Discovery of a Series of Potent, Selective, and Orally Bioavailable Nucleoside Inhibitors of CD73 That Demonstrates In Vivo Antitumor Activity.

CD73 (ecto-5'-nucleotidase) has emerged as an attractive target for cancer immunotherapy of many cancers. CD73 catalyzes the hydrolysis of adenosine monophosphate (AMP) into highly immunosuppressive adenosine that plays a critical role in tumor progression. Herein, we report our efforts in developing orally bioavailable and highly potent small-molecule CD73 inhibitors from the reported hit molecule 2 to lead molecule 20 and then finally to compound 49. Compound 49 was able to reverse AMP-mediated suppression of CD8+ T cells and completely inhibited CD73 activity in serum samples from various cancer patients. In preclinical in vivo studies, orally administered 49 showed a robust dose-dependent pharmacokinetic/pharmacodynamic (PK/PD) relationship that correlated with efficacy. Compound 49 also demonstrated the expected immune-mediated antitumor mechanism of action and was efficacious upon oral administration not only as a single agent but also in combination with either chemotherapeutics or checkpoint inhibitor in the mouse tumor model.

The glutaminase inhibitor telaglenastat enhances the antitumor activity of signal transduction inhibitors everolimus and cabozantinib in models of renal cell carcinoma.

Dysregulated metabolism is a hallmark of cancer that manifests through alterations in bioenergetic and biosynthetic pathways to enable tumor cell proliferation and survival. Tumor cells exhibit high rates of glycolysis, a phenomenon known as the Warburg effect, and an increase in glutamine consumption to support the tricarboxylic acid (TCA) cycle. Renal cell carcinoma (RCC) tumors express high levels of glutaminase (GLS), the enzyme required for the first step in metabolic conversion of glutamine to glutamate and the entry of glutamine into the TCA cycle. We found that RCC cells are highly dependent on glutamine for proliferation, and this dependence strongly correlated with sensitivity to telaglenstat (CB-839), an investigational, first-in-class, selective, orally bioavailable GLS inhibitor. Metabolic profiling of RCC cell lines treated with telaglenastat revealed a decrease in glutamine consumption, which was concomitant with a decrease in the production of glutamate and other glutamine-derived metabolites, consistent with GLS inhibition. Treatment of RCC cells with signal transduction inhibitors everolimus (mTOR inhibitor) or cabozantinib (VEGFR/MET/AXL inhibitor) in combination with telaglenastat resulted in decreased consumption of both glucose and glutamine and synergistic anti-proliferative effects. Treatment of mice bearing Caki-1 RCC xenograft tumors with cabozantinib plus telaglenastat resulted in reduced tumor growth compared to either agent alone. Enhanced anti-tumor activity was also observed with the combination of everolimus plus telaglenastat. Collectively, our results demonstrate potent, synergistic, anti-tumor activity of telaglenastat plus signal transduction inhibitors cabozantinib or everolimus via a mechanism involving dual inhibition of glucose and glutamine consumption.

LKB1 and KEAP1/NRF2 Pathways Cooperatively Promote Metabolic Reprogramming with Enhanced Glutamine Dependence in KRAS-Mutant Lung Adenocarcinoma.

In KRAS-mutant lung adenocarcinoma, tumors with LKB1 loss (KL) are highly enriched for concurrent KEAP1 mutations, which activate the KEAP1/NRF2 pathway (KLK). Here, we investigated the biological consequences of these cooccurring alterations and explored whether they conferred specific therapeutic vulnerabilities. Compared with KL tumors, KLK tumors exhibited increased expression of genes involved in glutamine metabolism, the tricarboxylic acid cycle, and the redox homeostasis signature. Using isogenic pairs with knockdown or overexpression of LKB1, KEAP1, and NRF2, we found that LKB1 loss results in increased energetic and redox stress marked by increased levels of intracellular reactive oxygen species and decreased levels of ATP, NADPH/NADP+ ratio, and glutathione. Activation of the KEAP1/NRF2 axis in LKB1-deficient cells enhanced cell survival and played a critical role in the maintenance of energetic and redox homeostasis in a glutamine-dependent manner. LKB1 and the KEAP1/NRF2 pathways cooperatively drove metabolic reprogramming and enhanced sensitivity to the glutaminase inhibitor CB-839 in vitro and in vivo. Overall, these findings elucidate the adaptive advantage provided by KEAP1/NRF2 pathway activation in KL tumors and support clinical testing of glutaminase inhibitor in subsets of KRAS-mutant lung adenocarcinoma. SIGNIFICANCE: In KRAS-mutant non-small cell lung cancer, LKB1 loss results in enhanced energetic/redox stress, which is tolerated, in part, through cooccurring KEAP1/NRF2-dependent metabolic adaptations, thus enhancing glutamine dependence and vulnerability to glutaminase inhibition.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/79/13/3251/F1.large.jpg.

