Coilin Modulates Nuclear Organization by Promoting Protein SLMOylation

Coilin was first identified as the marker protein for Cajal Bodies (CBs). However, CBs are only abundant in neuronal, embryonic, and cancer cells, while primary cells have few CBs. Even in cells with CBs, 70% of coilin is nucleoplasmic, not found in CBs. CBs are subnuclear domains that function in small nuclear ribonucleoprotein (snRNP) biogenesis as well as production of telomerase RNP, expression and 3'end processing of small nuclear RNAs (snRNAs), small nucleolar RNAs (snoRNAs), and small CB-associated RNAs (scaRNAs). The function of nucleoplasmic coilin is not well understood. We have previously shown that coilin promotes miRNA biogenesis by promoting phosphorylation of the Microprocessor component, DGCR8, at its S377 residue in both cells with and without CBs. We have since identified 7 additional serine residues of DGCR8 with decreased phosphorylation in coilin knockdown through phosphoproteomic analysis of HeLa cells. In addition to phosphorylation, the addition of a small ubiquitin-like modifier (SUMO), or SUMOylation, of DGCR8 also increases its stability. Because of coilin's role in the promotion of one post-translational modification, here we aimed to test the hypothesis that coilin knockdown alters protein SUMOylation and results in altered nuclear organization. We conducted a series of siRNA knockdowns, Western blots, and immunoprecipitation experiments. Here we show, for the first time, that coilin knockdown results in global decrease of protein SUMOylation in cells with (HeLa) and without CBs (WI-38), suggesting that regulation of protein SUMOylation may be a function of nucleoplasmic coilin. Specifically, we found that coilin knockdown results in decreased SUMOylation of DGCR8 and Speckled protein 100kDa (Sp100), a major component of the promyelocytic leukemia nuclear body (PML-NB). Sp100 is vital to PML-NB formation, and decreased SUMOylation of Sp100 decreases its stability. This information led us to the second part of our hypothesis. We used immunofluorescence and found that coilin knockdown results in decreased numbers of PML-NBs per cell. To summarize, we have identified coilin as a regulator of protein phosphorylation, SUMOylation, and nuclear organization.