Multiplexing the Identification of Microorganisms via Tandem Mass Tag Labeling Augmented by Interference Removal through a Novel Modification of the Expectation Maximization Algorithm.

Having fast, accurate, and broad spectrum methods for the identification of microorganisms is of paramount importance to public health, research, and safety. Bottom-up mass spectrometer-based proteomics has emerged as an effective tool for the accurate identification of microorganisms from microbial isolates. However, one major hurdle that limits the deployment of this tool for routine clinical diagnosis, and other areas of research such as culturomics, is the instrument time required for the mass spectrometer to analyze a single sample, which can take ∼1 h per sample, when using mass spectrometers that are presently used in most institutes. To address this issue, in this study, we employed, for the first time, tandem mass tags (TMTs) in multiplex identifications of microorganisms from multiple TMT-labeled samples in one MS/MS experiment. A difficulty encountered when using TMT labeling is the presence of interference in the measured intensities of TMT reporter ions. To correct for interference, we employed in the proposed method a modified version of the expectation maximization (EM) algorithm that redistributes the signal from ion interference back to the correct TMT-labeled samples. We have evaluated the sensitivity and specificity of the proposed method using 94 MS/MS experiments (covering a broad range of protein concentration ratios across TMT-labeled channels and experimental parameters), containing a total of 1931 true positive TMT-labeled channels and 317 true negative TMT-labeled channels. The results of the evaluation show that the proposed method has an identification sensitivity of 93-97% and a specificity of 100% at the species level. Furthermore, as a proof of concept, using an in-house-generated data set composed of some of the most common urinary tract pathogens, we demonstrated that by using the proposed method the mass spectrometer time required per sample, using a 1 h LC-MS/MS run, can be reduced to 10 and 6 min when samples are labeled with TMT-6 and TMT-10, respectively. The proposed method can also be used along with Orbitrap mass spectrometers that have faster MS/MS acquisition rates, like the recently released Orbitrap Astral mass spectrometer, to further reduce the mass spectrometer time required per sample.

Conformations of Bcs1L undergoing ATP hydrolysis suggest a concerted translocation mechanism for folded iron-sulfur protein substrate.

The human AAA-ATPase Bcs1L translocates the fully assembled Rieske iron-sulfur protein (ISP) precursor across the mitochondrial inner membrane, enabling respiratory Complex III assembly. Exactly how the folded substrate is bound to and released from Bcs1L has been unclear, and there has been ongoing debate as to whether subunits of Bcs1L act in sequence or in unison hydrolyzing ATP when moving the protein cargo. Here, we captured Bcs1L conformations by cryo-EM during active ATP hydrolysis in the presence or absence of ISP substrate. In contrast to the threading mechanism widely employed by AAA proteins in substrate translocation, subunits of Bcs1L alternate uniformly between ATP and ADP conformations without detectable intermediates that have different, co-existing nucleotide states, indicating that the subunits act in concert. We further show that the ISP can be trapped by Bcs1 when its subunits are all in the ADP-bound state, which we propose to be released in the apo form.

The thiol methyltransferase activity of TMT1A (METTL7A) is conserved across species.

Although few resistance mechanisms for histone deacetylase inhibitors (HDACis) have been described, we recently demonstrated that TMT1A (formerly METTL7A) and TMT1B (formerly METTL7B) can mediate resistance to HDACis with a thiol as the zinc-binding group by methylating and inactivating the drug. TMT1A and TMT1B are poorly characterized, and their normal physiological role has yet to be determined. As animal model systems are often used to determine the physiological function of proteins, we investigated whether the ability of these methyltransferases to methylate thiol-based HDACis is conserved across different species. We found that TMT1A was conserved across rats, mice, chickens, and zebrafish, displaying 85.7%, 84.8%, 60.7%, and 51.0% amino acid sequence identity, respectively, with human TMT1A. Because TMT1B was not found in the chicken or zebrafish, we focused our studies on the TMT1A homologs. HEK-293 cells were transfected to express mouse, rat, chicken, or zebrafish homologs of TMT1A and all conferred resistance to the thiol-based HDACIs NCH-51, KD-5170, and romidepsin compared to empty vector-transfected cells. Additionally, all homologs blunted the downstream effects of HDACi treatment such as increased p21 expression, increased acetylated histone H3, and cell cycle arrest. Increased levels of dimethylated romidepsin were also found in the culture medium of cells transfected to express any of the TMT1A homologs after a 24 h incubation with romidepsin compared to empty-vector transfected cells. Our results indicate that the ability of TMT1A to methylate molecules is conserved across species. Animal models may therefore be useful in elucidating the role of these enzymes in humans.

