A high-throughput screen for nucleolar function reveals a role for the signaling protein, SPRR3, in ribosome biogenesis

SPRR3 is a small, proline-rich protein that promotes cell proliferation. Overexpressed SPRR3 is associated with cancer and regulates AKT phosphorylation at serine 473. However, the specific cellular mechanisms by which SPRR3 drives proliferation are not fully understood. Using a genome-wide siRNA screen in MCF10A breast epithelial cells for decreased nucleolar number, we identified SPRR3 as a novel regulator of ribosome biogenesis. We used siRNA to deplete SPRR3 and found that it is required for transcription of the pre-ribosomal RNA (pre-rRNA), the earliest step in ribosome biogenesis. Furthermore, this reduction in pre-rRNA transcription triggers the nucleolar stress response (increased TP53 protein and CDKN1A mRNA levels) in both MCF10A cells and A549 lung carcinoma cells. Finally, SPRR3 depletion reduces AKT phosphorylation in both cell lines and correlates with lower levels of the RNAPI catalytic subunit POLR1A. In sum, we establish a new role for the non-nucleolar protein SPRR3 in ribosome biogenesis, specifically pre-rRNA transcription, via its ability to facilitate phosphorylation of AKT.