Ubiquitin proteomics identifies RNA polymerase I as a target of the Smc5/6 complex.

Ubiquitination controls numerous cellular processes, and its deregulation is associated with many pathologies. The Nse1 subunit in the Smc5/6 complex contains a RING domain with ubiquitin E3 ligase activity and essential functions in genome integrity. However, Nse1-dependent ubiquitin targets remain elusive. Here, we use label-free quantitative proteomics to analyze the nuclear ubiquitinome of nse1-C274A RING mutant cells. Our results show that Nse1 impacts the ubiquitination of several proteins involved in ribosome biogenesis and metabolism that, importantly, extend beyond canonical functions of Smc5/6. In addition, our analysis suggests a connection between Nse1 and RNA polymerase I (RNA Pol I) ubiquitination. Specifically, Nse1 and the Smc5/6 complex promote ubiquitination of K408 and K410 in the clamp domain of Rpa190, a modification that induces its degradation in response to blocks in transcriptional elongation. We propose that this mechanism contributes to Smc5/6-dependent segregation of the rDNA array, the locus transcribed by RNA Pol I.

Sumoylation of Smc5 Promotes Error-free Bypass at Damaged Replication Forks.

Replication of a damaged DNA template can threaten the integrity of the genome, requiring the use of various mechanisms to tolerate DNA lesions. The Smc5/6 complex, together with the Nse2/Mms21 SUMO ligase, plays essential roles in genome stability through undefined tasks at damaged replication forks. Various subunits within the Smc5/6 complex are substrates of Nse2, but we currently do not know the role of these modifications. Here we show that sumoylation of Smc5 is targeted to its coiled-coil domain, is upregulated by replication fork damage, and participates in bypass of DNA lesions. smc5-KR mutant cells display defects in formation of sister chromatid junctions and higher translesion synthesis. Also, we provide evidence indicating that Smc5 sumoylation modulates Mph1-dependent fork regression, acting synergistically with other pathways to promote chromosome disjunction. We propose that sumoylation of Smc5 enhances physical remodeling of damaged forks, avoiding the use of a more mutagenic tolerance pathway.

Characterization of cytoplasmic cyclin D1 as a marker of invasiveness in cancer.

Cyclin D1 (Ccnd1) is a proto-oncogen amplified in many different cancers and nuclear accumulation of Ccnd1 is a characteristic of tumor cells. Ccnd1 activates the transcription of a large set of genes involved in cell cycle progress and proliferation. However, Ccnd1 also targets cytoplasmic proteins involved in the regulation of cell migration and invasion. In this work, we have analyzed by immunohistochemistry the localization of Ccnd1 in endometrial, breast, prostate and colon carcinomas with different types of invasion. The number of cells displaying membranous or cytoplasmic Ccnd1 was significantly higher in peripheral cells than in inner cells in both collective and pushing invasion patterns of endometrial carcinoma, and in collective invasion pattern of colon carcinoma. Also, the cytoplasmic localization of Ccnd1 was higher when tumors infiltrated as single cells, budding or small clusters of cells. To evaluate cytoplasmic function of cyclin D1, we have built a variant (Ccnd1-CAAX) that remains attached to the cell membrane therefore sequestering this cyclin in the cytoplasm. Tumor cells harboring Ccnd1-CAAX showed high levels of invasiveness and metastatic potential compared to those containing the wild type allele of Ccnd1. However, Ccnd1-CAAX expression did not alter proliferative rates of tumor cells. We hypothesize that the role of Ccnd1 in the cytoplasm is mainly associated with the invasive capability of tumor cells. Moreover, we propose that subcellular localization of Ccnd1 is an interesting guideline to measure cancer outcome.