The novel ATR inhibitor M1774 induces replication protein overexpression and broad synergy with DNA-targeted anticancer drugs.

Ataxia Telangiectasia and Rad3-related (ATR) checkpoint kinase inhibitors are in clinical trials. Here we explored the molecular pharmacology and therapeutic combination strategies of the oral ATR inhibitor M1774 (Tuvusertib) with DNA damaging agents (DDAs). As single agent, M1774 suppressed cancer cell viability at nanomolar concentrations, showing greater activity than ceralasertib and berzosertib, but less potency than gartisertib and elimusertib in the small-cell lung cancer H146, H82, and DMS114 cell lines. M1774 also efficiently blocked the activation of the ATR-CHK1 checkpoint pathway caused by replication stress induced by TOP1 inhibitors. Combination with non-toxic dose of M1774 enhanced TOP1 inhibitor-induced cancer cell death by enabling unscheduled replication upon replicative damage, thereby increasing genome instability. Tandem mass tag (TMT)-based quantitative proteomics uncovered that M1774, in the presence of DDA, forces the expression of proteins activating replication (CDC45) and G2/M-progression (PLK1 and CCNB1). In particular, the fork protection complex proteins (TIMELESS and TIPIN) were enriched. Low dose of M1774 was found highly synergistic with a broad spectrum of clinical DDAs including TOP1 inhibitors (SN-38/irinotecan, topotecan, exatecan, and exatecan), the TOP2 inhibitor etoposide, cisplatin, the RNA polymerase II inhibitor lurbinectedin, and the PARP inhibitor talazoparib in various models including cancer cell lines, patient-derived organoids, and mouse xenograft models. Furthermore, we demonstrate that M1774 reverses chemoresistance to anticancer DDAs in cancer cells lacking SLFN11 expression, suggesting that SLFN11 can be utilized for patient selection in upcoming clinical trials.

The homeobox transcription factor DUXBL controls exit from totipotency.

In mice, exit from the totipotent two-cell (2C) stage embryo requires silencing of the 2C-associated transcriptional program. However, the molecular mechanisms involved in this process remain poorly understood. Here we demonstrate that the 2C-specific transcription factor double homeobox protein (DUX) mediates an essential negative feedback loop by inducing the expression of DUXBL to promote this silencing. We show that DUXBL gains accessibility to DUX-bound regions specifically upon DUX expression. Furthermore, we determine that DUXBL interacts with TRIM24 and TRIM33, members of the TRIM superfamily involved in gene silencing, and colocalizes with them in nuclear foci upon DUX expression. Importantly, DUXBL overexpression impairs 2C-associated transcription, whereas Duxbl inactivation in mouse embryonic stem cells increases DUX-dependent induction of the 2C-transcriptional program. Consequently, DUXBL deficiency in embryos results in sustained expression of 2C-associated transcripts leading to early developmental arrest. Our study identifies DUXBL as an essential regulator of totipotency exit enabling the first divergence of cell fates.

A spatial map of hepatic mitochondria uncovers functional heterogeneity shaped by nutrient-sensing signaling.

In the liver, mitochondria are exposed to different concentrations of nutrients due to their spatial positioning across the periportal and pericentral axis. How the mitochondria sense and integrate these signals to respond and maintain homeostasis is not known. Here, we combine intravital microscopy, spatial proteomics, and functional assessment to investigate mitochondrial heterogeneity in the context of liver zonation. We find that periportal and pericentral mitochondria are morphologically and functionally distinct; beta-oxidation is elevated in periportal regions, while lipid synthesis is predominant in the pericentral mitochondria. In addition, comparative phosphoproteomics reveals spatially distinct patterns of mitochondrial composition and potential regulation via phosphorylation. Acute pharmacological modulation of nutrient sensing through AMPK and mTOR shifts mitochondrial phenotypes in the periportal and pericentral regions, linking nutrient gradients across the lobule and mitochondrial heterogeneity. This study highlights the role of protein phosphorylation in mitochondrial structure, function, and overall homeostasis in hepatic metabolic zonation. These findings have important implications for liver physiology and disease.

Rewiring of RNA methylation by the oncometabolite fumarate in renal cell carcinoma.

