Loss of ATG4B and ATG4A results in two-stage cell cycle defects in pancreatic ductal adenocarcinoma cells.

Pancreatic ductal adenocarcinoma (PDAC) exhibits elevated levels of autophagy, which promote tumor progression and treatment resistance. ATG4B is an autophagy-related cysteine protease under consideration as a potential therapeutic target, but it is largely unexplored in PDAC. Here, we investigated the clinical and functional relevance of ATG4B expression in PDAC. Using two PDAC patient cohorts, we found that low ATG4B mRNA or protein expression is associated with worse patient survival outcomes, poorly differentiated PDAC tumors and a lack of survival benefit from adjuvant chemotherapy. In PDAC cell lines, ATG4B knockout reduced proliferation, abolished processing of LC3B (also known as MAP1LC3B), and reduced GABARAP and GABARAPL1 levels, but increased ATG4A levels. ATG4B and ATG4A double knockout lines displayed a further reduction in proliferation, characterized by delays in G1-S phase transition and mitosis. Pro-LC3B accumulated aberrantly at the centrosome with a concomitant increase in centrosomal proteins PCM1 and CEP131, which was rescued by exogenous ATG4B. The two-stage cell cycle defects following ATG4B and ATG4A loss have important therapeutic implications for PDAC.

Mass Spectrometry Acquisition and Fractionation Recommendations for TMT11 and TMT16 Labeled Samples.

Proteome coverage and accurate protein quantification are both important for evaluating biological systems; however, compromises between quantification, coverage, and mass spectrometry (MS) resources are often necessary. Consequently, experimental parameters that impact coverage and quantification must be adjusted, depending on experimental goals. Among these parameters is offline prefractionation, which is utilized in MS-based proteomics to decrease sample complexity resulting in higher overall proteome coverage upon MS analysis. Prefractionation leads to increases in required MS analysis time, although this is often mitigated by isobaric labeling using tandem-mass tags (TMT), which allow samples to be multiplexed. Here we evaluate common prefractionation schemes, TMT variants, and MS acquisition methods and their impact on protein quantification and coverage. Furthermore, we provide recommendations for experimental design depending on the experimental goals.

PeptideRanger: An R Package to Optimize Synthetic Peptide Selection for Mass Spectrometry Applications.

Targeted and semitargeted mass spectrometry-based approaches are reliable methods to consistently detect and quantify low abundance proteins including proteins of clinical significance. Despite their potential, the development of targeted and semitargeted assays is time-consuming and often requires the purchase of costly libraries of synthetic peptides. To improve the efficiency of this rate-limiting step, we developed PeptideRanger, a tool to identify peptides from protein of interest with physiochemical properties that make them more likely to be suitable for mass spectrometry analysis. PeptideRanger is a flexible, extensively annotated, and intuitive R package that uses a random forest model trained on a diverse data set of thousands of MS experiments spanning a variety of sample types profiled with different chromatography setups and instruments. To support a variety of applications and to leverage rapidly growing public MS databases, PeptideRanger can readily be retrained with experiment-specific data sets and customized to prioritize and filter peptides based on selected properties.

The proteome of clear cell ovarian carcinoma.

Clear cell ovarian carcinoma (CCOC) is the second most common subtype of epithelial ovarian carcinoma. Late-stage CCOC is not responsive to gold-standard chemotherapy and results in suboptimal outcomes for patients. In-depth molecular insight is urgently needed to stratify the disease and drive therapeutic development. We conducted global proteomics for 192 cases of CCOC and compared these with other epithelial ovarian carcinoma subtypes. Our results showed distinct proteomic differences in CCOC compared with other epithelial ovarian cancer subtypes including alterations in lipid and purine metabolism pathways. Furthermore, we report potential clinically significant proteomic subgroups within CCOC, suggesting the biologic plausibility of stratified treatment for this cancer. Taken together, our results provide a comprehensive understanding of the CCOC proteomic landscape to facilitate future understanding and research of this disease. © 2022 The Pathological Society of Great Britain and Ireland.

Class I HDAC inhibitors enhance YB-1 acetylation and oxidative stress to block sarcoma metastasis.

