Positive coactivator PC4 shows dynamic nucleolar distribution required for rDNA transcription and protein synthesis

The nucleolus is the site of rDNA transcription and ribosome biogenesis. Alterations in nucleolar function and architecture correlate with drastic heterochromatin rearrangement and global changes in gene expression. However, the precise mechanism that connects nucleolar function to heterochromatin organization and transcription is yet unknown. Here, we report that the RNA polymerase II (RNA pol II) transactivator and chromatin condenser, Positive Coactivator 4 (PC4), is a bona fide nucleolar protein. PC4 showed dynamic nucleolar accumulation, which is critical for rDNA transcription. The lysine acetyltransferase, KAT5 (Tip60) acetylates PC4 at K35, which facilitates nucleolar release of PC4 and concomitated inhibition of rDNA transcription. By employing PC4 mutant, which is defective in nucleolar accumulation, we found that nucleolar PC4 is crucial for RNA pol I-mediated rDNA transcription. To validate this significant novel role of PC4, in the context of nucleolus organization and function, at the organismal level, we looked into B cell-specific conditional knockout of Sub1 encoding PC4 in mice, which revealed that indeed the rDNA transcription and protein synthesis in B cells are severely repressed in the absence of PC4. Furthermore, PC4 CKO B cells were associated with the loss of H3K9me3-marked heterochromatin foci but not global H3K9me3 levels. LC-MS/MS analysis of the H3K9me3 chromatin complexes revealed that most non-histone heterochromatin proteins were reduced or absent in the constitutive heterochromatin of PC4 CKO B cells. These findings establish PC4 as a critical functional component of nucleolus for rDNA transcription.