Metabolic reprogramming is a hallmark of cancer that facilitates changes in many adaptive biological processes. Mutations in the tricarboxylic acid cycle enzyme fumarate hydratase (FH) lead to fumarate accumulation and cause hereditary leiomyomatosis and renal cell cancer (HLRCC). HLRCC is a rare, inherited disease characterized by the development of non-cancerous smooth muscle tumors of the uterus and skin, and an increased risk of an aggressive form of kidney cancer. Fumarate has been shown to inhibit 2-oxoglutarate-dependent dioxygenases (2OGDDs) involved in the hydroxylation of HIF1α, as well as in DNA and histone demethylation. However, the link between fumarate accumulation and changes in RNA post-transcriptional modifications has not been defined. Here, we determine the consequences of fumarate accumulation on the activity of different members of the 2OGDD family targeting RNA modifications. By evaluating multiple RNA modifications in patient-derived HLRCC cell lines, we show that mutation of FH selectively affects the levels of N6-methyladenosine (m6A), while the levels of 5-formylcytosine (f5C) in mitochondrial tRNA are unaffected. This supports the hypothesis of a differential impact of fumarate accumulation on distinct RNA demethylases. The observation that metabolites modulate specific subsets of RNA-modifying enzymes offers new insights into the intersection between metabolism and the epitranscriptome.

Staufen1 Represses the FOXA1-Regulated Transcriptome by Destabilizing FOXA1 mRNA in Colorectal Cancer Cells.

Transcription factors play key roles in development and disease by controlling gene expression. Forkhead box A1 (FOXA1), is a pioneer transcription factor essential for mouse development and functions as an oncogene in prostate and breast cancer. In colorectal cancer (CRC), FOXA1 is significantly downregulated and high FOXA1 expression is associated with better prognosis, suggesting potential tumor suppressive functions. We therefore investigated the regulation of FOXA1 expression in CRC, focusing on well-differentiated CRC cells, where FOXA1 is robustly expressed. Genome-wide RNA stability assays identified FOXA1 as an unstable mRNA in CRC cells. We validated FOXA1 mRNA instability in multiple CRC cell lines and in patient-derived CRC organoids, and found that the FOXA1 3'UTR confers instability to the FOXA1 transcript. RNA pulldowns and mass spectrometry identified Staufen1 (STAU1) as a potential regulator of FOXA1 mRNA. Indeed, STAU1 knockdown resulted in increased FOXA1 mRNA and protein expression due to increased FOXA1 mRNA stability. Consistent with these data, RNA-seq following STAU1 knockdown in CRC cells revealed that FOXA1 targets were upregulated upon STAU1 knockdown. Collectively, this study uncovers a molecular mechanism by which FOXA1 is regulated in CRC cells and provides insights into our understanding of the complex mechanisms of gene regulation in cancer.

Extracellular Proximity Labeling Reveals an Expanded Interactome for the Matrisome Protein TIMP2.

Classical methods of investigating protein-protein interactions (PPIs) are generally performed in non-living systems, yet in recent years new technologies utilizing proximity labeling (PL) have given researchers the tools to explore proximal PPIs in living systems. PL has distinct advantages over traditional protein interactome studies, such as the ability to identify weak and transient interactions in vitro and in vivo. Most PL studies are performed on targets within the cell or on the cell membrane. We have adapted the original PL method to investigate PPIs within the extracellular compartment, using both BioID2 and TurboID, that we term extracellular PL (ePL). To demonstrate the utility of this modified technique, we investigate the interactome of the widely expressed matrisome protein tissue inhibitor of metalloproteinases 2 (TIMP2). Tissue inhibitors of metalloproteinases (TIMPs) are a family of multi-functional proteins that were initially defined by their ability to inhibit the enzymatic activity of metalloproteinases (MPs), the major mediators of extracellular matrix (ECM) breakdown and turnover. TIMP2 exhibits a broad expression profile and is often abundant in both normal and diseased tissues. Understanding the functional transformation of matrisome regulators, like TIMP2, during the evolution of tissue microenvironments associated with disease progression is essential for the development of ECM-targeted therapeutics. Using carboxyl- and amino-terminal fusion proteins of TIMP2 with BioID2 and TurboID, we describe the TIMP2 proximal interactome. We also illustrate how the TIMP2 interactome changes in the presence of different stimuli, in different cell types, in unique culture conditions (2D vs 3D), and with different reaction kinetics (BioID2 vs. TurboID); demonstrating the power of this technique versus classical PPI methods. We propose that the screening of matrisome targets in disease models using ePL will reveal new therapeutic targets for further comprehensive studies.

