Wilhelm bernhard workshop on the cell nucleus

MICROSPECKLES, AN UNUSUAL NUCLEAR DOMAIN OF QUIESCENT (G₀ AND G₀₂) ONION CELLS, ARE TRANSIENT STORAGE SITES FOR snRNPs

Cui, P and Moreno Díaz de la Espina, S

Nuclear Matrix Laboratory, CIB CSIC, Madrid, Spain
Antibodies against U2B”, the sDMA epitope of Sm proteins and coilin, and p105, a protein of interchromatin granules (IGs), were used to investigate the nuclear distribution of the splicing factors in onion meristematic root cells in different physiological conditions. In steady-state proliferating cells, the spliceosomal components were distributed into a diffuse nucleoplasmic network, similar to that of IGs, and numerous Cajal bodies (CBs) by confocal microscopy. These two domains are the counterpart of the perichromatin fibrils and granules, IGs and CBs observed by EM after EDTA and bismuth oxynitrate stainings, and immunogold labelling with the same antibodies. The dormant cells of the root blastema in bulbs are stopped in both pre-replicative (G₀) and post replicative (G₂) states and after water activation they must undergo dramatic metabolic changes for progression through the first post-quiescence cell cycle and later cell division. They have a nuclear distribution of the proteins in small CBs and in an unusual nuclear domain, the micro-speckles, corresponding to storage sites for RNPs, which were rapidly mobilised after water imbibition, and showed a different ultrastructure from that of CBs and the interchromatin RNP network at the EM. The spliceosomal proteins relocated to a diffuse nucleoplasmic network and CBs when the cells were released from dormancy by water soaking and they re-started their proliferative activity. Inhibition of RNA synthesis by DRB treatment in proliferating cells demonstrated that the micro-speckles were not the morphological expression of a transcription block. Fractionation and confocal microscopy studies showed a differential association of the splicing factors with the nuclear matrix depending not only on the protein, but also on nuclear activity. Our results suggest a reversible relocation of the spliceosomal proteins between different sub-nuclear domains in physiological conditions.