Cajal bodies (CBs) are membraneless organelles involved in the assembly and maturation of small nuclear ribonucleoproteins (snRNPs). These processes rely on the survival motor neuron (SMN) protein and its partner coilin, which is essential for SMN localisation to CBs. Here (Ohazama et al., 2024), Hiroshi Maita and colleagues use structured illumination microscopy to analyse the dynamics of SMN and coilin within CBs. Following treatment with harmine (a topoisomerase inhibitor that disrupts CBs), coilin relocates to the peri-nucleolar compartment and separates from SMN, which forms nuclear foci. Removing harmine restores CBs, which then undergo a maturation process: in smaller CBs, SMN and coilin colocalise in granules, whereas in larger CBs, the two proteins segregate, with SMN forming a ring surrounded by coilin. Moreover, overexpression of the SMN mutants E134K, a mutation in the Tudor domain responsible for interaction with coilin, and Y272C, which causes a multimerisation defect, produces distinct effects after harmine treatment: E134K SMN fails to anchor coilin to CBs, whereas Y272C SMN forms enlarged CBs and partially anchors both coilin and snRNPs. Finally, overexpression of an E134K Y227C double mutant mimics the effect of E134K alone, underscoring the importance of the interaction between SMN and coilin for the assembly of CBs. These findings highlight the key role of multimerisation in the segregation of SMN from coilin and the dynamic nature of CBs.