Outcomes for metastatic Ewing sarcoma and osteosarcoma are dismal and have not changed for decades. Oxidative stress attenuates melanoma metastasis, and melanoma cells must reduce oxidative stress to metastasize. We explored this in sarcomas by screening for oxidative stress sensitizers, which identified the class I HDAC inhibitor MS-275 as enhancing vulnerability to reactive oxygen species (ROS) in sarcoma cells. Mechanistically, MS-275 inhibits YB-1 deacetylation, decreasing its binding to 5'-UTRs of NFE2L2 encoding the antioxidant factor NRF2, thereby reducing NFE2L2 translation and synthesis of NRF2 to increase cellular ROS. By global acetylomics, MS-275 promotes rapid acetylation of the YB-1 RNA-binding protein at lysine-81, blocking binding and translational activation of NFE2L2, as well as known YB-1 mRNA targets, HIF1A, and the stress granule nucleator, G3BP1. MS-275 dramatically reduces sarcoma metastasis in vivo, but an MS-275-resistant YB-1K81-to-alanine mutant restores metastatic capacity and NRF2, HIF1α, and G3BP1 synthesis in MS-275-treated mice. These studies describe a novel function for MS-275 through enhanced YB-1 acetylation, thus inhibiting YB-1 translational control of key cytoprotective factors and its pro-metastatic activity.

Complementary Methods for de Novo Monoclonal Antibody Sequencing to Achieve Complete Sequence Coverage.

Mass spectrometry is a powerful tool for de novo sequencing of novel proteins. Recent efforts in this area have mainly focused on liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Here, we present an alternative method, capillary electrophoresis tandem mass spectrometry (CE-MS/MS), for sequencing novel monoclonal antibodies. Using less than 200 ng in total of tryptic digest sample in a triplicated measurement, CE-MS/MS with pH-mediated focusing successfully sequenced mAb infliximab with 100% sequence coverage and 100% accuracy for the light chain and 96% coverage and 93% accuracy for the heavy chain. It was also demonstrated that CE-MS/MS gives comparable results, and in some cases, even better results, as compared to LC-MS/MS when used as a standalone technique. A combined workflow using both CE-MS/MS and LC-MS/MS was also used to sequence a novel antibody, anti-CD-176, resulting in the first proposed sequence for this mAb.

Bottom-up proteomics of envelope proteins extracted from spinach chloroplast via high organic content CE-MS.

A high organic content CE-MS/MS (HOCE-MS/MS) method was developed for the proteomic analysis of envelope proteins extracted from spinach leaves. Separation was performed in a 1-m long hydroxypropyl cellulose coated capillary, using 8% (v/v) formic acid in 70% (v/v) methanol and 22% water as the BGE. A flow-through microvial interface was used to couple the CE system with an Orbitrap Fusion Lumos mass spectrometer, and field-amplified sample stacking was used to improve the concentration sensitivity. Using this optimized method, 3579 peptides and 1141 proteins were identified using the Proteome Discoverer software with a 1% false discovery rate at the protein level. Relative to conventional aqueous CE, HOCE-MS did a better job of discovering hydrophobic peptides and provided more peptide and protein identifications. Relative to nano-LC-MS, it achieved comparable peptide and protein identification performance and detected peptides not identified by LC-MS: of the full set of peptides identified using the two techniques, 19% were identified only using HOCE-MS. It also outperformed nano-LC-MS with respect to the detection of low molecular weight peptides.

Dynamic pH barrage junction focusing of amino acids, peptides, and digested monoclonal antibodies in capillary electrophoresis-mass spectrometry.

Dynamic pH barrage junction focusing in CE enables effective signal enhancement, quantitative capture efficiencies, and straightforward optimization. The method is a technical variant of dynamic pH junction focusing. CE separation with dynamic pH barrage junction focusing is compatible with both optical and mass spectrometric detection. We developed a CE-MS/MS method using hydrophilic polyethyleneimine-coated capillaries and validated it for the qualitative analysis of amino acids, peptides, and tryptic peptides of digested monoclonal antibodies. The S/N of extracted ion electropherograms of zwitterionic analytes were enhanced by approximately two orders of magnitude with a tradeoff of a shortened separation window. Online focusing improved the MS signal intensity of a diluted antibody digest, enabling more precursor ions to be analyzed with subsequent tandem mass spectrometric identification. It also broadened the concentration range of protein digest samples for which adequate sequence coverage data can be obtained. With only 0.9 ng of digested infliximab sample loaded into the capillary, 76% and 100% sequence coverage was realized for antibody heavy and light chains, respectively, after online focusing. Full coverage was achieved with 9 ng of injected digest.

