prelamin A, we have been able to demo nstrate directly in vivo that prelamin A undergoes farnesylation and c arboxymethylation prior to conversion to lamin A, as is the case for o ther prenylated proteins. We have demonstrated previously that in the absence of isoprenylation, conversion of prelamin A to lamin A is bloc ked, but that unprocessed prelamin A is transported to the nucleus whe re it can still undergo maturation. Consistent with the implications o f these prior studies, we now demonstrate the presence of both subunit s of farnesyl-protein transferase in the nucleus.