ABSTRACT
Survival motor neuron protein (SMN), which is linked to spinal muscular atrophy, is a key component of the Gemin complex, which is essential for the assembly of small nuclear RNA-protein complexes (snRNPs). After 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 the Cajal bodies 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 have employed a super-resolution microscope alongside the β-carboline alkaloid harmine, which disrupts 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 in which 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.
Footnotes
Author contributions
Conceptualization: H.M.; Investigation: S.O., A.F., D.K., R.Y., H.M.; Resources: R.Y., S.N., H.M.; Data curation: S.O., H.M.; Writing - original draft: S.O., H.M.; Writing - review & editing: S.N., H.M.; Visualization: S.O., H.M.; Supervision: S.N., H.M.; Project administration: S.N., H.M.; Funding acquisition: S.N., H.M.
Funding
This study was supported by the Japan Science and Technology Agency Support for Pioneering Research Initiated by the Next Generation (JPMJSP2119 to S.O.) and the Japan Society for the Promotion of Science (KAKENHI 16K08225, 20H04687, 20K07028 and 23K06093 to H.M., and KAKENHI Grant-in-Aid for Transformative Research Areas 21H05274 to S.N.). This study was also supported by the Platform Project for Supporting Drug Discovery and Life Science Research [Basis for Supporting Innovative Drug Discovery and Life Science Research (BINDS)] from the Japan Agency for Medical Research and Development (JP21am0101001).
Data availability
Vector maps and sequences confirmed by Sanger sequencing have been deposited in Figshare (https://doi.org/10.6084/m9.figshare.26048449). All other relevant data can be found within the article and its supplementary information.