RawTools: Rapid and Dynamic Interrogation of Orbitrap Data Files for Mass Spectrometer System Management.

Optimizing the quality of proteomics data collected from a mass spectrometer (MS) requires careful selection of acquisition parameters and proper assessment of instrument performance. Software tools capable of extracting a broad set of information from raw files, including meta, scan, quantification, and identification data, are needed to provide guidance for MS system management. In this work, direct extraction and utilization of these data is demonstrated using RawTools, a standalone tool for extracting meta and scan data directly from raw MS files generated on Thermo Orbitrap instruments. RawTools generates summarized and detailed plain text outputs after parsing individual raw files, including scan rates and durations, duty cycle characteristics, precursor and reporter ion quantification, and chromatography performance. RawTools also contains a diagnostic module that includes an optional "preview" database search for facilitating informed decision-making related to optimization of MS performance based on a variety of metrics. RawTools has been developed in C# and utilizes the Thermo RawFileReader library and thus can process raw MS files with high speed and high efficiency on all major operating systems (Windows, MacOS, Linux). To demonstrate the utility of RawTools, the extraction of meta and scan data from both individual and large collections of raw MS files was carried out to identify problematic characteristics of instrument performance. Taken together, the combined rich feature-set of RawTools with the capability for interrogation of MS and experiment performance makes this software a valuable tool for proteomics researchers.

CDK12 regulates alternative last exon mRNA splicing and promotes breast cancer cell invasion.

CDK12 (cyclin-dependent kinase 12) is a regulatory kinase with evolutionarily conserved roles in modulating transcription elongation. Recent tumor genome studies of breast and ovarian cancers highlighted recurrent CDK12 mutations, which have been shown to disrupt DNA repair in cell-based assays. In breast cancers, CDK12 is also frequently co-amplified with the HER2 (ERBB2) oncogene. The mechanisms underlying functions of CDK12 in general and in cancer remain poorly defined. Based on global analysis of mRNA transcripts in normal and breast cancer cell lines with and without CDK12 amplification, we demonstrate that CDK12 primarily regulates alternative last exon (ALE) splicing, a specialized subtype of alternative mRNA splicing, that is both gene- and cell type-specific. These are unusual properties for spliceosome regulatory factors, which typically regulate multiple forms of alternative splicing in a global manner. In breast cancer cells, regulation by CDK12 modulates ALE splicing of the DNA damage response activator ATM and a DNAJB6 isoform that influences cell invasion and tumorigenesis in xenografts. We found that there is a direct correlation between CDK12 levels, DNAJB6 isoform levels and the migration capacity and invasiveness of breast tumor cells. This suggests that CDK12 gene amplification can contribute to the pathogenesis of the cancer.

Extending the Compatibility of the SP3 Paramagnetic Bead Processing Approach for Proteomics.

The diversity in protein and peptide biochemistry necessitates robust protocols and reagents for efficiently handling and enriching these molecules prior to analysis with mass spectrometry (MS) or other techniques. Further exploration of the paramagnetic bead-based approach, single-pot solid-phase-enhanced sample preparation (SP3), is carried out toward updating and extending previously described conditions and experimental workflows. The SP3 approach was tested in a wide range of experimental scenarios, including (1) binding solvents (acetonitrile, ethanol, isopropanol, acetone), (2) binding pH (acidic vs neutral), (3) solvent/lysate ratios (50-200%, v/v), (4) mixing and rinsing conditions (on-rack vs off-rack rinsing), (5) Enrichment of nondenatured proteins, and (6) capture of individual proteins from noncomplex mixtures. These results highlight the robust handling of proteins in a broad set of scenarios while also enabling the development of a modified SP3 workflow that offers extended compatibility. The modified SP3 approach is used in quantitative in-depth proteome analyses to compare it with commercial paramagnetic bead-based HILIC methods (MagReSyn) and across multiple binding conditions (e.g., pH and solvent during binding). Together, these data reveal the extensive quantitative coverage of the proteome possible with SP3 independent of the binding approach utilized. The results further establish the utility of SP3 for the unbiased handling of peptides and proteins for proteomic applications.

Parsing and Quantification of Raw Orbitrap Mass Spectrometer Data Using RawQuant.

