The spinal muscular atrophy disease gene product, SMN: A link between snRNP biogenesis and the Cajal (coiled) body

The spliceosomal snRNAs U1, U2, U4, and U5 are synthesized in the nucleus, exported to the cytoplasm to assemble with Sm proteins, and reimported to t he nucleus as ribonucleoprotein particles. Recently two novel proteins invo lved in biogenesis of small nuclear ribonucleoproteins (snRNPs) were identi fied, the Spinal muscular atrophy disease gene product (SMN) and its associ ated protein SIP1. It was previously reported that in HeLa cells, SMN and S IP1 form discrete foci located next to Cajal (coiled) bodies, the so-called "gemini of coiled bodies" or "gems." An intriguing feature of gems is that they do not appear to contain snRNPs, Here we show that gems are present i n a variable but small proportion of rapidly proliferating cells in culture . In the vast majority of cultured cells and in all primary neurons analyze d, SMN and SIP1 colocalize precisely with snRNPs in the Cajal body, The pre sence of SMN and SIP1 in Cajal bodies is confirmed by immunoelectron micros copy and by microinjection of antibodies that interfere with the integrity of the structure. The association of SMN with snRNPs and coilin persists du ring cell division, but at the end of mitosis there is a lag period between assembly of new Cajal bodies in the nucleus and detection of SMN in these structures, suggesting that SMN is targeted to preformed Cajal bodies. Fina lly, treatment of cells with leptomycin B (a drug that blocks export of U s nRNAs to the cytoplasm and consequently import of new snRNPs into the nucle us) is shown to deplete snRNPs (but not SMN or SIP1) from the Cajal body. T his suggests that snRNPs flow through the Cajal body during their biogenesi s pathway.