IS THE SPHERE ORGANELLE COILED BODY A UNIVERSAL NUCLEAR-COMPONENT

We present evidence for the essential homology of four nuclear organel les that have previously been described under four different names: co iled bodies in mammalian somatic nuclei, prenucleolar bodies in nuclei assembled in vitro in Xenopus egg extract, sphere organelles in amphi bian germinal vesicles (GVs), and Binnenkorper in insect GVs. Each of these organelles contains coilin or a coilin-related protein plus a va riety of small nuclear ribonucleoproteins. We suggest that the sphere organelle/coiled body is a ''universal'' nuclear component in the sens e that it is involved in common nuclear processes and hence will be fo und in one form or another in most eukaryotic cells. We postulate that it functions in the assembly and sorting of snRNP complexes for three RNA processing pathways: pre-mRNA splicing, rRNA processing, and hist one pre-mRNA 3' end formation. Specifically, the sphere organelle/coil ed body may be the initial site for assembly of processing complexes, which are then sorted to other places in the nucleus, where the actual RNA processing takes place. (C) 1995 Wiley-Liss, Inc.