Effective analysis of protein samples by mass spectrometry (MS) requires careful selection and optimization of a range of experimental parameters. As the output from the primary detection device, the "raw" MS data file can be used to gauge the success of a given sample analysis. However, the closed-source nature of the standard raw MS file can complicate effective parsing of the data contained within. To ease and increase the range of analyses possible, the RawQuant tool was developed to enable parsing of raw MS files derived from Thermo Orbitrap instruments to yield meta and scan data in an openly readable text format. RawQuant can be commanded to export user-friendly files containing MS1, MS2, and MS3 metadata as well as matrices of quantification values based on isobaric tagging approaches. In this study, the utility of RawQuant is demonstrated in several scenarios: (1) reanalysis of shotgun proteomics data for the identification of the human proteome, (2) reanalysis of experiments utilizing isobaric tagging for whole-proteome quantification, and (3) analysis of a novel bacterial proteome and synthetic peptide mixture for assessing quantification accuracy when using isobaric tags. Together, these analyses successfully demonstrate RawQuant for the efficient parsing and quantification of data from raw Thermo Orbitrap MS files acquired in a range of common proteomics experiments. In addition, the individual analyses using RawQuant highlights parametric considerations in the different experimental sets and suggests targetable areas to improve depth of coverage in identification-focused studies and quantification accuracy when using isobaric tags.

The histone methyltransferase EZH2 is a therapeutic target in small cell carcinoma of the ovary, hypercalcaemic type.

Small cell carcinoma of the ovary, hypercalcaemic type (SCCOHT) is a rare but aggressive and untreatable malignancy affecting young women. We and others recently discovered that SMARCA4, a gene encoding the ATPase of the SWI/SNF chromatin-remodelling complex, is the only gene recurrently mutated in the majority of SCCOHT. The low somatic complexity of SCCOHT genomes and the prominent role of the SWI/SNF chromatin-remodelling complex in transcriptional control of genes suggest that SCCOHT cells may rely on epigenetic rewiring for oncogenic transformation. Herein, we report that approximately 80% (19/24) of SCCOHT tumour samples have strong expression of the histone methyltransferase EZH2 by immunohistochemistry, with the rest expressing variable amounts of EZH2. Re-expression of SMARCA4 suppressed the expression of EZH2 in SCCOHT cells. In comparison to other ovarian cell lines, SCCOHT cells displayed hypersensitivity to EZH2 shRNAs and two selective EZH2 inhibitors, GSK126 and EPZ-6438. EZH2 inhibitors induced cell cycle arrest, apoptosis, and cell differentiation in SCCOHT cells, along with the induction of genes involved in cell cycle regulation, apoptosis, and neuron-like differentiation. EZH2 inhibitors suppressed tumour growth and improved the survival of mice bearing SCCOHT xenografts. Therefore, our data suggest that loss of SMARCA4 creates a dependency on the catalytic activity of EZH2 in SCCOHT cells and that pharmacological inhibition of EZH2 is a promising therapeutic strategy for treating this disease. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

The clonal and mutational evolution spectrum of primary triple-negative breast cancers.

Primary triple-negative breast cancers (TNBCs), a tumour type defined by lack of oestrogen receptor, progesterone receptor and ERBB2 gene amplification, represent approximately 16% of all breast cancers. Here we show in 104 TNBC cases that at the time of diagnosis these cancers exhibit a wide and continuous spectrum of genomic evolution, with some having only a handful of coding somatic aberrations in a few pathways, whereas others contain hundreds of coding somatic mutations. High-throughput RNA sequencing (RNA-seq) revealed that only approximately 36% of mutations are expressed. Using deep re-sequencing measurements of allelic abundance for 2,414 somatic mutations, we determine for the first time-to our knowledge-in an epithelial tumour subtype, the relative abundance of clonal frequencies among cases representative of the population. We show that TNBCs vary widely in their clonal frequencies at the time of diagnosis, with the basal subtype of TNBC showing more variation than non-basal TNBC. Although p53 (also known as TP53), PIK3CA and PTEN somatic mutations seem to be clonally dominant compared to other genes, in some tumours their clonal frequencies are incompatible with founder status. Mutations in cytoskeletal, cell shape and motility proteins occurred at lower clonal frequencies, suggesting that they occurred later during tumour progression. Taken together, our results show that understanding the biology and therapeutic responses of patients with TNBC will require the determination of individual tumour clonal genotypes.

Investigating Acquisition Performance on the Orbitrap Fusion When Using Tandem MS/MS/MS Scanning with Isobaric Tags.

