Radiomics and artificial intelligence for risk stratification of pulmonary nodules: Ready for primetime?

Pulmonary nodules are ubiquitously found on computed tomography (CT) imaging either incidentally or via lung cancer screening and require careful diagnostic evaluation and management to both diagnose malignancy when present and avoid unnecessary biopsy of benign lesions. To engage in this complex decision-making, clinicians must first risk stratify pulmonary nodules to determine what the best course of action should be. Recent developments in imaging technology, computer processing power, and artificial intelligence algorithms have yielded radiomics-based computer-aided diagnosis tools that use CT imaging data including features invisible to the naked human eye to predict pulmonary nodule malignancy risk and are designed to be used as a supplement to routine clinical risk assessment. These tools vary widely in their algorithm construction, internal and external validation populations, intended-use populations, and commercial availability. While several clinical validation studies have been published, robust clinical utility and clinical effectiveness data are not yet currently available. However, there is reason for optimism as ongoing and future studies aim to target this knowledge gap, in the hopes of improving the diagnostic process for patients with pulmonary nodules.

Rates of Downstream Procedures and Complications Associated With Lung Cancer Screening in Routine Clinical Practice : A Retrospective Cohort Study.

BACKGROUND: Lung cancer screening (LCS) using low-dose computed tomography (LDCT) reduces lung cancer mortality but can lead to downstream procedures, complications, and other potential harms. Estimates of these events outside NLST (National Lung Screening Trial) have been variable and lacked evaluation by screening result, which allows more direct comparison with trials. OBJECTIVE: To identify rates of downstream procedures and complications associated with LCS. DESIGN: Retrospective cohort study. SETTING: 5 U.S. health care systems. PATIENTS: Individuals who completed a baseline LDCT scan for LCS between 2014 and 2018. MEASUREMENTS: Outcomes included downstream imaging, invasive diagnostic procedures, and procedural complications. For each, absolute rates were calculated overall and stratified by screening result and by lung cancer detection, and positive and negative predictive values were calculated. RESULTS: Among the 9266 screened patients, 1472 (15.9%) had a baseline LDCT scan showing abnormalities, of whom 140 (9.5%) were diagnosed with lung cancer within 12 months (positive predictive value, 9.5% [95% CI, 8.0% to 11.0%]; negative predictive value, 99.8% [CI, 99.7% to 99.9%]; sensitivity, 92.7% [CI, 88.6% to 96.9%]; specificity, 84.4% [CI, 83.7% to 85.2%]). Absolute rates of downstream imaging and invasive procedures in screened patients were 31.9% and 2.8%, respectively. In patients undergoing invasive procedures after abnormal findings, complication rates were substantially higher than those in NLST (30.6% vs. 17.7% for any complication; 20.6% vs. 9.4% for major complications). LIMITATION: Assessment of outcomes was retrospective and was based on procedural coding. CONCLUSION: The results indicate substantially higher rates of downstream procedures and complications associated with LCS in practice than observed in NLST. Diagnostic management likely needs to be assessed and improved to ensure that screening benefits outweigh potential harms. PRIMARY FUNDING SOURCE: National Cancer Institute and Gordon and Betty Moore Foundation.

Social vulnerability and lung malignancy mortality.

INTRODUCTION: Lung cancer is a major cause of death in the United States. Social determinants of health (SDOH) are important factors that impact the treatment and prognosis of lung cancer. The social vulnerability index (SVI) is a validated measure of SDOH. This cross-sectional study aimed to investigate the impact of the SVI on lung cancer mortality using descriptive epidemiology. METHODS: Mortality data for lung malignancies from 2014 to 2018 was obtained from the CDC database and was age-adjusted and standardized to the population in the year 2000. The SVI for the same years was obtained from the CDC Agency for Toxic Substances and Disease Registry database. Age-adjusted mortality rates (AAMR) were estimated for each SVI quartile (SVI-Q) and demographic subgroup. RESULTS: We found that counties in SVI-Q4 (most vulnerable) had a higher cumulative AAMR compared to counties in SVI-Q1 (least vulnerable), accounting for a 4.48 excess death rate per 100,000 person-years. AAMR among males in SVI-Q4 was higher compared to SVI-Q1, accounting for a 9.96 excess death rate per 100,000 person-years, whereas no mortality differences were observed for female populations between SVI-Q4 and SVI-Q1. AAMR in SVI-Q4 was higher for both Hispanic and non-Hispanic populations, except for American Indian/Alaska Native populations. Similar trends were observed in both metropolitan and non-metropolitan counties. CONCLUSION: Our study suggests that the SVI may play a significant role in lung cancer mortality and highlights the need for interventions targeting vulnerable populations to improve outcomes.

