Mammalian Toll-like receptors (TLRs) 3, 7, 8 and 9 initiate immune responses to infection by recognizing microbial nucleic acids1, 2; however, these responses come at the cost of potential autoimmunity due to inappropriate recognition of self nucleic acid3. The localization of TLR9 and TLR7 to intracellular compartments appears to play a role in facilitating responses to viral nucleic acids while maintaining tolerance to self nucleic acid, yet the cell biology regulating the trafficking and localization of these receptors remains poorly understood4-6. Here, we define the route by which TLR9 and TLR7 exit the endoplasmic reticulum (ER) and traffic to endolysosomes. Surprisingly, the ectodomains of TLR9 and TLR7 are cleaved in the endolysosome, such that no full-length protein is detectable in the compartment where ligand is recognized. Remarkably, though both the full-length and cleaved forms of TLR9 are capable of binding ligand, only the processed form recruits MyD88 upon activation, arguing that this truncated receptor, rather than the full-length form, is functional. Furthermore, conditions that prevent receptor proteolysis, including forced TLR9 surface localization, render the receptor non-functional. We propose that ectodomain cleavage represents a strategy to restrict receptor activation to endolysosomal compartments and prevent TLRs from responding to self nucleic acid.

Receptor-interacting protein kinase 1 (RIPK1) and RIPK3 trigger pro-inflammatory cell death termed "necroptosis." Studies with RIPK3-deficient mice or the RIPK1 inhibitor necrostatin-1 suggest that necroptosis exacerbates pathology in many disease models. We engineered mice expressing catalytically inactive RIPK3 D161N or RIPK1 D138N to determine the need for the active kinase in the whole animal. Unexpectedly, RIPK3 D161N promoted lethal RIPK1- and caspase-8-dependent apoptosis. In contrast, mice expressing RIPK1 D138N were viable and, like RIPK3-deficient mice, resistant to tumor necrosis factor (TNF)-induced hypothermia. Cells expressing RIPK1 D138N were resistant to TNF-induced necroptosis, whereas TNF-induced signaling pathways promoting gene transcription were unperturbed. Our data indicate that the kinase activity of RIPK3 is essential for necroptosis but also governs whether a cell activates caspase-8 and dies by apoptosis.

Ubiquitination by the anaphase-promoting complex (APC/C) is essential for proliferation in all eukaryotes. The human APC/C promotes the degradation of mitotic regulators by assembling K11-linked ubiquitin chains, the formation of which is initiated by its E2 UbcH10. Here, we identify the conserved Ube2S as a K11-specific chain elongating E2 for human and Drosophila APC/C. Ube2S depends on the cell cycledependent association with the APC/C activators Cdc20 and Cdh1 for its activity. While depletion of Ube2S already inhibits APC/C in cells, the loss of the complete UbcH10/Ube2S-module leads to dramatic stabilization of APC/C substrates, severe spindle defects, and a strong mitotic delay. Ube2S and UbcH10 are tightly co-regulated in the cell cycle by APC/C-dependent degradation. We conclude that UbcH10 and Ube2S constitute a physiological E2-module for APC/C, the activity of which is required for spindle assembly and cell division.K11-linked chain ͉ proteasome ͉ ubiquitin

Packaging of proteins from the ER into COPII-vesicles is essential for secretion. In cells, most COPII-vesicles are ~60-80nm in diameter, yet some must increase their size to accommodate 300-400nm procollagen fibers or chylomicrons. Impaired COPII function results in collagen deposition defects, cranio-lenticulo-sutural dysplasia, or chylomicron retention disease, but mechanisms to enlarge COPII-coats have remained elusive. Here, we have identified the ubiquitin ligase Cul3Klhl12 as a regulator of COPII coat formation. Cul3Klhl12 catalyzes the monoubiquitination of the COPII-component Sec31 and drives the assembly of large COPII coats. As a result, ubiquitination by Cul3Klhl12 is essential for collagen export, yet less important for the transport of small cargo. We conclude that monoubiquitination controls the size and function of a vesicle coat.

Ubiquitin chains of different topologies trigger distinct functional consequences, including protein degradation and reorganization of complexes. The assembly of most ubiquitin chains is promoted by E2s, yet how these enzymes achieve linkage specificity is poorly understood. We have discovered that the K11-specific Ube2S orients the donor ubiquitin through an essential non-covalent interaction that occurs in addition to the thioester bond at the E2 active site. The E2-donor ubiquitin complex transiently recognizes the acceptor ubiquitin, primarily through electrostatic interactions. The recognition of the acceptor ubiquitin surface around Lys11, but not around other lysines, generates a catalytically competent active site, which is composed of residues of both Ube2S and ubiquitin. Our studies suggest that monomeric E2s promote linkage-specific ubiquitin chain formation through substrate-assisted catalysis.

Polyubiquitination is a posttranslational modification where ubiquitin chains containing isopeptide bonds linking one of seven ubiquitin lysines with the C terminus of an adjoining ubiquitin are covalently attached to proteins. While functions of K48- and K63-linked polyubiquitin are understood, the role(s) of noncanonical K11-linked chains is less clear. A crystal structure of K11-linked diubiquitin demonstrates a distinct conformation from K48- or K63-linked diubiquitin. We engineered a K11 linkage-specific antibody and use it to demonstrate that K11 chains are highly upregulated in mitotic human cells precisely when substrates of the ubiquitin ligase anaphase-promoting complex (APC/C) are degraded. These chains increased with proteasomal inhibition, suggesting they act as degradation signals in vivo. Inhibition of the APC/C strongly impeded the formation of K11-linked chains, suggesting that a single ubiquitin ligase is the major source of mitotic K11-linked chains. Our results underscore the importance of K11-linked ubiquitin chains as critical regulators of mitotic protein degradation.

Receptor-interacting protein kinase 1 (RIPK1) promotes cell survival-mice lacking RIPK1 die perinatally, exhibiting aberrant caspase-8-dependent apoptosis and mixed lineage kinase-like (MLKL)-dependent necroptosis. However, mice expressing catalytically inactive RIPK1 are viable, and an ill-defined pro-survival function for the RIPK1 scaffold has therefore been proposed. Here we show that the RIP homotypic interaction motif (RHIM) in RIPK1 prevents the RHIM-containing adaptor protein ZBP1 (Z-DNA binding protein 1; also known as DAI or DLM1) from activating RIPK3 upstream of MLKL. Ripk1 mice that expressed mutant RIPK1 with critical RHIM residues IQIG mutated to AAAA died around birth and exhibited RIPK3 autophosphorylation on Thr231 and Ser232, which is a hallmark of necroptosis, in the skin and thymus. Blocking necroptosis with catalytically inactive RIPK3(D161N), RHIM mutant RIPK3, RIPK3 deficiency, or MLKL deficiency prevented lethality in Ripk1 mice. Loss of ZBP1, which engages RIPK3 in response to certain viruses but previously had no defined role in development, also prevented perinatal lethality in Ripk1 mice. Consistent with the RHIM of RIPK1 functioning as a brake that prevents ZBP1 from engaging the RIPK3 RHIM, ZBP1 interacted with RIPK3 in Ripk1Mlkl macrophages, but not in wild-type, Mlkl or Ripk1Ripk3 macrophages. Collectively, these findings indicate that the RHIM of RIPK1 is critical for preventing ZBP1/RIPK3/MLKL-dependent necroptosis during development.

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