Dsc E3 ligase localization to the Golgi requires the ATPase Cdc48 and cofactor Ufd1 for activation of sterol regulatory element-binding protein in fission yeast

AAA-ATPase valosin-containing protein binds the transcription factor SREBP1 and promotes its proteolytic activation by rhomboid protease RHBDL4

Valosin-containing protein (VCP) is a member of AAA-ATPase superfamily involved in various cellular functions. To investigate the pathophysiological role of VCP in metabolic disorders, we generated knock-in mice bearing an A232E mutation in VCP, a known human VCP pathogenic variant. When heterozygous mutant mice (A232E/+) were fed a high-fat diet, we observed that fatty liver was ameliorated and the proteolytic processing of the transcription factor sterol regulatory element-binding protein 1 (SREBP1) was impaired. Further co-immunoprecipitation analysis in wildtype mice revealed interactions of VCP with SREBP1 and a rhomboid protease, RHBDL4, in the liver, and these interactions were attenuated in A232E/+ mice. Consistent with these results, we show that knockdown or chemical inhibition of VCP or RHBDL4 in human hepatocytes impaired the proteolytic processing of SREBP1. Finally, we found that knockdown of E3 ligases such as glycoprotein 78 and HMG-CoA reductase degradation protein 1 disrupted the interaction of VCP with SREBP1 and impaired the proteolytic processing of SREBP1. These results suggest that VCP recognizes ubiquitinylated SREBP1 and recruits it to RHBDL4 to promote its proteolytic processing. The present study reveals a novel proteolytic processing pathway of SREBP1 and may lead to development of new therapeutic strategies to treat fatty liver diseases.

Cdc48 influence on separase levels is independent of mitosis and suggests translational sensitivity of separase

Cdc48 (p97/VCP) is a AAA-ATPase that can extract ubiquitinated proteins from their binding partners and can cooperate with the proteasome for their degradation. A fission yeast cdc48 mutant (cdc48-353) shows low levels of the cohesin protease, separase, and pronounced chromosome segregation defects in mitosis. Separase initiates chromosome segregation when its binding partner securin is ubiquitinated and degraded. The low separase levels in the cdc48-353 mutant have been attributed to a failure to extract ubiquitinated securin from separase, resulting in co-degradation of separase along with securin. If true, Cdc48 would be important in mitosis. In contrast, we show here that low separase levels in the cdc48-353 mutant are independent of mitosis. Moreover, we find no evidence of enhanced separase degradation in the mutant. Instead, we suggest that the cdc48-353 mutant uncovers specific requirements for separase translation. Our results highlight a need to better understand how this key mitotic enzyme is synthesized.

The role of rhomboid superfamily members in protein homeostasis: Mechanistic insight and physiological implications

Cells are equipped with protein quality control pathways in order to maintain a healthy proteome; a process known as protein homeostasis. Dysfunction in protein homeostasis leads to the development of many diseases that are associated with proteinopathies. Recently, the rhomboid superfamily has attracted much attention concerning their involvement in protein homeostasis. While their functional role has become much clearer in the last few years, their systemic significance in mammals remains elusive. Here we delineate the current knowledge of rhomboids in protein quality control and how these functions are integrated at the organismal level.

AMPKα Subunit Ssp2 and Glycogen Synthase Kinases Gsk3/Gsk31 are involved in regulation of sterol regulatory element-binding protein (SREBP) activity in fission yeast

