Molecular compartmentalization in a syncytium: restricted mobility of proteins within the sea urchin skeletogenic mesenchyme

Multinucleated cells, or syncytia, are found in diverse taxa. Their biological function is often associated with the compartmentalization of biochemical or cellular activities within the syncytium. How such compartments are generated and maintained is poorly understood. The sea urchin embryonic skeleton is secreted by a syncytium, and local patterns of skeletal growth are associated with distinct sub-domains of gene expression within the syncytium. For such molecular compartments to be maintained and to control local patterns of skeletal growth: 1) the mobility of TFs must be restricted to produce stable differences in the transcriptional states of nuclei within the syncytium, and 2) the mobility of biomineralization proteins must also be restricted to produce regional differences in skeletal growth patterns. To test these predictions, we expressed fluorescently-tagged forms of transcription factors and biomineralization proteins in sub-domains of the skeletogenic syncytium. We found that both classes of proteins have restricted mobility within the syncytium and identified motifs that limit their mobility. Our findings have general implications for understanding the functional and molecular compartmentalization of syncytia. Summary statement Transcription factors and effector proteins have limited mobility within the skeletogenic syncytium of the sea urchin embryo.