IDENTIFICATION OF AN INTERCHROMOSOMAL COMPARTMENT BY POLYMERIZATION OF NUCLEAR-TARGETED VIMENTIN

A number of structural and functional subnuclear compartments have bee n described, including regions exclusive of chromosomes previously hyp othesized to form a reactive nuclear space. We have now explored this accessible nuclear space and interchromosomal nucleoplasmic domains ex perimentally using Xenopus vimentin engineered to contain a nuclear lo calization signal (NLS-vimentin). In stably transfected human cells in cubated at 37 degrees C, the NLS-vimentin formed a restricted number o f intranuclear speckles, At 28 degrees C, the optimal temperature for assembly of the amphibian protein, NLS-vimentin progressively extended with time out from the speckles into strictly orientated intranuclear filamentous arrays. This enabled us to observe the development of a s ystem of interconnecting channel-like areas. Quantitative analysis bas ed on 3-D imaging microscopy revealed that these arrays were localized almost exclusively outside of chromosome territories. During mitosis the filaments disassembled and dispersed throughout the cytoplasm, whi le in anaphase-telophase the vimentin was recruited back into the nucl eus and reassembled into filaments at the chromosome surfaces, in dist ributions virtually identical to those observed in the previous interp hase. The filaments also colocalized with specific nuclear RNAs, coile d bodies and PML bodies, all situated outside of chromosome territorie s, thereby interlinking these structures. This strongly implies that t hese nuclear entities coexist in the same interconnected nuclear compa rtment. The assembling NLS-vimentin is restricted to and can be used t o delineate, at least in part, the formerly proposed reticular interch romosomal domain compartment (ICD), The properties of NLS-vimentin mak e it an excellent tool for performing structural and functional studie s on this compartment.