Academic Appointments

  • Basic Life Science Research Associate, Biology

All Publications

  • Defining chaperone and ubiquitination circuits cooperating in nuclear and cytoplasmic protein quality control Nature Samant, R. S., Livingston, C. M., Sontag, E. M., Frydman, J. 2018; in press
  • Distinct proteostasis circuits cooperate in nuclear and cytoplasmic protein quality control. Nature Samant, R. S., Livingston, C. M., Sontag, E. M., Frydman, J. 2018; 563 (7731): 407–11


    Protein misfolding is linked to a wide array of human disorders, including Alzheimer's disease, Parkinson's disease and type II diabetes1,2. Protective cellular protein quality control (PQC) mechanisms have evolved to selectively recognize misfolded proteins and limit their toxic effects3-9, thus contributing to the maintenance of the proteome (proteostasis). Here we examine how molecular chaperones and the ubiquitin-proteasome system cooperate to recognize and promote the clearance of soluble misfolded proteins. Using a panel of PQC substrates with distinct characteristics and localizations, we define distinct chaperone and ubiquitination circuitries that execute quality control in the cytoplasm and nucleus. In the cytoplasm, proteasomal degradation of misfolded proteins requires tagging with mixed lysine 48 (K48)- and lysine 11 (K11)-linked ubiquitin chains. A distinct combination of E3 ubiquitin ligases and specific chaperones is required to achieve each type of linkage-specific ubiquitination. In the nucleus, however, proteasomal degradation of misfolded proteins requires only K48-linked ubiquitin chains, and is thus independent of K11-specific ligases and chaperones. The distinct ubiquitin codes for nuclear and cytoplasmic PQC appear to be linked to the function of the ubiquilin protein Dsk2, which is specifically required to clear nuclear misfolded proteins. Our work defines the principles of cytoplasmic and nuclear PQC as distinct, involving combinatorial recognition by defined sets of cooperating chaperones and E3 ligases. A better understanding of how these organelle-specific PQC requirements implement proteome integrity has implications for our understanding of diseases linked to impaired protein clearance and proteostasis dysfunction.

    View details for PubMedID 30429547

  • Mechanisms and Functions of Spatial Protein Quality Control. Annual review of biochemistry Sontag, E. M., Samant, R. S., Frydman, J. 2017


    A healthy proteome is essential for cell survival. Protein misfolding is linked to a rapidly expanding list of human diseases, ranging from neurodegenerative diseases to aging and cancer. Many of these diseases are characterized by the accumulation of misfolded proteins in intra- and extracellular inclusions, such as amyloid plaques. The clear link between protein misfolding and disease highlights the need to better understand the elaborate machinery that manages proteome homeostasis, or proteostasis, in the cell. Proteostasis depends on a network of molecular chaperones and clearance pathways involved in the recognition, refolding, and/or clearance of aberrant proteins. Recent studies reveal that an integral part of the cellular management of misfolded proteins is their spatial sequestration into several defined compartments. Here, we review the properties, function, and formation of these compartments. Spatial sequestration plays a central role in protein quality control and cellular fitness and represents a critical link to the pathogenesis of protein aggregation-linked diseases.

    View details for DOI 10.1146/annurev-biochem-060815-014616

    View details for PubMedID 28489421

  • Sorting out the trash: the spatial nature of eukaryotic protein quality control CURRENT OPINION IN CELL BIOLOGY Sontag, E. M., Vonk, W. I., Frydman, J. 2014; 26: 139-146


    Failure to maintain protein homeostasis is associated with aggregation and cell death, and underies a growing list of pathologies including neurodegenerative diseases, aging, and cancer. Misfolded proteins can be toxic and interfere with normal cellular functions, particularly during proteotoxic stress. Accordingly, molecular chaperones, the ubiquitin-proteasome system (UPS) and autophagy together promote refolding or clearance of misfolded proteins. Here we discuss emerging evidence that the pathways of protein quality control (PQC) are intimately linked to cell architecture, and sequester proteins into spatially and functionally distinct PQC compartments. This sequestration serves a number of functions, including enhancing the efficiency of quality control; clearing the cellular milieu of potentially toxic species and facilitating asymmetric inheritance of damaged proteins to promote rejuvenation of daughter cells.