The GSK3 Signaling Axis Regulates Adaptive Glutamine Metabolism in Lung Squamous Cell Carcinoma.

Altered metabolism is a hallmark of cancer growth, forming the conceptual basis for development of metabolic therapies as cancer treatments. We performed in vivo metabolic profiling and molecular analysis of lung squamous cell carcinoma (SCC) to identify metabolic nodes for therapeutic targeting. Lung SCCs adapt to chronic mTOR inhibition and suppression of glycolysis through the GSK3α/ß signaling pathway, which upregulates glutaminolysis. Phospho-GSK3α/ß protein levels are predictive of response to single-therapy mTOR inhibition while combinatorial treatment with the glutaminase inhibitor CB-839 effectively overcomes therapy resistance. In addition, we identified a conserved metabolic signature in a broad spectrum of hypermetabolic human tumors that may be predictive of patient outcome and response to combined metabolic therapies targeting mTOR and glutaminase.

Inhibition of arginase by CB-1158 blocks myeloid cell-mediated immune suppression in the tumor microenvironment.

BACKGROUND: Myeloid cells are an abundant leukocyte in many types of tumors and contribute to immune evasion. Expression of the enzyme arginase 1 (Arg1) is a defining feature of immunosuppressive myeloid cells and leads to depletion of L-arginine, a nutrient required for T cell and natural killer (NK) cell proliferation. Here we use CB-1158, a potent and orally-bioavailable small-molecule inhibitor of arginase, to investigate the role of Arg1 in regulating anti-tumor immunity. METHODS: CB-1158 was tested for the ability to block myeloid cell-mediated inhibition of T cell proliferation in vitro, and for tumor growth inhibition in syngeneic mouse models of cancer as a single agent and in combination with other therapies. Tumors from animals treated with CB-1158 were profiled for changes in immune cell subsets, expression of immune-related genes, and cytokines. Human tumor tissue microarrays were probed for Arg1 expression by immunohistochemistry and immunofluorescence. Cancer patient plasma samples were assessed for Arg1 protein and L-arginine by ELISA and mass spectrometry, respectively. RESULTS: CB-1158 blocked myeloid cell-mediated suppression of T cell proliferation in vitro and reduced tumor growth in multiple mouse models of cancer, as a single agent and in combination with checkpoint blockade, adoptive T cell therapy, adoptive NK cell therapy, and the chemotherapy agent gemcitabine. Profiling of the tumor microenvironment revealed that CB-1158 increased tumor-infiltrating CD8+ T cells and NK cells, inflammatory cytokines, and expression of interferon-inducible genes. Patient tumor samples from multiple histologies expressed an abundance of tumor-infiltrating Arg1+ myeloid cells. Plasma samples from cancer patients exhibited elevated Arg1 and reduced L-arginine compared to healthy volunteers. CONCLUSIONS: These results demonstrate that Arg1 is a key mediator of immune suppression and that inhibiting Arg1 with CB-1158 shifts the immune landscape toward a pro-inflammatory environment, blunting myeloid cell-mediated immune evasion and reducing tumor growth. Furthermore, our results suggest that arginase blockade by CB-1158 may be an effective therapy in multiple types of cancer and combining CB-1158 with standard-of-care chemotherapy or other immunotherapies may yield improved clinical responses.

Capzimin is a potent and specific inhibitor of proteasome isopeptidase Rpn11.

The proteasome is a vital cellular machine that maintains protein homeostasis, which is of particular importance in multiple myeloma and possibly other cancers. Targeting of proteasome 20S peptidase activity with bortezomib and carfilzomib has been widely used to treat myeloma. However, not all patients respond to these compounds, and those who do eventually suffer relapse. Therefore, there is an urgent and unmet need to develop new drugs that target proteostasis through different mechanisms. We identified quinoline-8-thiol (8TQ) as a first-in-class inhibitor of the proteasome 19S subunit Rpn11. A derivative of 8TQ, capzimin, shows >5-fold selectivity for Rpn11 over the related JAMM proteases and >2 logs selectivity over several other metalloenzymes. Capzimin stabilized proteasome substrates, induced an unfolded protein response, and blocked proliferation of cancer cells, including those resistant to bortezomib. Proteomic analysis revealed that capzimin stabilized a subset of polyubiquitinated substrates. Identification of capzimin offers an alternative path to develop proteasome inhibitors for cancer therapy.