The Performance of an Extended Next Generation Sequencing Panel Using Endobronchial Ultrasound-Guided Fine Needle Aspiration Samples in Non-Squamous Non-Small Cell Lung Cancer: A Pragmatic Study.

INTRODUCTION/BACKGROUND: Samples from endobronchial ultrasound-guided fine needle aspiration (EBUS-TBNA) are frequently used for next generation sequencing (NGS) in patients with non-small cell lung cancer (NSCLC) to look for genetic driver mutations. The objective of the current study was to evaluate the performance of extended NGS panels using EBUS-TBNA samples in a real-world setting and identify factors associated with the success of NGS. MATERIALS AND METHODS: This study included all patients who underwent EBUS and were diagnosed with non-squamous NSCLC with mediastinal metastasis from 2016 to 2019 at the University of Pennsylvania. We reviewed demographic information, imaging studies, procedure reports, pathology and NGS reports. Logistic regression was used to analyze factors associated with the success of NGS panels. RESULTS: The success rates of NGS using EBUS-TBNA samples were 92.5%, and 91.5% for DNA and RNA NGS panels respectively. Samples from higher N stage (N2 and N3 lymph nodes) and with higher tumor cellularity (>25%) resulted in higher success rate for DNA NGS. The effect of tumor cellularity remained borderline significant after entering multivariable logistic regression. The short-axis diameter of the sampled lymph node on CT scan, FDG-avidity on PET CT and >3 EBUS passes per lymph node during the procedure were not associated with NGS success. CONCLUSION: Both DNA and RNA extended-panel NGS had high performance using EBUS-TBNA samples. Sampling more advanced nodal stations and obtaining samples with higher tumor cellularity were associated with higher success rate of DNA NGS. Other imaging or procedural factors did not affect NGS performance.

Percentage Up to Date With Chest Computed Tomography Among Those Eligible for Lung Cancer Screening.

INTRODUCTION: Authors aimed to calculate the percentage up-to-date with testing in the context of lung cancer screening across 5 healthcare systems and evaluate differences according to patient and health system characteristics. METHODS: Lung cancer screening‒eligible individuals receiving care within the five systems in the Population-based Research to Optimize the Screening Process Lung consortium from October 1, 2018 to September 30, 2019 were included in analyses. Data collection was completed on June 15, 2021; final analyses were completed on April 1, 2022. Chest computed tomography scans and patient characteristics were obtained through electronic health records and used to calculate the percentage completing a chest computed tomography scan in the previous 12 months (considered up-to-date). The association of patient and healthcare system factors with being up-to-date was evaluated with adjusted prevalence ratios and 95% CIs using log-binomial regression models. RESULTS: There were 29,417 individuals eligible for lung cancer screening as of September 30, 2019; 8,333 (28.3%) were up-to-date with testing. Those aged 65-74 years (prevalence ratio=1.19; CI=1.15, 1.24, versus ages 55-64), those with chronic obstructive pulmonary disease (prevalence ratio=2.05; CI=1.98, 2.13), and those in higher SES census tracts (prevalence ratio=1.22; CI=1.16, 1.30, highest quintile versus lowest) were more likely to be up-to-date. Currently smoking (prevalence ratio=0.91; CI=0.88, 0.95), having a BMI ≥30 kg/m2 (prevalence ratio=0.83; CI=0.77, 0.88), identifying as Native Hawaiian or other Pacific Islander (prevalence ratio=0.79; CI=0.68, 0.92), and having a decentralized lung cancer screening program (prevalence ratio=0.77; CI=0.74, 0.80) were inversely associated with being up-to-date. CONCLUSIONS: The percentage up-to-date with testing among those eligible for lung cancer screening is well below up-to-date estimates for other types of cancer screening, and disparities in lung cancer screening participation remain.

Diagnostic Yield of Cone-beam-Derived Augmented Fluoroscopy and Ultrathin Bronchoscopy Versus Conventional Navigational Bronchoscopy Techniques.

