The processing pathway of prelamin A.

The conversion of mammalian prelamin A to mature lamin A proceeds through the removal of 18 amino acids from the carboxyl terminus. The initial step in this processing is the isoprenylation of a CAAX box cysteine. This proteolytic event is distinctive for prelamin A among the known prenylated mammalian proteins. Since the carboxyl terminus of prelamin A is removed during maturation, it is not obvious that this protein would undergo the two reactions subsequent to prenylation observed in other CAAX box proteins--the endoproteolytic removal of the carboxyl-terminal 3 amino acids and the subsequent methylation of the now carboxyl-terminal cysteine. To characterize the maturation of prelamin A further, we have developed a CHO-K1 cell line that possesses a dexamethasone-inducible human prelamin A against a genetic background of high mevalonate uptake. Utilizing this cell line in association with antibodies specific to the transgenic prelamin A, we have been able to demonstrate directly in vivo that prelamin A undergoes farnesylation and carboxymethylation prior to conversion to lamin A, as is the case for other prenylated proteins. We have demonstrated previously that in the absence of isoprenylation, conversion of prelamin A to lamin A is blocked, but that unprocessed prelamin A is transported to the nucleus where it can still undergo maturation. Consistent with the implications of these prior studies, we now demonstrate the presence of both subunits of farnesyl-protein transferase in the nucleus.