Discovery of an Inhibitor of the Proteasome Subunit Rpn11.

The proteasome plays a crucial role in degradation of normal proteins that happen to be constitutively or inducibly unstable, and in this capacity it plays a regulatory role. Additionally, it degrades abnormal/damaged/mutant/misfolded proteins, which serves a quality-control function. Inhibitors of the proteasome have been validated in the treatment of multiple myeloma, with several FDA-approved therapeutics. Rpn11 is a Zn2+-dependent metalloisopeptidase that hydrolyzes ubiquitin from tagged proteins that are trafficked to the proteasome for degradation. A fragment-based drug discovery (FBDD) approach was utilized to identify fragments with activity against Rpn11. Screening of a library of metal-binding pharmacophores (MBPs) revealed that 8-thioquinoline (8TQ, IC50 value ∼2.5 µM) displayed strong inhibition of Rpn11. Further synthetic elaboration of 8TQ yielded a small molecule compound (35, IC50 value ∼400 nM) that is a potent and selective inhibitor of Rpn11 that blocks proliferation of tumor cells in culture.

Targeted Inhibition of EGFR and Glutaminase Induces Metabolic Crisis in EGFR Mutant Lung Cancer.

Cancer cells exhibit increased use of nutrients, including glucose and glutamine, to support the bioenergetic and biosynthetic demands of proliferation. We tested the small-molecule inhibitor of glutaminase CB-839 in combination with erlotinib on epidermal growth factor receptor (EGFR) mutant non-small cell lung cancer (NSCLC) as a therapeutic strategy to simultaneously impair cancer glucose and glutamine utilization and thereby suppress tumor growth. Here, we show that CB-839 cooperates with erlotinib to drive energetic stress and activate the AMP-activated protein kinase (AMPK) pathway in EGFR (del19) lung tumors. Tumor cells undergo metabolic crisis and cell death, resulting in rapid tumor regression in vivo in mouse NSCLC xenografts. Consistently, positron emission tomography (PET) imaging with 18F-fluoro-2-deoxyglucose (18F-FDG) and 11C-glutamine (11C-Gln) of xenografts indicated reduced glucose and glutamine uptake in tumors following treatment with CB-839 + erlotinib. Therefore, PET imaging with 18F-FDG and 11C-Gln tracers can be used to non-invasively measure metabolic response to CB-839 and erlotinib combination therapy.

Clinical activity of carfilzomib correlates with inhibition of multiple proteasome subunits: application of a novel pharmacodynamic assay.

While proteasome inhibition is a validated therapeutic approach for multiple myeloma (MM), inhibition of individual constitutive proteasome (c20S) and immunoproteasome (i20S) subunits has not been fully explored owing to a lack of effective tools. We utilized the novel proteasome constitutive/immunoproteasome subunit enzyme-linked immunosorbent (ProCISE) assay to quantify proteasome subunit occupancy in samples from five phase I/II and II trials before and after treatment with the proteasome inhibitor carfilzomib. Following the first carfilzomib dose (15-56 mg/m(2) ), dose-dependent inhibition of c20S and i20S chymotrypsin-like active sites was observed [whole blood: ≥67%; peripheral blood mononuclear cells (PBMCs): ≥75%]. A similar inhibition profile was observed in bone marrow-derived CD138(+) tumour cells. Carfilzomib-induced proteasome inhibition was durable, with minimal recovery in PBMCs after 24 h but near-complete recovery between cycles. Importantly, the ProCISE assay can be used to quantify occupancy of individual c20S and i20S subunits. We observed a relationship between MM patient response (n = 29), carfilzomib dose and occupancy of multiple i20S subunits, where greater occupancy was associated with an increased likelihood of achieving a clinical response at higher doses. ProCISE represents a new tool for measuring proteasome inhibitor activity in clinical trials and relating drug action to patient outcomes.

Discovery of a First-in-Class, Potent, Selective, and Orally Bioavailable Inhibitor of the p97 AAA ATPase (CB-5083).

