Abstract
Targeting of nuclear lamins to the inner nuclear envelope membrane requires a nuclear localization signal and CaaX motif-dependent posttranslational modifications, including isoprenylation and carboxyl methylation. These modifications, although necessary for membrane targeting, are not sufficient to mediate stable association with membranes. We show that two variants of lamin B3 (i.e., B3a and B3b) exist in Xenopus oocytes. They are encoded by two alternatively spliced, developmentally regulated mRNAs. The two lamin variants differ greatly in their membrane association in meiotically matured eggs. The presence of an extra cysteine residue (as a potential palmitoylation site) and a basic cluster in conjunction with the CaaX motif function as secondary targeting signals responsible for the stable membrane association of lamin B3b in Xenopus eggs. Moreover, transfection experiments with Green Fluorescent Protein lamin tail chimeras and with a Green Fluorescent Protein N-Ras chimera show that these secondary motifs are sufficient to target proteins to the inner nuclear membrane and/or the plasma membrane. Implications for the intracellular trafficking of doubly lipidated proteins are discussed.