Lamin A is not synthesized as a larger precursor polypeptide.

Isolation of rat liver nuclei in the presence of N-ethylmaleimide (NEM) led to the recovery in the final nuclear matrix of a higher molecular weight form of lamin A. The 2 kDa larger form was identified as lamin A by isoelectric point determination, recognition by an anti-lamin A and C monoclonal antibody and peptide mapping using V8 protease and N-chlorosuccinimide. The 2 kDa extension was tentatively localized to the carboxy-terminus of lamin A. Pulse-chase labeling and immunoprecipitation studies using baby hamster kidney cells showed that lysis of the cells in the presence of NEM allowed the recovery of a stable higher molecular weight form of lamin A. We conclude from these results that NEM prevented the degradation of the native form of lamin A previously thought to represent a higher molecular weight transient precursor form.