KDM3B inhibitors disrupt the oncogenic activity of PAX3-FOXO1 in fusion-positive rhabdomyosarcoma.

Fusion-positive rhabdomyosarcoma (FP-RMS) is an aggressive pediatric sarcoma driven primarily by the PAX3-FOXO1 fusion oncogene, for which therapies targeting PAX3-FOXO1 are lacking. Here, we screen 62,643 compounds using an engineered cell line that monitors PAX3-FOXO1 transcriptional activity identifying a hitherto uncharacterized compound, P3FI-63. RNA-seq, ATAC-seq, and docking analyses implicate histone lysine demethylases (KDMs) as its targets. Enzymatic assays confirm the inhibition of multiple KDMs with the highest selectivity for KDM3B. Structural similarity search of P3FI-63 identifies P3FI-90 with improved solubility and potency. Biophysical binding of P3FI-90 to KDM3B is demonstrated using NMR and SPR. P3FI-90 suppresses the growth of FP-RMS in vitro and in vivo through downregulating PAX3-FOXO1 activity, and combined knockdown of KDM3B and KDM1A phenocopies P3FI-90 effects. Thus, we report KDM inhibitors P3FI-63 and P3FI-90 with the highest specificity for KDM3B. Their potent suppression of PAX3-FOXO1 activity indicates a possible therapeutic approach for FP-RMS and other transcriptionally addicted cancers.

SMARCD1 is a "Goldilocks" metastasis modifier.

Breast cancer is the most frequently diagnosed cancer worldwide, constituting around 15% of all diagnosed cancers in 2023. The predominant cause of breast cancer-related mortality is metastasis to distant essential organs, and a lack of metastasis-targeted therapies perpetuates dismal outcomes for late-stage patients. However, through our use of meiotic genetics to study inherited transcriptional network regulation, we have identified a new class of "Goldilocks" genes that are promising candidates for the development of metastasis-targeted therapeutics. Building upon previous work that implicated the CCR4-NOT RNA deadenylase complex in metastasis, we now demonstrate that the RNA-binding proteins (RNA-BPs) NANOS1, PUM2, and CPSF4 also regulate metastatic potential. Using cell lines, 3D culture, mouse models, and clinical data, we pinpoint Smarcd1 mRNA as a key target of all three RNA-BPs. Strikingly, both high and low expression of Smarcd1 is associated with positive clinical outcomes, while intermediate expression significantly reduces the probability of survival. Applying the theory of "essential genes" from evolution, we identify an additional 50 genes that span several cellular processes and must be maintained within a discrete window of expression for metastasis to occur. In the case of Smarcd1, small perturbations in its expression level significantly reduce metastasis in laboratory mouse models and alter splicing programs relevant to the ER+/HER2-enriched breast cancer subtype. The identification of subtype-specific "Goldilocks" metastasis modifier genes introduces a new class of genes and potential catalogue of novel targets that, when therapeutically "nudged" in either direction, may significantly improve late-stage patient outcomes.

The Methyltransferases METTL7A and METTL7B Confer Resistance to Thiol-Based Histone Deacetylase Inhibitors.

Histone deacetylase inhibitors (HDACi) are part of a growing class of epigenetic therapies used for the treatment of cancer. Although HDACis are effective in the treatment of T-cell lymphomas, treatment of solid tumors with this class of drugs has not been successful. Overexpression of the multidrug resistance protein P-glycoprotein (P-gp), encoded by ABCB1, is known to confer resistance to the HDACi romidepsin in vitro, yet increased ABCB1 expression has not been associated with resistance in patients, suggesting that other mechanisms of resistance arise in the clinic. To identify alternative mechanisms of resistance to romidepsin, we selected MCF-7 breast cancer cells with romidepsin in the presence of the P-gp inhibitor verapamil to reduce the likelihood of P-gp-mediated resistance. The resulting cell line, MCF-7 DpVp300, does not express P-gp and was found to be selectively resistant to romidepsin but not to other HDACis such as belinostat, panobinostat, or vorinostat. RNA-sequencing analysis revealed upregulation of the mRNA coding for the putative methyltransferase, METTL7A, whose paralog, METTL7B, was previously shown to methylate thiol groups on hydrogen sulfide and captopril. As romidepsin has a thiol as the zinc-binding moiety, we hypothesized that METTL7A could inactivate romidepsin and other thiol-based HDACis via methylation of the thiol group. We demonstrate that expression of METTL7A or METTL7B confers resistance to thiol-based HDACis and that both enzymes are capable of methylating thiol-containing HDACis. We thus propose that METTL7A and METTL7B confer resistance to thiol-based HDACis by methylating and inactivating the zinc-binding thiol.