The AAA-ATPase p97 plays vital roles in mechanisms of protein homeostasis, including ubiquitin-proteasome system (UPS) mediated protein degradation, endoplasmic reticulum-associated degradation (ERAD), and autophagy. Herein we describe our lead optimization efforts focused on in vitro potency, ADME, and pharmaceutical properties that led to the discovery of a potent, ATP-competitive, D2-selective, and orally bioavailable p97 inhibitor 71, CB-5083. Treatment of tumor cells with 71 leads to significant accumulation of markers associated with inhibition of UPS and ERAD functions, which induces irresolvable proteotoxic stress and cell death. In tumor bearing mice, oral administration of 71 causes rapid accumulation of markers of the unfolded protein response (UPR) and subsequently induces apoptosis leading to sustained antitumor activity in in vivo xenograft models of both solid and hematological tumors. 71 has been taken into phase 1 clinical trials in patients with multiple myeloma and solid tumors.

Antitumor activity of the glutaminase inhibitor CB-839 in triple-negative breast cancer.

Glutamine serves as an important source of energy and building blocks for many tumor cells. The first step in glutamine utilization is its conversion to glutamate by the mitochondrial enzyme glutaminase. CB-839 is a potent, selective, and orally bioavailable inhibitor of both splice variants of glutaminase (KGA and GAC). CB-839 had antiproliferative activity in a triple-negative breast cancer (TNBC) cell line, HCC-1806, that was associated with a marked decrease in glutamine consumption, glutamate production, oxygen consumption, and the steady-state levels of glutathione and several tricarboxylic acid cycle intermediates. In contrast, no antiproliferative activity was observed in an estrogen receptor-positive cell line, T47D, and only modest effects on glutamine consumption and downstream metabolites were observed. Across a panel of breast cancer cell lines, GAC protein expression and glutaminase activity were elevated in the majority of TNBC cell lines relative to receptor positive cells. Furthermore, the TNBC subtype displayed the greatest sensitivity to CB-839 treatment and this sensitivity was correlated with (i) dependence on extracellular glutamine for growth, (ii) intracellular glutamate and glutamine levels, and (iii) GAC (but not KGA) expression, a potential biomarker for sensitivity. CB-839 displayed significant antitumor activity in two xenograft models: as a single agent in a patient-derived TNBC model and in a basal like HER2(+) cell line model, JIMT-1, both as a single agent and in combination with paclitaxel. Together, these data provide a strong rationale for the clinical investigation of CB-839 as a targeted therapeutic in patients with TNBC and other glutamine-dependent tumors.

Nonproteasomal targets of the proteasome inhibitors bortezomib and carfilzomib: a link to clinical adverse events.

PURPOSE: Bortezomib (Velcade), a dipeptide boronate 20S proteasome inhibitor and an approved treatment option for multiple myeloma, is associated with a treatment-emergent, painful peripheral neuropathy (PN) in more than 30% of patients. Carfilzomib, a tetrapeptide epoxyketone proteasome inhibitor, currently in clinical investigation in myeloma, is associated with low rates of PN. We sought to determine whether PN represents a target-mediated adverse drug reaction (ADR). EXPERIMENTAL DESIGN: Neurodegenerative effects of proteasome inhibitors were assessed in an in vitro model utilizing a differentiated neuronal cell line. Secondary targets of both inhibitors were identified by a multifaceted approach involving candidate screening, profiling with an activity-based probe, and database mining. Secondary target activity was measured in rats and patients receiving both inhibitors. RESULTS: Despite equivalent levels of proteasome inhibition, only bortezomib reduced neurite length, suggesting a nonproteasomal mechanism. In cell lysates, bortezomib, but not carfilzomib, significantly inhibited the serine proteases cathepsin G (CatG), cathepsin A, chymase, dipeptidyl peptidase II, and HtrA2/Omi at potencies near or equivalent to that for the proteasome. Inhibition of CatG was detected in splenocytes of rats receiving bortezomib and in peripheral blood mononuclear cells derived from bortezomib-treated patients. Levels of HtrA2/Omi, which is known to be involved in neuronal survival, were upregulated in neuronal cells exposed to both proteasome inhibitors but was inhibited only by bortezomib exposure. CONCLUSION: These data show that bortezomib-induced neurodegeneration in vitro occurs via a proteasome-independent mechanism and that bortezomib inhibits several nonproteasomal targets in vitro and in vivo, which may play a role in its clinical ADR profile.

