Residual Cajal bodies in coilin knockout mice fail to recruit Sm snRNPs and SMN, the spinal muscular atrophy gene product

Cajal bodies (CBs) are nuclear suborganelles involved in the biogenesis of small nuclear ribonucleoproteins (snRNPs). In addition to snRNPs, they are highly enriched in basal transcription and cell cycle factors, the nucleola r proteins fibrillarin (Fb) and Nopp140 (Nopp), the survival motor neuron ( SMN) protein complex, and the CB marker protein, p80 coilin. We report the generation of knockout mice lacking the COOH-terminal 487 amino acids of co ilin. Northern and Western blot analyses demonstrate that we have successfu lly removed the full-length coilin protein from the knockout animals. Some homozygous mutant animals are viable, but their numbers are reduced signifi cantly when crossed to inbred backgrounds. Analysis of tissues and cell lin es from mutant animals reveals the presence of extranucleolar foci that con tain Fb and Nopp but not other typical nucleolar markers. These so-called " residual" CBs neither condense Sm proteins nor recruit members of the SMN p rotein complex. Transient expression of wild-type mouse coilin in knockout cells results in formation of CBs and restores these missing epitopes. Our data demonstrate that full-length coilin is essential for proper formation and/or maintenance of CBs and that recruitment of snRNP and SMN complex pro teins to these nuclear subdomains requires sequences within the coilin COOH terminus.