Dissecting the role of SMN multimerization in its dissociation from the Cajal body using harmine as a tool compound.

SMN, linked to spinal muscular atrophy, is a key component of the Gemin complex essential for snRNP assembly. Following initial snRNP assembly in the cytoplasm, both snRNPs and SMN migrate to the nucleus and associate with Cajal bodies, where final snRNP maturation occurs. It is assumed that SMN must be free from the Cajal bodies for continuous snRNP biogenesis. Previous observation of the SMN granules docked in CB suggests the existence of a separation mechanism. However, the precise processes that regulate the spatial separation of SMN-complexes from Cajal bodies remain unclear. Here we employed a super-resolution microscope alongside the beta-carboline alkaloid harmine, which disrupted the Cajal body in a reversible manner. Upon removal of harmine, SMN and Coilin first appear as small, interconnected condensates. The SMN condensates mature into spheroidal structures encircled by Coilin, eventually segregating into distinct condensates. Expression of a multimerization-deficient SMN mutant leads to enlarged, atypical Cajal bodies where SMN is unable to segregate into separate condensates. These findings underscore the importance of multimerization in facilitating the segregation of SMN from Coilin within Cajal bodies.