In order to study mechanisms and regulation of RNA polymerase II (RNAPII) ubiquitylation and degradation, highly purified factors were used to reconstitute RNAPII ubiquitylation in vitro. We show that arrested RNAPII elongation complexes are the preferred substrates for ubiquitylation. Accordingly, not only DNA-damage-dependent but also DNA-damage-independent transcriptional arrest results in RNAPII ubiquitylation in vivo. Def1, known to be required for damage-induced degradation of RNAPII, stimulates ubiquitylation of RNAPII only in an elongation complex. Ubiquitylation of RNAPII is dependent on its C-terminal repeat domain (CTD). Moreover, CTD phosphorylation at serine 5, a hallmark of the initiating polymerase, but not at serine 2, a hallmark of the elongating polymerase, completely inhibits ubiquitylation. In agreement with this, ubiquitylated RNAPII is hypophosphorylated at serine 5 in vivo, and mutation of the serine 5 phosphatase SSU72 inhibits RNAPII degradation. These results identify several mechanisms that confine ubiquitylation of RNAPII to the forms of the enzyme that arrest during elongation.

The proteasome degrades proteins modified by polyubiquitylation, so correctly controlled ubiquitylation is crucial to avoid unscheduled proteolysis of essential proteins. The mechanism regulating proteolysis of RNAPII has been controversial since two distinct ubiquitin ligases (E3s), Rsp5 (and its human homologue NEDD4) and Elongin-Cullin complex, have both been shown to be required for its DNA-damage-induced polyubiquitylation. Here we show that these E3s work sequentially in a two-step mechanism. First, Rsp5 adds mono-ubiquitin, or sometimes a ubiquitin chain linked via ubiquitin lysine 63 that does not trigger proteolysis. When produced, the K63 chain can be trimmed to mono-ubiquitylation by an Rsp5-associated ubiquitin protease, Ubp2. Based on this mono-ubiquitin moiety on RNAPII, an Elc1/Cul3 complex then produces a ubiquitin chain linked via lysine 48, which can trigger proteolysis. Likewise, for correct polyubiquitylation of human RNAPII, NEDD4 cooperates with the ElonginA/B/C-Cullin 5 complex. These data indicate that RNAPII polyubiquitylation requires cooperation between distinct, sequentially acting ubiquitin ligases, and raise the intriguing possibility that other members of the large and functionally diverse family of NEDD4-like ubiquitin ligases also require the assistance of a second E3 when targeting proteins for degradation.elongin ͉ NEDD4 ͉ Rsp5 ͉ ubiquitylation P rotein ubiquitylation plays a crucial role in virtually all cell regulatory pathways. Mono-ubiquitylation commonly alters the activity of the target protein, or tags it for interaction with other factors, while the effect of polyubiquitylation depends on the type of ubiquitin chain being added. Ubiquitin lysine 48 (K48) chains most often result in degradation of the target protein by the proteasome, whereas other chains, such as those occurring through K63, are typically signals for proteolysisindependent pathways (1, 2).One interesting substrate for protein ubiquitylation is RNA-PII, which transcribes all protein-encoding genes in eukaryotes. Ubiquitylation and degradation of RNAPII was first thought to occur specifically in response to DNA damage (3-5), but more recent experiments have shown that RNAPII arrested during transcript elongation as a result of other transcription obstacles is also prone to ubiquitylation and degradation (6). Thus, degradation of RNAPII may be a ''last resort,'' used to clear active genes of persistently arrested RNAPII elongation complexes (6-9). Interestingly, the proteasome is nuclear and can be found on the coding region of genes by chromatin-immunoprecipitation (10), so RNAPII proteolysis may well occur on the DNA.We have reconstituted RNAPII ubiquitylation in vitro with highly purified, physiologically relevant yeast, or human, ubiquitylation factors, respectively (6,11,12). The yeast HECT E3 Rsp5 binds RNAPII via the flexible C-terminal repeat domain (CTD) of the Rpb1 subunit (13), but modifies the polymerase in the main body of the Rpb1 subunit (6,14). Mutation of RSP5 (rsp5-1; temperature-sensitiv...

No abstract

Transcriptional arrest triggers ubiquitylation of RNA polymerase II (RNAPII). We mapped the yeast RNAPII ubiquitylation sites and found that they play an important role in elongation and the DNA-damage response. One site lies in a protein domain that is unordered in free RNAPII, but ordered in the elongating form, helping explain the preferential ubiquitylation of this form. The other site is >125 Angstroms away, yet mutation of either site affects ubiquitylation of the other, in vitro and in vivo. The basis for this remarkable coupling was uncovered: an Rsp5 (E3) dimer assembled on the RNAPII C-terminal domain (CTD). The ubiquitylation sites bind Ubc5 (E2), which in turn binds Rsp5 to allow modification. Evidence for folding of the CTD compatible with this mechanism of communication between distant sites is provided. These data reveal the specificity and mechanism of RNAPII ubiquitylation and demonstrate that E2s can play a crucial role in substrate recognition.

Dictyostelium RacH localizes predominantly to membranes of the nuclear envelope, endoplasmic recticulum and Golgi apparatus. To investigate the role of this protein, we generated knockout and overexpressor strains. RacH-deficient cells displayed 50% reduced fluid-phase uptake and a moderate exocytosis defect, but phagocytosis was unaffected. Detailed examination of the endocytic pathway revealed defective acidification of early endosomes and reduced secretion of acid phosphatase in the presence of sucrose. The distribution of the postlysosomal marker vacuolin was altered, with a high proportion of cells showing a diffuse vesicular pattern in contrast to the wild-type strain, where few intensely stained vacuoles predominate. Cytokinesis, cell motility, chemotaxis and development appeared largely unaffected. In a cell-free system, RacH stimulates actin polymerization, suggesting that this protein is involved in actin-based trafficking of vesicular compartments. We also investigated the determinants of subcellular localization of RacH by expression of green-fluorescent-proteintagged chimeras in which the C-terminus of RacH and the plasma-membrane-targeted RacG were exchanged, the insert region was deleted or the net positive charge of the hypervariable region was increased. We show that several regions of the molecule, not only the hypervariable region, determine targeting of RacH. Overexpression of mistargeted RacH mutants did not recapitulate the phenotypes of a strain overexpressing nonmutated RacH, indicating that the function of this protein is in great part related to its subcellular localization.