Cajal bodies (CBs) are nuclear organelles involved in the maturation of spliceosomal small nuclear ribonucleoproteins (snRNPs). They concentrate coilin, snRNPs and the survival motor neuron protein (SMN). Dysfunction of CB assembly occurs in spinal muscular atrophy (SMA). Here, we demonstrate that SMN is a SUMO1 target that has a small ubiquitin-related modifier (SUMO)-interacting motif (SIM)-like motif in the Tudor domain. The expression of SIM-like mutant constructs abolishes the interaction of SMN with the spliceosomal SmD1 (also known as SNRPD1), severely decreases SMN-coilin interaction and prevents CB assembly. Accordingly, the SMN SIM-like-mediated interactions are important for CB biogenesis and their dysfunction can be involved in SMA pathophysiology.