ИСТИНА

Nuclear lamina is a main structural component of nuclear skeleton, which provides mechanical rigidity of the cell nucleus. It also takes part in the chromosome organization by linking special chromosome loci to nuclear lamina. Association of chromosomes with nuclear lamina is correlated with transcription activity and replication timing, and lamina might plays an important role in regulation of this processes. On he other hand, tight connection of late-replicationg heterochromatin to lamina may create additional topological constrains during replication. It can be overcome by temporally dissociating replicating chromatin from lamina. We studied correlation between spatial lamina organization and replication status of preipheral heterochromatin using several expreimental models. First, we analysed distribution of replicating chromatin loci relative to nuclear lamina at ultrastructural level in cell lines expressing GFP-PCNA (Leonhard et al, 2000). Asynchronous cell culture was labeled with mouse-anti GFP Abs, which were detected with Nanogold-coupled secondary Abs and silver enhancement. Cells with replication pattern 3 were selected under bright-field microscope and sectioned. We found that in many cases peripherally located labeled replication sites moved away fron nuclear lamina to a distance of 0.2-0.3 mkm. Interestingly, this detachment was observed mainly when the label concentrated on the distal part of replicating chromatin domain facing nuclear lamina. Next, we studied intranuclear position of large artificial heterochromatic locus that demonstartes orderly relocalization from its peripheral position in G1 to more central location just before and during its replication (Li et al, 1998). Surprisingly, we found that this locus never lost its association with nuclear lamina even during replication. Rather, movement of the locus towards the central nuclear region was accompanied by formation of deep intranuclear invaginations of nuclear envelope. These invaginations always contained lamins A and B. Apparently, we failed to observe complete dissociation of this locus from lamina during replication due to its complex replicative organization, where individual replicative clusters fire asynchronously. Nevertheless, our studies demonstrate dynamics of chromatin-nuclear envelope interactions and high plasticity of nuclear lamina during replication.