RNA polymerase II subunit 3 is retained in the cytoplasm by its interaction with HCR, the psoriasis vulgaris candidate gene product.

Here, we show that the subcellular localization of alpha-like RNA polymerase II core subunit 3 (RPB3) is regulated during muscle differentiation. We have recently demonstrated that the expression of RPB3 is regulated during muscle differentiation and that, inside RNA polymerase II (RNAP II), it is directly involved in contacting regulatory proteins such as the myogenic transcription factor Myogenin and activating transcription factor ATF4. We show for the first time, that RPB3, in addition to its presence and role inside the RNAP II core enzyme, accumulates in the cytoplasm of cycling myogenic cells and migrates to the nucleus upon induction of the differentiation program. Furthermore, using human RPB3 as bait in a yeast two-hybrid system, we have isolated a novel RPB3 cytoplasmic interacting protein, HCR. HCR, previously identified as alpha-helix coiled-coil rod homologue, is one of the psoriasis vulgaris (PV) candidate genes. In cycling myogenic C2C7 cells, we show that the RPB3 protein directly interacts with HCR within the cytoplasm. Finally, knocking down HCR expression by RNA interference, we demonstrate that HCR acts as cytoplasmic docking site for RPB3.