THE ROLE OF ISOPRENYLATION IN MEMBRANE ATTACHMENT OF NUCLEAR LAMINS -A SINGLE-POINT MUTATION PREVENTS PROTEOLYTIC CLEAVAGE OF THE LAMIN-A PRECURSOR AND CONFERS MEMBRANE-BINDING PROPERTIES

Mature A- and B-type lamins differ in the extent to which they interac t with the nuclear membrane and thus represent an interesting model fo r studying the role of isoprenylation and carboxyl-methylation in memb rane attachment. Both A- and B-type lamins are isoprenylated and carbo xyl-methylated shortly after synthesis, but A-type lamins undergo a fu rther proteolytic cleavage which results in the loss of the hydrophobi cally modified C terminus. Here, we have constructed mutants of chicke n lamin A that differ in their abilities to serve as substrates for di fferent post-translational processing events occurring at the C termin us of the wild-type precursor. In addition to studying full-length pro teins, we have analyzed C-terminal end domains of lamin A, either alon e or after fusion to reporter proteins. Mutant proteins were expressed in mammalian cells, and their membrane association was analyzed by im munofluorescence microscopy and subcellular fractionation. Our results provide information on the substrate specificity and subcellular loca lization of the lamin-h-specific protease. Moreover, they indicate tha t hydrophobic modifications of the C-terminal end domains account for the differential membrane-binding properties of A- and B-type lamins. Thus, some of the integral membrane proteins implicated in anchoring B -type lamins to the membrane may function as receptors for the isopren ylated and carboxyl-methylated C terminus.