Methods for isobaric-tagged peptide analysis (e.g., TMT, iTRAQ), such as the synchronous precursor selection (SPS) tandem MS/MS/MS (MS3) approach, enable maintenance of reporter ion accuracy and precision by reducing the ratio compression caused by coisolated precursor ions. However, the decreased throughput of the MS3 approach necessitates careful optimization of acquisition strategies and methods to ensure maximal proteome coverage. We present a systematic analysis of acquisition parameters used to analyze isobaric-tagged peptide samples on current generation Orbitrap mass spectrometer (MS) hardware. In contrast with previously reported works, we demonstrate the limited utility of acquiring reporter ion data in the ion trap analyzer; ion trap acquisition had only a minimal increase in identification depth and reduced quantification precision. We establish that despite the significantly increased scan rate afforded through the use of higher energy collisional dissociation (HCD) in MS3-based ion trap isobaric tag analyses, the reduced quantification precision and reporter ion yields negate the potential benefits in proteome coverage. Lastly, using optimized parameter sets, we further demonstrate the limited utility of the ion trap detector versus the Orbitrap for reporter ion detection in an in-depth analysis of a complex proteome sample. Together, these data will serve as a valuable resource to researchers undertaking analysis on current generation Orbitrap instrumentation with isobaric tags.

Evaluating the Characteristics of Reporter Ion Signal Acquired in the Orbitrap Analyzer for Isobaric Mass Tag Proteome Quantification Experiments.

Multiplexed quantification with isobaric chemical tags (e.g., TMT, iTRAQ) provides a robust and efficient means to comparatively examine proteome dynamics between several biological states using a mass spectrometer (MS). The quantitative nature of isobaric tags necessitates strict validation of the observed ion signals in the chosen MS detector before differential patterns are extracted between biological states. We present an in-depth analysis of isobaric tag data acquired on current generation Orbitrap MS hardware to illustrate pitfalls in acquisition settings that can negatively impact results. We establish, for the first time, the presence of a notch, a region of no observed values, in the reporter ion distributions from isobaric-labeled peptide mixtures acquired on these instruments. We determine that this notch is present in published data across a wide range of instruments of the same or different type and is isolated to the Orbitrap mass analyzer. We demonstrate that the impact of the notch can be minimized using manipulations of Orbitrap scan parameters and on-column injection amounts. Lastly, using a mixture of synthetic standard peptides we investigated the impact on identification rates and quantification precision. Together, these data highlight an important phenomenon that negatively impacts peptide identification and quantification in the Orbitrap analyzer as well as outlining guidelines to follow to ensure minimization of MS-induced artifacts in isobaric tag experiments resulting from the notch.

A transgenic mouse model demonstrating the oncogenic role of mutations in the polycomb-group gene EZH2 in lymphomagenesis.

The histone methyltransferase EZH2 is frequently mutated in germinal center-derived diffuse large B-cell lymphoma and follicular lymphoma. To further characterize these EZH2 mutations in lymphomagenesis, we generated a mouse line where EZH2(Y641F) is expressed from a lymphocyte-specific promoter. Spleen cells isolated from the transgenic mice displayed a global increase in trimethylated H3K27, but the mice did not show an increased tendency to develop lymphoma. As EZH2 mutations often coincide with other mutations in lymphoma, we combined the expression of EZH2(Y641F) by crossing these transgenic mice with Eµ-Myc transgenic mice. We observed a dramatic acceleration of lymphoma development in this combination model of Myc and EZH2(Y641F). The lymphomas show histologic features of high-grade disease with a shift toward a more mature B-cell phenotype, increased cycling and gene expression, and epigenetic changes involving important pathways in B-cell regulation and function. Furthermore, they initiate disease in secondary recipients. In summary, EZH2(Y641F) can collaborate with Myc to accelerate lymphomagenesis demonstrating a cooperative role of EZH2 mutations in oncogenesis. This murine lymphoma model provides a new tool to study global changes in the epigenome caused by this frequent mutation and a promising model system for testing novel treatments.

Recurrent DICER1 hotspot mutations in endometrial tumours and their impact on microRNA biogenesis.

