De Novo Formation of a Subnuclear Body

The mammalian cell nucleus contains structurally stable functional compartments. We show here that one of them, the Cajal body ( CB), can be formed de novo. Immobilization on chromatin of both CB structural components, such as coilin, and functional components of the CB, such as the SMN complex, spliceosomal small nuclear ribonucleoproteins ( RNPs), small nucleolar RNPs, and small Cajal body- specific RNPs, is sufficient for the formation of a morphologically normal and apparently functional CB. Biogenesis of the CB does not follow a hierarchical assembly pathway and exhibits hallmarks of a self- organizing structure.