Nuclear lamins A and B1: Different pathways of assembly during nuclear envelope formation in living cells

At the end of mitosis, the nuclear lamins assemble to form the nuclear lami na during nuclear envelope formation in daughter cells. We have fused A- an d B-type nuclear lamins to the green fluorescent protein to study this proc ess in living cells. The results reveal that the A- and B-type lamins exhib it different pathways of assembly. In the early stages of mitosis, both lam ins are distributed throughout the cytoplasm in a diffusible (nonpolymerize d) state, as demonstrated by fluorescence recovery after photobleaching (FR AP). During the anaphase-telophase transition, lamin B1 begins to become co ncentrated at the surface of the chromosomes. As the chromosomes reach the spindle poles, virtually all of the detectable lamin B1 has accumulated at their surfaces. Subsequently, this lamin rapidly encloses the entire perime ter of the region containing decondensing chromosomes in each daughter cell . By this time, lamin B1 has assembled into a relatively stable polymer, as indicated by FRAP analyses and insolubility in detergent/high ionic streng th solutions. In contrast, the association of lamin A with the nucleus begi ns only after the major components of the nuclear envelope including pore c omplexes are assembled in daughter cells. Initially lamin A is found in an unpolymerized state throughout the nucleoplasm of daughter cell nuclei in e arly G1 and only gradually becomes incorporated into the peripheral lamina during the first few hours of this stage of the cell cycle. In later stages of G1, FRAP analyses suggest that both green fluorescent protein lamins A and B1 form higher order polymers throughout interphase nuclei.