BACKGROUND: Pulmonary nodules suspicious for lung cancer are frequently diagnosed. Evaluating and optimizing the diagnostic yield of lung nodule biopsy is critical as innovation in bronchoscopy continues to progress. METHODS: This is a retrospective cohort study. Consecutive patients undergoing guided bronchoscopy for suspicious pulmonary nodule(s) between February 2020 and July 2021 were included. The cone-beam computed tomography (CBCT)+ radial endobronchial ultrasound (r-EBUS) group had their procedure using CBCT-derived augmented fluoroscopy along with r-EBUS. The CBCT+ ultrathin bronchoscope (UTB)+r-EBUS group had the same procedure but with the use of an ultrathin bronchoscope. The r-EBUS group underwent r-EBUS guidance without CBCT or augmented fluoroscopy. We used multivariable logistic regression to compare diagnostic yield, adjusting for confounding variables. RESULTS: A total of 116 patients were included. The median pulmonary lesion diameter was 19.5 mm (interquartile range, 15.0 to 27.5 mm), and 91 (78.4%) were in the peripheral half of the lung. Thirty patients (25.9%) underwent CBCT+UTB, 27 (23.3%) CBCT, and 59 (50.9%) r-EBUS alone with unadjusted diagnostic yields of 86.7%, 70.4%, and 42.4%, respectively ( P <0.001). The adjusted diagnostic yields were 85.0% (95% CI, 68.6% to 100%), 68.3% (95% CI, 50.1% to 86.6%), and 44.5% (95% CI, 31.0% to 58.0%), respectively. There was significantly more virtual navigational bronchoscopy use in the r-EBUS group (45.8%) compared with the CBCT+UTB (13.3%) and CBCT (18.5%) groups, respectively. CBCT procedures required dose area product radiation doses of 7602.5 µGym 2 . CONCLUSION: Compared with the r-EBUS group, CBCT + UTB + r-EBUS was associated with higher navigational success, fewer nondiagnostic biopsy results, and a higher diagnostic yield. CBCT procedures are associated with a considerable radiation dose.

Artificial Intelligence Tool for Assessment of Indeterminate Pulmonary Nodules Detected with CT.

Background Limited data are available regarding whether computer-aided diagnosis (CAD) improves assessment of malignancy risk in indeterminate pulmonary nodules (IPNs). Purpose To evaluate the effect of an artificial intelligence-based CAD tool on clinician IPN diagnostic performance and agreement for both malignancy risk categories and management recommendations. Materials and Methods This was a retrospective multireader multicase study performed in June and July 2020 on chest CT studies of IPNs. Readers used only CT imaging data and provided an estimate of malignancy risk and a management recommendation for each case without and with CAD. The effect of CAD on average reader diagnostic performance was assessed using the Obuchowski-Rockette and Dorfman-Berbaum-Metz method to calculate estimates of area under the receiver operating characteristic curve (AUC), sensitivity, and specificity. Multirater Fleiss κ statistics were used to measure interobserver agreement for malignancy risk and management recommendations. Results A total of 300 chest CT scans of IPNs with maximal diameters of 5-30 mm (50.0% malignant) were reviewed by 12 readers (six radiologists, six pulmonologists) (patient median age, 65 years; IQR, 59-71 years; 164 [55%] men). Readers' average AUC improved from 0.82 to 0.89 with CAD (P < .001). At malignancy risk thresholds of 5% and 65%, use of CAD improved average sensitivity from 94.1% to 97.9% (P = .01) and from 52.6% to 63.1% (P < .001), respectively. Average reader specificity improved from 37.4% to 42.3% (P = .03) and from 87.3% to 89.9% (P = .05), respectively. Reader interobserver agreement improved with CAD for both the less than 5% (Fleiss κ, 0.50 vs 0.71; P < .001) and more than 65% (Fleiss κ, 0.54 vs 0.71; P < .001) malignancy risk categories. Overall reader interobserver agreement for management recommendation categories (no action, CT surveillance, diagnostic procedure) also improved with CAD (Fleiss κ, 0.44 vs 0.52; P = .001). Conclusion Use of computer-aided diagnosis improved estimation of indeterminate pulmonary nodule malignancy risk on chest CT scans and improved interobserver agreement for both risk stratification and management recommendations. © RSNA, 2022 Online supplemental material is available for this article. See also the editorial by Yanagawa in this issue.

Racial Disparities in Adherence to Annual Lung Cancer Screening and Recommended Follow-Up Care: A Multicenter Cohort Study.