YAP localization mediates mechanical adaptation of human cancer cells during extravasation in vivo.

Biophysical profiling of primary tumors has revealed that individual tumor cells fall along a highly heterogeneous continuum of mechanical phenotypes. One idea is that a subset of tumor cells is "softer" to facilitate detachment and escape from the primary site, a step required to initiate metastasis. However, it has also been postulated that cells must be able to deform and generate sufficient force to exit into distant sites. Here, we aimed to dissect the mechanical changes that occur during extravasation and organ colonization. Using multiplexed methods of intravital microscopy and optical tweezer based active microrheology, we obtained longitudinal images and mechanical profiles of cells during organ colonization in vivo. We determined that cells were softer, more liquid like upon exit of the vasculature but stiffened and became more solid like once in the new organ microenvironment. We also determined that a YAP mediated mechanogenotype influenced the global dissemination in our in vivo and in vitro models and that reducing mechanical heterogeneity could reduce extravasation. Moreover, our high throughput analysis of mechanical phenotypes of patient samples revealed that this mechanics was in part regulated by the external hydrodynamic forces that the cancer cells experienced within capillary mimetics. Our findings indicate that disseminated cancer cells can keep mutating with a continuum landscape of mechano-phenotypes, governed by the YAP-mediated mechanosensing of hydrodynamic flow.

The thiol methyltransferase activity of TMT1A (METTL7A) is conserved across species.

Although few resistance mechanisms for histone deacetylase inhibitors (HDACis) have been described, we recently demonstrated that TMT1A (formerly METTL7A) and TMT1B (formerly METTL7B) can mediate resistance to HDACis with a thiol as the zinc-binding group by methylating and inactivating the drug. TMT1A and TMT1B are poorly characterized, and their normal physiological role has yet to be determined. As animal model systems are often used to determine the physiological function of proteins, we investigated whether the ability of these methyltransferases to methylate thiol-based HDACis is conserved across different species. We found that TMT1A was conserved across rats, mice, chickens, and zebrafish, displaying 85.7%, 84.8%, 60.7% and 51.0% amino acid sequence identity, respectively, with human TMT1A. Because TMT1B was not found in the chicken or zebrafish, we focused our studies on the TMT1A homologs. HEK-293 cells were transfected to express mouse, rat, chicken, or zebrafish homologs of TMT1A and all conferred resistance to the thiol-based HDACIs NCH-51, KD-5170 and romidepsin compared to empty vector-transfected cells. Additionally, all homologs blunted the downstream effects of HDACi treatment such as increased p21 expression, increased acetylated histone H3, and cell cycle arrest. Increased levels of dimethylated romidepsin were also found in the culture medium of cells transfected to express any of the TMT1A homologs after a 24 h incubation with romidepsin compared to empty-vector transfected cells. Our results indicate that the ability of TMT1A to methylate molecules is conserved across species. Animal models may therefore be useful in elucidating the role of these enzymes in humans.

The TDRD3-USP9X complex and MIB1 regulate TOP3B homeostasis and prevent deleterious TOP3B cleavage complexes.

TOP3B is stabilized by TDRD3. Hypothesizing that TDRD3 recruits a deubiquitinase, we find that TOP3B interacts with USP9X via TDRD3. Inactivation of USP9X destabilizes TOP3B, and depletion of both TDRD3 and USP9X does not promote further TOP3B ubiquitylation. Additionally, we observe that MIB1 mediates the ubiquitylation and proteasomal degradation of TOP3B by directly interacting with TOP3B independently of TDRD3. Combined depletion of USP9X, TDRD3 and MIB1 causes no additional increase in TOP3B levels compared to MIB1 knockdown alone indicating that the TDRD3-USP9X complex works downstream of MIB1. To comprehend why cells degrade TOP3B in the absence of TDRD3, we measured TOP3Bccs. Lack of TDRD3 increases TOP3Bccs in DNA and RNA, and induced R-loops, γH2AX and growth defect. Biochemical experiments confirm that TDRD3 increases the turnover of TOP3B. Our work provides molecular insights into the mechanisms by which TDRD3 protect cells from deleterious TOP3Bccs which are otherwise removed by TRIM41.