Carfilzomib can induce tumor cell death through selective inhibition of the chymotrypsin-like activity of the proteasome.

Carfilzomib is a proteasome inhibitor in clinical development that primarily targets the chymotrypsin-like (CT-L) subunits in both the constitutive proteasome (c20S) and the immunoproteasome (i20S). To investigate the impact of inhibiting the CT-L activity with carfilzomib, we set out to quantitate the levels of CT-L subunits beta5 from the c20S and LMP7 from the i20S in normal and malignant hematopoietic cells. We found that the i20S is a major form of the proteasome expressed in cells of hematopoietic origin, including multiple myeloma (MM) CD138+ tumor cells. Although specific inhibition of either LMP7 or beta5 alone was insufficient to produce an antitumor response, inhibition of all proteasome subunits was cytotoxic to both hematologic tumor cells and peripheral blood mononuclear cells. However, selective inhibition of both beta5 and LMP7 was sufficient to induce an antitumor effect in MM, non-Hodgkin lymphoma, and leukemia cells while minimizing the toxicity toward nontransformed cells. In MM tumor cells, CT-L inhibition alone was sufficient to induce proapoptotic sequelae, including proteasome substrate accumulation, Noxa and caspase 3/7 induction, and phospho-eIF2alpha suppression. These data support a hypothesis that hematologic tumor cells are uniquely sensitive to CT-L inhibition and provide a mechanistic understanding of the clinical safety profile and antitumor activity of proteasome inhibitors.

A selective inhibitor of the immunoproteasome subunit LMP7 blocks cytokine production and attenuates progression of experimental arthritis.

The immunoproteasome, a distinct class of proteasome found predominantly in monocytes and lymphocytes, is known to shape the antigenic repertoire presented on class I major histocompatibility complexes (MHC-I). However, a specific role for the immunoproteasome in regulating other facets of immune responses has not been established. We describe here the characterization of PR-957, a selective inhibitor of low-molecular mass polypeptide-7 (LMP7, encoded by Psmb8), the chymotrypsin-like subunit of the immunoproteasome. PR-957 blocked presentation of LMP7-specific, MHC-I-restricted antigens in vitro and in vivo. Selective inhibition of LMP7 by PR-957 blocked production of interleukin-23 (IL-23) by activated monocytes and interferon-gamma and IL-2 by T cells. In mouse models of rheumatoid arthritis, PR-957 treatment reversed signs of disease and resulted in reductions in cellular infiltration, cytokine production and autoantibody levels. These studies reveal a unique role for LMP7 in controlling pathogenic immune responses and provide a therapeutic rationale for targeting LMP7 in autoimmune disorders.

Design and synthesis of an orally bioavailable and selective peptide epoxyketone proteasome inhibitor (PR-047).

Proteasome inhibition has been validated as a therapeutic modality in the treatment of multiple myeloma and non-Hodgkin's lymphoma. Carfilzomib, an epoxyketone currently undergoing clinical trials in malignant diseases, is a highly selective inhibitor of the chymotrypsin-like (CT-L) activity of the proteasome. A chemistry effort was initiated to discover orally bioavailable analogues of carfilzomib, which would have potential for improved dosing flexibility and patient convenience over intravenously administered agents. The lead compound, 2-Me-5-thiazole-Ser(OMe)-Ser(OMe)-Phe-ketoepoxide (58) (PR-047), selectively inhibited CT-L activity of both the constitutive proteasome (beta5) and immunoproteasome (LMP7) and demonstrated an absolute bioavailability of up to 39% in rodents and dogs. It was well tolerated with repeated oral administration at doses resulting in >80% proteasome inhibition in most tissues and elicited an antitumor response equivalent to intravenously administered carfilzomib in multiple human tumor xenograft and mouse syngeneic models. The favorable pharmacologic profile supports its further development for the treatment of malignant diseases.

Antitumor activity of PR-171, a novel irreversible inhibitor of the proteasome.