Rationale: Black patients receive recommended lung cancer screening (LCS) follow-up care less frequently than White patients, but it is unknown if this racial disparity persists across both decentralized and centralized LCS programs. Objectives: To determine adherence to American College of Radiology Lung Imaging Reporting and Data System (Lung-RADS) recommendations among individuals undergoing LCS at either decentralized or centralized programs and to evaluate the association of race with LCS adherence. Methods: We performed a multicenter retrospective cohort study of patients receiving LCS at five heterogeneous U.S. healthcare systems. We calculated adherence to annual LCS among patients with a negative baseline screen (Lung-RADS 1 or 2) and recommended follow-up care among those with a positive baseline screen (Lung-RADS 3, 4A, 4B, or 4X) stratified by type of LCS program and evaluated the association between race and adherence using multivariable modified Poisson regression. Results: Of the 6,134 total individuals receiving LCS, 5,142 (83.8%) had negative baseline screens, and 992 (16.2%) had positive baseline screens. Adherence to both annual LCS (34.8% vs. 76.1%; P < 0.001) and recommended follow-up care (63.9% vs. 74.6%; P < 0.001) was lower at decentralized compared with centralized programs. Among individuals with negative baseline screens, a racial disparity in adherence was observed only at decentralized screening programs (interaction term, P < 0.001). At decentralized programs, Black race was associated with 27% reduced adherence to annual LCS (adjusted relative risk [aRR], 0.73; 95% confidence interval [CI], 0.63-0.84), whereas at centralized programs, no effect by race was observed (aRR, 0.98; 95% CI, 0.91-1.05). In contrast, among those with positive baseline screens, there was no significant difference by race for adherence to recommended follow-up care by type of LCS program (decentralized aRR, 0.95; 95% CI, 0.81-1.11; centralized aRR, 0.81; 95% CI, 0.71-0.93; interaction term, P = 0.176). Conclusions: In this large multicenter study of individuals screened for lung cancer, adherence to both annual LCS and recommended follow-up care was greater at centralized screening programs. Black patients were less likely to receive annual LCS than White patients at decentralized compared with centralized LCS programs. Our results highlight the need for further study of healthcare system-level mechanisms to optimize longitudinal LCS care.

Impact of the COVID-19 Pandemic on Treatment Patterns for Patients With Metastatic Solid Cancer in the United States.

BACKGROUND: The COVID-19 pandemic has led to delays in patients seeking care for life-threatening conditions; however, its impact on treatment patterns for patients with metastatic cancer is unknown. We assessed the COVID-19 pandemic's impact on time to treatment initiation (TTI) and treatment selection for patients newly diagnosed with metastatic solid cancer. METHODS: We used an electronic health record-derived longitudinal database curated via technology-enabled abstraction to identify 14 136 US patients newly diagnosed with de novo or recurrent metastatic solid cancer between January 1 and July 31 in 2019 or 2020. Patients received care at approximately 280 predominantly community-based oncology practices. Controlled interrupted time series analyses assessed the impact of the COVID-19 pandemic period (April-July 2020) on TTI, defined as the number of days from metastatic diagnosis to receipt of first-line systemic therapy, and use of myelosuppressive therapy. RESULTS: The adjusted probability of treatment within 30 days of diagnosis was similar across periods (January-March 2019 = 41.7%, 95% confidence interval [CI] = 32.2% to 51.1%; April-July 2019 = 42.6%, 95% CI = 32.4% to 52.7%; January-March 2020 = 44.5%, 95% CI = 30.4% to 58.6%; April-July 2020 = 46.8%, 95% CI= 34.6% to 59.0%; adjusted percentage-point difference-in-differences = 1.4%, 95% CI = -2.7% to 5.5%). Among 5962 patients who received first-line systemic therapy, there was no association between the pandemic period and use of myelosuppressive therapy (adjusted percentage-point difference-in-differences = 1.6%, 95% CI = -2.6% to 5.8%). There was no meaningful effect modification by cancer type, race, or age. CONCLUSIONS: Despite known pandemic-related delays in surveillance and diagnosis, the COVID-19 pandemic did not affect TTI or treatment selection for patients with metastatic solid cancers.

Impact of the COVID-19 pandemic on treatment patterns for US patients with metastatic solid cancer.

