Ubiquitin-mediated proteolysis plays a central role in protein degradation in eukaryotes. The largest known class of ubiquitin ligases is Cullin-RING ubiquitin ligases (CRLs). CRL activation requires the conjugation of the ubiquitin-like protein Nedd8 to the Cullin. CRLs are deneddylated by the COP9 signalosome (CSN), which keeps CRLs from becoming hyperactive and destabilizing its own components. A recent study in humans reported that a conserved protein, SAP130, interacts with fully assembled CRL complexes and with CSN (Menon et al., 2008 BMC Biochem). Human SAP130 functions in three different cellular contexts: as a component of the STAGA and TFTC general transcription complexes; as a component of the U2 splicing complex; and binding to CRLs and CSN. The function of human SAP130 in CRL regulation is unknown, as siRNA knockdown or overexpression was not reported to produce noticeable changes in CRL activity. To determine if the C. elegans SAP130 ortholog TAG-203 is required for CRL function, we analyzed
tag-203(RNAi) animals. We examined
tag-203 RNAi phenotypes to determine if they matched those of cullin mutants.
tag-203 RNAi causes embryonic lethality, however, the early embryonic phenotypes associated with
cul-2 and
cul-3 mutants were not observed. L1 larvae subjected to
tag-203 feeding RNAi develop enlarged cells in several mitotically dividing lineages, including seam cells, P cells, and somatic gonadal cells. The enlargement of these cells is superficially similar to that observed in
cul-4 mutants, where cells enlarge due to re-replication of genomic DNA. However, the DNA content of the enlarged
tag-203(RNAi) cells is not increased to the extent seen in
cul-4 mutants. Depletion of
tag-203 in mutants of the CRL regulators CAND-1 and CSN, enhances the enlarged cell phenotype, suggesting genetic interaction. To clarify which of the potential roles for TAG-203 (transcription, splicing, or CRL regulation) is linked to the cell enlargement, we inactivated other components of the transcription and splicing complexes in which SAP130 functions. Inactivation of
trr-1, which encodes a predicted component of the STAGA and TFTC transcription complexes, did not produce enlarged cells. However, inactivation of multiple splicing components produced enlarged cell phenotypes that were similar to that of
tag-203 RNAi. This suggests that TAG-203's role as a splicing component is critical for preventing the cell enlargement rather than a direct role in regulating CRLs.