Clinical studies with bortezomib have validated the proteasome as a therapeutic target for the treatment of multiple myeloma and non-Hodgkin's lymphoma. However, significant toxicities have restricted the intensity of bortezomib dosing. Here we describe the antitumor activity of PR-171, a novel epoxyketone-based irreversible proteasome inhibitor that is currently in clinical development. In comparison to bortezomib, PR-171 exhibits equal potency but greater selectivity for the chymotrypsin-like activity of the proteasome. In cell culture, PR-171 is more cytotoxic than bortezomib following brief treatments that mimic the in vivo pharmacokinetics of both molecules. Hematologic tumor cells exhibit the greatest sensitivity to brief exposure, whereas solid tumor cells and nontransformed cell types are less sensitive to such treatments. Cellular consequences of PR-171 treatment include the accumulation of proteasome substrates and induction of cell cycle arrest and/or apoptosis. Administration of PR-171 to animals results in the dose-dependent inhibition of the chymotrypsin-like proteasome activity in all tissues examined with the exception of the brain. PR-171 is well tolerated when administered for either 2 or 5 consecutive days at doses resulting in >80% proteasome inhibition in blood and most tissues. In human tumor xenograft models, PR-171 mediates an antitumor response that is both dose and schedule dependent. The antitumor efficacy of PR-171 delivered on 2 consecutive days is stronger than that of bortezomib administered on its clinical dosing schedule. These studies show the tolerability, efficacy, and dosing flexibility of PR-171 and provide validation for the clinical testing of PR-171 in the treatment of hematologic malignancies using dose-intensive schedules.

i-SNAREs: inhibitory SNAREs that fine-tune the specificity of membrane fusion.

A new functional class of SNAREs, designated inhibitory SNAREs (i-SNAREs), is described here. An i-SNARE inhibits fusion by substituting for or binding to a subunit of a fusogenic SNAREpin to form a nonfusogenic complex. Golgi-localized SNAREs were tested for i-SNARE activity by adding them as a fifth SNARE together with four other SNAREs that mediate Golgi fusion reactions. A striking pattern emerges in which certain subunits of the cis-Golgi SNAREpin function as i-SNAREs that inhibit fusion mediated by the trans-Golgi SNAREpin, and vice versa. Although the opposing distributions of the cis- and trans-Golgi SNAREs themselves could provide for a countercurrent fusion pattern in the Golgi stack, the gradients involved would be strongly sharpened by the complementary countercurrent distributions of the i-SNAREs.

Drug discovery in the ubiquitin regulatory pathway.

The ubiquitin system has been implicated in the pathogenesis of numerous disease states, including oncogenesis, inflammation, viral infection, CNS disorders and metabolic dysfunction. Ubiquitin conjugation and deconjugation to substrate proteins is carried out by multiple families of proteins, each with a defined role in the enzymatic cascade. This conjugation-deconjugation system parallels the kinase-phosphatase system in that both alter protein function by the addition and removal of post-translational modifiers. Our understanding of ubiquitin biology and strategies to interfere pharmacologically with the ubiquitin regulatory machinery is progressing rapidly. In light of increased interest in ubiquitin pathways as drug targets, we review the ubiquitin enzymatic cascades, highlighting therapeutic opportunities and enzymatic mechanisms. We also discuss the challenges of targeting this class of enzymes with small molecules, as well as current approaches and progress in drug discovery.

Distinct SNARE complexes mediating membrane fusion in Golgi transport based on combinatorial specificity.

Syntaxin-5 (Sed5) is the only syntaxin needed for transport into and across the yeast Golgi, raising the question of how a single syntaxin species could mediate vesicle transport in both the anterograde and the retrograde direction within the stack. Sed5 is known to combine with two light chains (Bos1 and Sec22) to form the t-SNARE needed to receive vesicles from the endoplasmic reticulum. However, the yeast Golgi contains several other potential light chains with which Sed5 could potentially combine to form other t-SNAREs. To explore the degree of specificity in the choice of light chains by a t-SNARE, we undertook a comprehensive examination of the capacity of all 21 Sed5-based t-SNAREs that theoretically could assemble in the yeast Golgi to fuse with each of the 7 potential v-SNAREs also present in this organelle. Only one additional of these 147 combinations was fusogenic. This functional proteomic strategy thereby revealed a previously uncharacterized t-SNARE in which Sed5 is the heavy chain and Gos1 and Ykt6 are the light chains, and whose unique cognate v-SNARE is Sft1. Immunoprecipitation experiments confirmed the existence of this complex in vivo. Fusion mediated by this second Golgi SNAREpin is topologically restricted, and existing genetic and morphologic evidence implies that it is used for transport across the Golgi stack. From this study, together with the previous functional proteomic analyses which have tested 275 distinct quaternary SNARE combinations, it follows that the fusion potential and transport pathways of the yeast cell can be read out from its genome sequence according to the SNARE hypothesis with a predictive accuracy of about 99.6%.