BACKGROUND: The COVID-19 pandemic has led to delays in patients seeking care for life-threatening conditions; however, its impact on treatment patterns for patients with metastatic cancer is unknown. We assessed the COVID-19 pandemic's impact on time to treatment initiation (TTI) and treatment selection for patients newly diagnosed with metastatic solid cancer. METHODS: We used an electronic health record-derived longitudinal database curated via technology-enabled abstraction to identify 14,136 US patients newly diagnosed with de novo or recurrent metastatic solid cancer between January 1 and July 31 in 2019 or 2020. Patients received care at ∼280 predominantly community-based oncology practices. Controlled interrupted time series analyses assessed the impact of the COVID-19 pandemic period (April-July 2020) on TTI, defined as the number of days from metastatic diagnosis to receipt of first-line systemic therapy, and use of myelosuppressive therapy. RESULTS: The adjusted probability of treatment within 30 days of diagnosis [95% confidence interval] was similar across periods: January-March 2019 41.7% [32.2%, 51.1%]; April-July 2019 42.6% [32.4%, 52.7%]; January-March 2020 44.5% [30.4%, 58.6%]; April-July 2020 46.8% [34.6%, 59.0%]; adjusted percentage-point difference-in-differences 1.4% [-2.7%, 5.5%]. Among 5,962 patients who received first-line systemic therapy, there was no association between the pandemic period and use of myelosuppressive therapy (adjusted percentage-point difference-in-differences 1.6% [-2.6%, 5.8%]). There was no meaningful effect modification by cancer type, race, or age. CONCLUSIONS: Despite known pandemic-related delays in surveillance and diagnosis, the COVID-19 pandemic did not impact time to treatment initiation or treatment selection for patients with metastatic solid cancers.

Immune Checkpoint Inhibitor Uptake in Real-World Patients With Malignant Pleural Mesothelioma.

INTRODUCTION: Since the July 2017 National Comprehensive Cancer Network (NCCN) malignant pleural mesothelioma (MPM) guideline revision recommended second-line immune checkpoint inhibitors (ICIs), studies have suggested a greater response to ICI among patients with nonepithelioid MPM. Nevertheless, little is known regarding adoption of ICI in routine practice and if uptake differs by histologic subtype. Our objectives were to evaluate the real-world uptake of second-line ICI among patients with MPM and to reveal its association with histologic subtype. METHODS: This was a multicenter, retrospective cohort study of real-world patients with MPM receiving at least two lines of systemic therapy between 2011 and 2019. We found the uptake of second-line ICI over time and evaluated the association between histologic subtype and ICI use, adjusting for relevant patient demographic and clinical factors. RESULTS: Among the 426 patients with MPM in our cohort, 310 had epithelioid and 116 nonepithelioid histologic subtype. The median age was 73 years (interquartile range: 67-78). Overall, 144 patients (33.8%) received second-line ICI and 282 (66.2%) traditional chemotherapy. ICI uptake began in early 2015 before the NCCN guideline revision and increased rapidly to 2019. After the 2017 NCCN guideline revision, patients with nonepithelioid MPM histologic subtypes had more than 3 times the odds of receiving second-line ICI (OR = 3.26; 95% confidence interval: 1.41-7.54). CONCLUSIONS: Among real-world patients with MPM, second-line ICI uptake began over two years before the 2017 NCCN guideline recommendations and was associated with nonepithelioid histologic subtype after contemporary studies suggested increased clinical benefit in this population, reflecting prompt integration of scientific discovery into clinical practice.

Comparative effectiveness of second-line immune checkpoint inhibitor therapy versus chemotherapy for malignant pleural mesothelioma.

OBJECTIVES: Second-line immune checkpoint inhibition (ICI) was recently shown to have a survival advantage over placebo in malignant pleural mesothelioma (MPM), but the survival comparison to chemotherapy in patients with MPM receiving routine clinical care is unknown. Our objective was to examine the effectiveness of second-line ICI versus chemotherapy on overall survival (OS) outcomes in real-world patients with advanced MPM. MATERIALS AND METHODS: We performed a multicenter retrospective cohort study of real-world adult patients with advanced MPM who received first-line platinum-based chemotherapy and at least two total lines of systemic therapy. Patients received either second-line chemotherapy (gemcitabine and/or vinorelbine) or ICI therapy (pembrolizumab or nivolumab ± ipilimumab). The primary outcome was OS, defined as the time from second-line therapy initiation to death or end of the observation period. We used Kaplan-Meier methods and Cox proportional hazards modeling with adjustment for relevant patient demographic and clinical variables to compare OS between the two second-line treatment groups. RESULTS: Of the 176 patients with MPM, the median age was 75 years (IQR: 69-79.5 years), and most were white (77%), male (74%), and had epithelioid histology (67%). Thirty-five percent (61) received second-line chemotherapy and 65% (115) ICI therapy (80 pembrolizumab, 31 nivolumab, and 4 nivolumab + ipilimumab). Treatment with ICI was associated with significantly longer median OS compared to chemotherapy (8.7 vs 5.0 months, p=0.001; adjusted hazard ratio: 0.52, 95% CI: 0.34-0.81). The estimated 12-month OS probability was 36.7% (95% CI: 27.6%-45.8%) and 15.6% (95% CI: 7.7%-26.1%) in the ICI and chemotherapy groups, respectively. CONCLUSION: In this "real-world" population of patients with MPM, treatment with ICI was associated with improved OS outcomes compared to chemotherapy in the second-line setting, in contrast with a recent clinical trial. Our findings suggest that ICI may benefit patients with advanced MPM and progression after initial platinum-based chemotherapy.

