Professional Education

  • Doctor of Philosophy, University of Iowa, Biochemistry (2019)
  • Bachelor of Science, Yonsei University, Systems Biology (2013)

Research & Scholarship

Lab Affiliations


All Publications

  • Novel PRPF31 gene loss of function for retinitis pigmentosa 11 Chemudupati, T., Sun, Y., Yang, J., Ng, B., Mahajan, V. B. ASSOC RESEARCH VISION OPHTHALMOLOGY INC. 2020
  • Structural Insights into the Unique Activation Mechanisms of a Non-classical Calpain and Its Disease-Causing Variants. Cell reports Velez, G., Sun, Y. J., Khan, S., Yang, J., Herrmann, J., Chemudupati, T., MacLaren, R. E., Gakhar, L., Wakatsuki, S., Bassuk, A. G., Mahajan, V. B. 2020; 30 (3): 881


    Increased calpain activity is linked to neuroinflammation including a heritable retinal disease caused by hyper-activating mutations in the calcium-activated calpain-5 (CAPN5) protease. Although structures for classical calpains are known, the structure of CAPN5, a non-classical calpain, remains undetermined. Here we report the 2.8A crystal structure of the human CAPN5 protease core (CAPN5-PC). Compared to classical calpains, CAPN5-PC requires high calcium concentrations for maximal activity. Structure-based phylogenetic analysis and multiple sequence alignment reveal that CAPN5-PC contains three elongated flexible loops compared to its classical counterparts. The presence of a disease-causing mutation (c.799G>A, p.Gly267Ser) on the unique PC2L2 loop reveals a function in this region for regulating enzymatic activity. This mechanism could be transferred to distant calpains, using synthetic calpain hybrids, suggesting an evolutionary mechanism for fine-tuning calpain function by modifying flexible loops. Further, the open (inactive) conformation of CAPN5-PC provides structural insight into CAPN5-specific residues that can guide inhibitor design.

    View details for DOI 10.1016/j.celrep.2019.12.077

    View details for PubMedID 31968260

  • A physics-based energy function allows the computational redesign of a PDZ domain. Scientific reports Opuu, V., Sun, Y. J., Hou, T., Panel, N., Fuentes, E. J., Simonson, T. 2020; 10 (1): 11150


    Computational protein design (CPD) can address the inverse folding problem, exploring a large space of sequences and selecting ones predicted to fold. CPD was used previously to redesign several proteins, employing a knowledge-based energy function for both the folded and unfolded states. We show that a PDZ domain can be entirely redesigned using a "physics-based" energy for the folded state and a knowledge-based energy for the unfolded state. Thousands of sequences were generated by Monte Carlo simulation. Three were chosen for experimental testing, based on their low energies and several empirical criteria. All three could be overexpressed and had native-like circular dichroism spectra and 1D-NMR spectra typical of folded structures. Two had upshifted thermal denaturation curves when a peptide ligand was present, indicating binding and suggesting folding to a correct, PDZ structure. Evidently, the physical principles that govern folded proteins, with a dash of empirical post-filtering, can allow successful whole-protein redesign.

    View details for DOI 10.1038/s41598-020-67972-w

    View details for PubMedID 32636412

    View details for PubMedCentralID PMC7341745

  • SGEF forms a complex with Scribble and Dlg1 and regulates epithelial junctions and contractility JOURNAL OF CELL BIOLOGY Awadia, S., Hug, F., Arnold, T. H., Goicoechea, S. M., Sun, Y., Hou, T., Kreider-Letterman, G., Massimi, P., Banks, L., Fuentes, E. J., Miller, A. L., Garcia-Mata, R. 2019; 218 (8): 2699?2725


    The canonical Scribble polarity complex is implicated in regulation of epithelial junctions and apical polarity. Here, we show that SGEF, a RhoG-specific GEF, forms a ternary complex with Scribble and Dlg1, two members of the Scribble complex. SGEF targets to apical junctions in a Scribble-dependent fashion and functions in the regulation of actomyosin-based contractility and barrier function at tight junctions as well as E-cadherin-mediated formation of adherens junctions. Surprisingly, SGEF does not control the establishment of polarity. However, in 3D cysts, SGEF regulates the formation of a single open lumen. Interestingly, SGEF's nucleotide exchange activity regulates the formation and maintenance of adherens junctions, and in cysts the number of lumens formed, whereas SGEF's scaffolding activity is critical for regulation of actomyosin contractility and lumen opening. We propose that SGEF plays a key role in coordinating junctional assembly and actomyosin contractility by bringing together Scribble and Dlg1 and targeting RhoG activation to cell-cell junctions.