Sterol regulatory element-binding protein (SREBP), a highly conserved family of membrane-bound transcription factors, is an essential regulator for cellular cholesterol and lipid homeostasis in mammalian cells. Sre1, the homolog of SREBP in the fission yeast Schizosaccharomyces pombe (S. pombe), regulates genes involved in the transcriptional responses to low sterol as well as low oxygen. Previous study reported that casein kinase 1 family member Hhp2 phosphorylated the Sre1 N-terminal transcriptional factor domain (Sre1N) and accelerated Sre1N degradation, and other kinases might exist for regulating the Sre1 function. To gain insight into the mechanisms underlying the Sre1 activity and to identify additional kinases involved in regulation of Sre1 function, we developed a luciferase reporter system to monitor the Sre1 activity through its binding site called SRE2 in living yeast cells. Here we showed that both ergosterol biosynthesis inhibitors and hypoxia-mimic CoCl₂ caused a dose-dependent increase in the Sre1 transcription activity, concurrently, these induced transcription activities were almost abolished in Δsre1 cells. Surprisingly, either AMPKα Subunit Ssp2 deletion or Glycogen Synthase Kinases Gsk3/Gsk31 double deletion significantly suppressed ergosterol biosynthesis inhibitors- or CoCl₂-induced Sre1 activity. Notably, the Δssp2Δgsk3Δgsk31 mutant showed further decreased Sre1 activity when compared with their single or double deletion. Consistently, the Δssp2Δgsk3Δgsk31 mutant showed more marked temperature sensitivity than any of their single or double deletion. Moreover, the fluorescence of GFP-Sre1N localized at the nucleus in wild-type cells, but significantly weaker nuclear fluorescence of GFP-Sre1N was observed in Δssp2, Δgsk3Δgsk31, Δssp2Δgsk3, Δssp2Δgsk31 or Δssp2Δgsk3Δgsk31 cells. On the other hand, the immunoblot showed a dramatic decrease in GST-Sre1N levels in the Δgsk3Δgsk31 or the Δssp2Δgsk3Δgsk31 cells but not in the Δssp2 cells. Altogether, our findings suggest that Gsk3/Gsk31 may regulate Sre1N degradation, while Ssp2 may regulate not only the degradation of Sre1N but also its translocation to the nucleus.

Sorting of a multi-subunit ubiquitin ligase complex in the endolysosome system

The yeast Dsc E3 ligase complex has long been recognized as a Golgi-specific protein ubquitination system. It shares a striking sequence similarity to the Hrd1 complex that plays critical roles in the ER-associated degradation pathway. Using biochemical purification and mass spectrometry, we identified two novel Dsc subunits, which we named as Gld1 and Vld1. Surprisingly, Gld1 and Vld1 do not coexist in the same complex. Instead, they compete with each other to form two functionally independent Dsc subcomplexes. The Vld1 subcomplex takes the AP3 pathway to reach the vacuole membrane, whereas the Gld1 subcomplex travels through the VPS pathway and is cycled between Golgi and endosomes by the retromer. Thus, instead of being Golgi-specific, the Dsc complex can regulate protein levels at three distinct organelles, namely Golgi, endosome, and vacuole. Our study provides a novel model of achieving multi-tasking for transmembrane ubiquitin ligases with interchangeable trafficking adaptors.

Proteins perform many tasks and, to remain healthy, each cell must ensure that its proteins are in good condition and present at the right levels. Plants, animals and fungi all largely deal with damaged, or otherwise unneeded, proteins by tagging them with a small marker called ubiquitin. The tagged proteins are then rapidly destroyed, which prevents them from harming the cells.

Enzymes known as E3 ligases attach ubiquitin to proteins. Yet, the number of E3 ligases is dwarfed by the number of proteins modified with ubiquitin. For instance, humans have approximately 20,000 different proteins, about one third of which are found in or on cell membranes. However, there are only around 600 E3 ligases, and only about 50 of them are associated with cell membranes. This is further complicated by the fact that proteins are also present in distinct compartments within the cell.

The Dsc complex, for example, is an E3 ligase from yeast that is found within a compartment of the cell known as the Golgi. It was thus expected to only attach ubiquitin to Golgi proteins. Yet some recent studies showed that the Dsc complex could also tag proteins present in two other compartments of yeast cells: the endosome and vacuole. How can the Dsc complex act on proteins in three distinct compartments?

The Dsc complex is actually made from multiple proteins, and Yang et al. now report two new protein components. Biochemical and genetic tools showed that these two proteins do not co-exist in the same Dsc complex. Instead, they compete with each other to form two different kinds of Dsc complexes, which Yang et al. refer to as subcomplexes.

Further work showed that the two new proteins determine the route taken by the Dsc complex along the cell’s protein transport pathway. One subcomplex is transported to the vacuole and the other cycles between the Golgi and endosomes. Thus, by changing just one component, the Dsc complex can be sent to different locations within the cell.

These findings describe a new mechanism that enables E3 ligases to multi-task on a wide range of proteins, even across distinct compartments of the cell. Future work will determine whether plant and animal cells also use a similar strategy. Since defects in protein quality control contribute to many human diseases, such as Alzheimer's and Parkinson's disease, working out how E3 ligases work is important for the field of biomedicine.