Triblock copolymer micelle model of spherical paraspeckles

Paraspeckles are nuclear bodies composed of architectural RNA (arcRNA) and RNA-binding proteins. In the wild type, the blocks at the two terminal regions of arcRNAs compose the shell of paraspeckles and the middle region between the two terminal blocks composes the core, analogous to micelles of ABC triblock copolymers. We here use an extension of the theory of polymer micelles to predict the structure and size of paraspeckles as one decreases the length of one of the terminal blocks of arcRNA by CRISPR/Cas9, assuming that paraspeckles are spherical. Our theory predicts that when the length of the edited terminal blocks is larger than a critical value, paraspeckles show discontinuous transitions between the structure in which all the edited terminal blocks are localized in the shell and the structure in which all the edited terminal blocks are localized in the core at a threshold value of the transcription rate of arcRNA. In contrast, when the length of the edited terminal blocks is smaller than the critical value, the population of edited terminal blocks in the shell decreases continuously as one increases the transcription rate of arcRNA. The size of paraspeckles increases as one decreases the length of the edited terminal blocks. These predictions are consistent with our experiments.