FATE OF SPECIFIC NUCLEOLAR PERICHROMOSOMAL PROTEINS DURING MITOSIS - CELLULAR-DISTRIBUTION AND ASSOCIATION WITH U3 SNORNA

In mammalian cells, the nucleoli disintegrate during mitosis and some nucleolar proteins disperse at the periphery of all chromosomes formin g a novel class of chromosomal passenger proteins. The nucleolar compo nents which participate in the formation of this perichromosomal layer have been investigated to elucidate the role of these perichromosomal proteins in the assembly and disassembly of the nucleoli. i) Electron microscopy immunolabelling reveals that these proteins are predominan tly located in the granular component of the nucleoli during interphas e. ii) Immunoprecipitation data suggest that they are distributed at t he chromosome periphery in association with U3 small nucleolar RNA (sn oRNA). In addition, the distribution of U3 snoRNA visualized by in sit u hybridization, is similar to that observed for the perichromosomal p roteins. iii) In cells which possess a nucleolar remnant during mitosi s, U3 snoRNA and perichromosomal proteins were found both in the peric hromosomal layer and in the nucleolar remnant. iv) Some of these prote ins are conserved from yeast to man such as fibrillarin and a protein of 52 kDa. v) The location of these proteins observed in yeast by conf ocal microscopy shows that they are not dispersed during mitosis. Thei r partition between the two daughter cells is performed by scission of nucleolar structures forming a rod during the budding process. Theref ore RNP complexes related to the processing steps of ribosome biogenes is in mammalian cells quit the nucleolus in late Gz and associate with the chromosome periphery until late telophase. They associate in the perichromosomal layer in human and PtK1 cells and both in the perichro mosomal layer and the nucleolar remnant in CHO cells.