DICER1 plays a critical role in microRNA (miRNA) biogenesis. Recurrent somatic 'hotspot' mutations at the four metal-binding sites within the RNase IIIb domain of DICER1 were identified in ovarian sex cord-stromal tumours and have since been described in other paediatric tumours. In this study, we screened the RNase IIIb domain of DICER1 in 290 endometrial tumours and identified six cases with hotspot mutations, including two cases affected by an atypical G1809R mutation directly adjacent to a metal-binding site. Using Illumina and Sanger targeted resequencing, we observed and validated biallelic DICER1 mutations in several cases with hotspot mutations. Through in vitro DICER1 cleavage assays, small RNA deep sequencing and real-time PCR, we demonstrated that mutations adding a positively charged side chain to residue 1809 have similar detrimental effects on 5p miRNA production to mutations at the metal-binding sites. As expected, 5p miRNAs were globally reduced in tumours and cell lines with hotspot mutations. Pathway analysis of gene expression profiles indicated that genes de-repressed due to loss of 5p miRNAs are strongly associated with pathways regulating the cell cycle. Using a Dicer1-null mouse cell line model, we found that expression of DICER1 hotspot mutants promoted cell proliferation, whereas wild-type (WT) DICER1 inhibited cell proliferation. Furthermore, targets of let-7 family miRNAs are enriched among the up-regulated genes, suggesting that loss of let-7 may be impacting downstream pathways. Our results reveal that DICER1 hotspot mutations are implicated in common malignancies and may constitute a unique oncogenic pathway.

Recurrent somatic DICER1 mutations in nonepithelial ovarian cancers.

BACKGROUND: Germline truncating mutations in DICER1, an endoribonuclease in the RNase III family that is essential for processing microRNAs, have been observed in families with the pleuropulmonary blastoma-family tumor and dysplasia syndrome. Mutation carriers are at risk for nonepithelial ovarian tumors, notably sex cord-stromal tumors. METHODS: We sequenced the whole transcriptomes or exomes of 14 nonepithelial ovarian tumors and noted closely clustered mutations in the region of DICER1 encoding the RNase IIIb domain of DICER1 in four samples. We then sequenced this region of DICER1 in additional ovarian tumors and in certain other tumors and queried the effect of the mutations on the enzymatic activity of DICER1 using in vitro RNA cleavage assays. RESULTS: DICER1 mutations in the RNase IIIb domain were found in 30 of 102 nonepithelial ovarian tumors (29%), predominantly in Sertoli-Leydig cell tumors (26 of 43, or 60%), including 4 tumors with additional germline DICER1 mutations. These mutations were restricted to codons encoding metal-binding sites within the RNase IIIb catalytic centers, which are critical for microRNA interaction and cleavage, and were somatic in all 16 samples in which germline DNA was available for testing. We also detected mutations in 1 of 14 nonseminomatous testicular germ-cell tumors, in 2 of 5 embryonal rhabdomyosarcomas, and in 1 of 266 epithelial ovarian and endometrial carcinomas. The mutant DICER1 proteins had reduced RNase IIIb activity but retained RNase IIIa activity. CONCLUSIONS: Somatic missense mutations affecting the RNase IIIb domain of DICER1 are common in nonepithelial ovarian tumors. These mutations do not obliterate DICER1 function but alter it in specific cell types, a novel mechanism through which perturbation of microRNA processing may be oncogenic. (Funded by the Terry Fox Research Institute and others.).

The 3' overhangs at Tetrahymena thermophila telomeres are packaged by four proteins, Pot1a, Tpt1, Pat1, and Pat2.

Although studies with the ciliate Tetrahymena thermophila have played a central role in advancing our understanding of telomere biology and telomerase mechanisms and composition, the full complement of Tetrahymena telomere proteins has not yet been identified. Previously, we demonstrated that in Tetrahymena, the telomeric 3' overhang is protected by a three-protein complex composed of Pot1a, Tpt1, and Pat1. Here we show that Tpt1 and Pat1 associate with a fourth protein, Pat2 (Pot1 associated Tetrahymena 2). Mass spectrometry of proteins copurifying with Pat1 or Tpt1 identified peptides from Pat2, Pot1a, Tpt1, and Pat1. The lack of other proteins copurifying with Pat1 or Tpt1 implies that the overhang is protected by a four-protein Pot1a-Tpt1-Pat1-Pat2 complex. We verified that Pat2 localizes to telomeres, but we were unable to detect direct binding to telomeric DNA. Cells depleted of Pat2 continue to divide, but the telomeres exhibit gradual shortening. The lack of growth arrest indicates that, in contrast to Pot1a and Tpt1, Pat2 is not required for the sequestration of the telomere from the DNA repair machinery. Instead, Pat2 is needed to regulate telomere length, most likely by acting in conjunction with Pat1 to allow telomerase access to the telomere.