The ER Repeat Protein Yt521-B Localizes to a Novel Subnuclear Compartment

The characterization of distinct subnuclear domains suggests a dynamic nuclear framework supporting gene expression and DNA replication. Here, we show that the glutamic acid/arginine-rich domain protein YT521-B localizes to a novel subnuclear structure, the YT bodies. YT bodies are dynamic compartments, which first appear at the beginning of S-phase in the cell cycle and disperse during mitosis. Furthermore, in untreated cells of the human cell line MCF7 they were undetectable and appeared only after drug- induced differentiation. YT bodies contain transcriptionally active sites and are in close contact to other subnuclear structures such as speckles and coiled bodies. YT bodies disperse upon actinomycin D treatment, whereas other transcriptional inhibitors such as α-amanitin or DRB have little effect. On the basis of our experiments, we propose that YT521-B may participate in the assembly of genes into transcription centers, thereby allowing efficient regulation of gene expression.