    View details for DOI 10.1016/

    View details for Web of Science ID 000331860400018

    View details for PubMedID 24463332

  • Exogenous delivery of chaperonin subunit fragment ApiCCT1 modulates mutant Huntingtin cellular phenotypes PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Sontag, E. M., Joachimiak, L. A., Tan, Z., Tomlinson, A., Housman, D. E., Glabe, C. G., Potkin, S. G., Frydman, J., Thompson, L. M. 2013; 110 (8): 3077-3082


    Aggregation of misfolded proteins is characteristic of a number of neurodegenerative diseases, including Huntington disease (HD). The CCT/TRiC (chaperonin containing TCP-1/TCP-1 ring) chaperonin complex can inhibit aggregation and cellular toxicity induced by expanded repeat Huntingtin (mHtt) fragments. The substrate-binding apical domain of CCT/TRiC subunit CCT1, ApiCCT1, is sufficient to inhibit aggregation of expanded repeat mHtt fragments in vitro, providing therapeutic promise for HD. However, a key hurdle in considering ApiCCT1 as a potential treatment is in delivery. Because ApiCCT1 has a region of similarity to the HIV Tat protein cell-transduction domain, we tested whether recombinant ApiCCT1 (ApiCCT1(r)) protein could enter cells following exogenous delivery and modulate an established panel of mHtt-mediated cell-based phenotypes. Cell fractionation studies demonstrate that exogenous ApiCCT1(r) can penetrate cell membranes and can localize to the nucleus, consistent with a strategy that can target both cytosolic and nuclear pathogenic events in HD. ApiCCT1(r) application does indeed modulate HD cellular phenotypes by decreasing formation of visible inclusions, fibrillar oligomers, and insoluble mHtt derived from expression of a truncated mHtt exon 1 fragment. ApiCCT1(r) also delays the onset of inclusion body formation as visualized via live imaging. ApiCCT1(r) reduces mHtt-mediated toxicity in immortalized striatal cells derived from full-length knock-in HD mice, suggesting that therapeutic benefit may extend beyond effects on aggregation. These studies provide the basis for a potentially robust and unique therapeutic strategy to target mHtt-mediated protein pathogenesis.

    View details for DOI 10.1073/pnas.1222663110

    View details for Web of Science ID 000315954400093

    View details for PubMedID 23365139

  • Detection of Mutant Huntingtin Aggregation Conformers and Modulation of SDS-Soluble Fibrillar Oligomers by Small Molecules. Journal of Huntington's disease Sontag, E. M., Lotz, G. P., Yang, G., Sontag, C. J., Cummings, B. J., Glabe, C. G., Muchowski, P. J., Thompson, L. M. 2012; 1 (1): 119-132


    The Huntington's disease (HD) mutation leads to a complex process of Huntingtin (Htt) aggregation into multimeric species that eventually form visible inclusions in cytoplasm, nuclei and neuronal processes. One hypothesis is that smaller, soluble forms of amyloid proteins confer toxic effects and contribute to early cell dysfunction. However, analysis of mutant Htt aggregation intermediates to identify conformers that may represent toxic forms of the protein and represent potential drug targets remains difficult. We performed a detailed analysis of aggregation conformers in multiple in vitro, cell and ex vivo models of HD. Conformation-specific antibodies were used to identify and characterize aggregation species, allowing assessment of multiple conformers present during the aggregation process. Using a series of assays together with these antibodies, several forms could be identified. Fibrillar oligomers, defined as having a β-sheet rich conformation, are observed in vitro using recombinant protein and in protein extracts from cells in culture or mouse brain and shown to be globular, soluble and non-sedimentable structures. Compounds previously described to modulate visible inclusion body formation and reduce toxicity in HD models were also tested and consistently found to alter the formation of fibrillar oligomers. Interestingly, these compounds did not alter the rate of visible inclusion formation, indicating that fibrillar oligomers are not necessarily the rate limiting step of inclusion body formation. Taken together, we provide insights into the structure and formation of mutant Htt fibrillar oligomers that are modulated by small molecules with protective potential in HD models.

    View details for DOI 10.3233/JHD-2012-129004

    View details for PubMedID 24086178