Recent evidence shows that the COOH-terminal CaaX motif of lamins is necessary to target newly synthesized proteins to the nuclear envelope membranes. Isoprenylation at the CaaX-cysteine has been taken to explain the different fates of A- and B-type lamins during cell division. A-type lamins, which loose their isoprenylation shortly after incorporation into the lamina structure, become freely soluble upon mitotic nuclear envelope breakdown. Somatic B-type lamins, in contrast, are permanently isoprenylated and, although depolymerized during mitosis, remain associated with remnants of nuclear envelope membranes. However, Xenopus lamin B3, the major B-type lamin of amphibian oocytes and eggs, becomes soluble after nuclear envelope breakdown in meiotic metaphase. Here we show that Xenopus lamin B3 is permanently isoprenylated and carboxyl methylated in oocytes (interphase) and eggs (meiotic metaphase). When transfected into mouse L cells Xenopus lamin B3 is integrated into the host lamina and responds to cell cycle signals in a normal fashion. Notably, the ectopically expressed Xenopus lamin does not form heterooligomers with the endogenous lamins as revealed by a coprecipitation experiment with mitotic lamins. In contrast to the situation in amphibian eggs, a significant portion of lamin B3 remains associated with membranes during mitosis. We conclude from these data that the CaaX motif-mediated modifications, although necessary, are not sufficient for a stable association of lamins with membranes and that additional factors are involved in lamin-membrane binding.

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