Identification of coilin interactors reveals coordinated control of Cajal body number and substructure

The cell nucleus contains distinct biomolecular condensates that form at specific genetic loci, organize chromosomes in 3D space, and regulate RNA processing. Among these, Cajal bodies (CBs) require key “scaffolding” proteins for their assembly, which is not fully understood. Here we employ proximity biotinylation, mass spectrometry, and functional screening to comprehensively identify and test functions of CB components. We document 144 protein interactors of coilin, of which 70 were newly detected, and establish 25 players needed for CB assembly and/or maintenance. Surprisingly, depletion of nine coilin interactors – mostly constituents of the 60S ribosome (RPLs) – increased CB number and caused subdomains defined by coilin and the Survival of Motor Neuron protein (SMN) to merge. These phenotypes were traceable to altered nuclear levels of dimethylarginine. Our data implicate RPL24 and other players in the regulation of CBs by modulating post-translational modifications. Moreover, the prevalence of transcription factors among the identified components highlights roles for gene activity in CB assembly and nuclear positioning.