Linear (-)-Zampanolide: Flexibility in Conformation-Activity Relationships.

Through an understanding of the conformational preferences of the polyketide natural product (-)-zampanolide, and the structural motifs that control these preferences, we developed a linear zampanolide analogue that exhibits potent cytotoxicity against cancer cell lines. This discovery provides a set of three structural handles for further structure-activity relationship (SAR) studies of this potent microtubule-stabilizing agent. Moreover, it provides additional evidence of the complex relationship between ligand preorganization, conformational flexibility, and biological potency. In contrast to medicinal chemistry dogma, these results demonstrate that increased overall conformational flexibility is not necessarily detrimental to protein binding affinity and biological activity.

A spatial map of hepatic mitochondria uncovers functional heterogeneity shaped by nutrient-sensing signaling.

In the liver, mitochondria are exposed to different concentrations of nutrients due to their spatial positioning across the periportal (PP) and pericentral (PC) axis. How these mitochondria sense and integrate these signals to respond and maintain homeostasis is not known. Here, we combined intravital microscopy, spatial proteomics, and functional assessment to investigate mitochondrial heterogeneity in the context of liver zonation. We found that PP and PC mitochondria are morphologically and functionally distinct; beta-oxidation was elevated in PP regions, while lipid synthesis was predominant in the PC mitochondria. In addition, comparative phosphoproteomics revealed spatially distinct patterns of mitochondrial composition and potential regulation via phosphorylation. Acute pharmacological modulation of nutrient sensing through AMPK and mTOR shifted mitochondrial phenotypes in the PP and PC regions, linking nutrient gradients across the lobule and mitochondrial heterogeneity. This study highlights the role of protein phosphorylation in mitochondrial structure, function, and overall homeostasis in hepatic metabolic zonation. These findings have important implications for liver physiology and disease.

J-domain Proteins form Binary Complexes with Hsp90 and Ternary Complexes with Hsp90 and Hsp70.

Hsp90 and Hsp70 are highly conserved molecular chaperones that help maintain proteostasis by participating in protein folding, unfolding, remodeling and activation of proteins. Both chaperones are also important for cellular recovery following environmental stresses. Hsp90 and Hsp70 function collaboratively for the remodeling and activation of some client proteins. Previous studies using E. coli and S. cerevisiae showed that residues in the Hsp90 middle domain directly interact with a region in the Hsp70 nucleotide binding domain, in the same region known to bind J-domain proteins. Importantly, J-domain proteins facilitate and stabilize the interaction between Hsp90 and Hsp70 both in E. coli and S. cerevisiae. To further explore the role of J-domain proteins in protein reactivation, we tested the hypothesis that J-domain proteins participate in the collaboration between Hsp90 and Hsp70 by simultaneously interacting with Hsp90 and Hsp70. Using E. coli Hsp90, Hsp70 (DnaK), and a J-domain protein (CbpA), we detected a ternary complex containing all three proteins. The interaction involved the J-domain of CbpA, the DnaK binding region of E. coli Hsp90, and the J-domain protein binding region of DnaK where Hsp90 also binds. Additionally, results show that E. coli Hsp90 interacts with E. coli J-domain proteins, DnaJ and CbpA, and that yeast Hsp90, Hsp82, interacts with a yeast J-domain protein, Ydj1. Together these results suggest that the complexes may be transient intermediates in the pathway of collaborative protein remodeling by Hsp90 and Hsp70.

Vaccine plus microbicide effective in preventing vaginal SIV transmission in macaques.

The human immunodeficiency virus epidemic continues in sub-Saharan Africa, and particularly affects adolescent girls and women who have limited access to antiretroviral therapy. Here we report that the risk of vaginal simian immunodeficiency virus (SIV)mac251 acquisition is reduced by more than 90% using a combination of a vaccine comprising V1-deleted (V2 enhanced) SIV envelope immunogens with topical treatment of the zinc-finger inhibitor SAMT-247. Following 14 weekly intravaginal exposures to the highly pathogenic SIVmac251, 80% of a cohort of 20 macaques vaccinated and treated with SAMT-247 remained uninfected. In an arm of 18 vaccinated-only animals without microbicide, 40% of macaques remained uninfected. The combined SAMT-247/vaccine regimen was significantly more effective than vaccination alone. By analysing immune correlates of protection, we show that, by increasing zinc availability, SAMT-247 increases natural killer cytotoxicity and monocyte efferocytosis, and decreases T-cell activation to augment vaccine-induced protection.