Malignant Mesothelioma: Has Anything Changed?

Malignant pleural mesothelioma is a rare cancer associated with asbestos exposure and portends a dismal prognosis. Its worldwide incidence has been increasing, and treatment options are currently suboptimal and noncurative. However, since the turn of the century, several encouraging steps have been made toward improving outcomes for mesothelioma patients. An increased understanding of disease pathophysiology has led to more accurate diagnosis and staging, and the establishment of the standard of care first-line pemetrexed/platin doublet chemotherapy regimen in 2003 initially revolutionized treatment. While significant debate remains regarding the preferred approach to surgical and radiation therapy in the context of multimodal therapy, recent breakthroughs in immunotherapy offer hope for another paradigm shift in the near future. This review will summarize the current clinical approach to diagnosis, staging, and treatment of malignant pleural mesothelioma.

Factors Associated With Discharge Home Among Medical ICU Patients in an Early Mobilization Program.

OBJECTIVES: One goal of early mobilization programs is to facilitate discharge home after an ICU hospitalization, but little is known about which factors are associated with this outcome. Our objective was to evaluate factors associated with discharge home among medical ICU patients in an early mobilization program who were admitted to the hospital from home. DESIGN: Retrospective cohort study of medical ICU patients in an early mobilization program. SETTING: Tertiary care center medical ICU. PATIENTS: Medical ICU patients receiving early mobilization who were community-dwelling prior to admission. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: A comprehensive set of baseline, ICU-related, and mobilization-related factors were tested for their association with discharge home using multivariable logistic regression. Among the analytic cohort (n = 183), the mean age was 61.9 years (sd 16.67 yr) and the mean Acute Physiology and Chronic Health Evaluation II score was 23.5 (sd 7.11). Overall, 65.0% of patients were discharged home after their critical illness. In multivariable analysis, each incremental increase in the maximum level of mobility achieved (range, 1-6) during the medical ICU stay was associated with nearly a 50% greater odds of discharge home (odds ratio, 1.46; 95% CI, 1.13-1.88), whereas increased age (odds ratio, 0.95; 95% CI, 0.93-0.98) and greater hospital length of stay (odds ratio, 0.94; 95% CI, 0.90-0.99) were associated with decreased odds of discharge home. Prehospital ambulatory status was not associated with discharge home. CONCLUSIONS: Among medical ICU patients who resided at home prior to their ICU admission, the maximum level of mobility achieved in the medical ICU was the factor most strongly associated with discharge back home. Identification of this factor upon ICU-to-ward transfer may help target mobilization plans on the ward to facilitate a goal of discharge home.

A conserved 20S proteasome assembly factor requires a C-terminal HbYX motif for proteasomal precursor binding.

Dedicated chaperones facilitate the assembly of the eukaryotic proteasome, but how they function remains largely unknown. Here we show that a yeast 20S proteasome assembly factor, Pba1-Pba2, requires a previously overlooked C-terminal hydrophobic-tyrosine-X (HbYX) motif for function. HbYX motifs in proteasome activators open the 20S proteasome entry pore, but Pba1-Pba2 instead binds inactive proteasomal precursors. We discovered an archaeal ortholog of this factor, here named PbaA, that also binds preferentially to proteasomal precursors in a HbYX motif-dependent fashion using the same proteasomal α-ring surface pockets as are bound by activators. PbaA and the related PbaB protein can be induced to bind mature 20S proteasomes if the active sites in the central chamber are occupied by inhibitors. Our data are consistent with an allosteric mechanism in which the maturation of the proteasome active sites determines the binding of assembly chaperones, potentially shielding assembly intermediates or misassembled complexes from nonproductive associations until assembly is complete.