    View details for DOI 10.1083/jcb.201811114

    View details for Web of Science ID 000478788200019

    View details for PubMedID 31248911

    View details for PubMedCentralID PMC6683736

  • Novel mutations in the 3-box motif of the BACK domain of KLHL7 associated with nonsyndromic autosomal dominant retinitis pigmentosa. Orphanet journal of rare diseases Oh, J. K., Lima de Carvalho, J. R., Sun, Y. J., Ragi, S., Yang, J., Levi, S. R., Ryu, J., Bassuk, A. G., Mahajan, V. B., Tsang, S. H. 2019; 14 (1): 295


    Mutations in the Kelch-like protein 7 (KLHL7) represent a recently described and, to date, poorly characterized etiology of inherited retinal dystrophy. Dominant mutations in KLHL7 are a cause of isolated, non-syndromic retinitis pigmentosa (RP). In contrast, recessive loss-of-function mutations are known to cause Crisponi or Bohring-Opitz like cold induced sweating syndrome-3 (BOS-3). In this study, the phenotype and progression of five unrelated patients with KLHL7 mediated autosomal dominant RP (adRP) are characterized. Clinical evaluation of these patients involved a complete ophthalmic exam, full-field electroretinography (ffERG), and imaging, including fundus photography, spectral domain optical coherence tomography (SD-OCT), short wavelength fundus autofluorescence (SW-AF), and near-infrared fundus autofluorescence (NIR-AF). Molecular diagnoses were performed using whole-exome sequencing or gene panel testing. Disease progression was monitored in three patients with available data for a mean follow up time of 4.5?±?2.9?years. Protein modeling was performed for all variants found in this study in addition to those documented in the literature for recessive loss-of-function alleles causing Crisponi or Bohring-Opitz like cold-induced sweating syndrome.Genetic testing in three patients identified two novel variants within the 3-box motif of the BACK domain: c.472?T?>?C:p.(Cys158Arg) and c.433A?>?T:p.(Asn145Tyr). Clinical imaging demonstrated hyperautofluorescent ring formation on both SW-AF and NIR-AF in three patients, with diffuse peripheral and peripapillary atrophy seen in all but one case. SD-OCT demonstrated a phenotypic spectrum, from parafoveal atrophy of the outer retina with foveal sparing to widespread retinal thinning and loss of photoreceptors. Incidence of cystoid macular edema was high with four of five patients affected. Protein modeling of dominant alleles versus recessive loss-of-function alleles showed dominant alleles localized to the BTB and BACK domains while recessive alleles were found in the Kelch domain.We report the phenotype in five patients with KLHL7 mediated adRP, two novel coding variants, and imaging biomarkers using SW-AF and NIR-AF. These findings may influence future gene-based therapies for adRP and pave the way for mechanistic studies that elucidate the pathogenesis of KLHL7-mediated RP.

    View details for DOI 10.1186/s13023-019-1275-2

    View details for PubMedID 31856884

  • A Simple PB/LIE Free Energy Function Accurately Predicts the Peptide Binding Specificity of the Tiam1 PDZ Domain. Frontiers in molecular biosciences Panel, N., Sun, Y. J., Fuentes, E. J., Simonson, T. 2017; 4: 65


    PDZ domains generally bind short amino acid sequences at the C-terminus of target proteins, and short peptides can be used as inhibitors or model ligands. Here, we used experimental binding assays and molecular dynamics simulations to characterize 51 complexes involving the Tiam1 PDZ domain and to test the performance of a semi-empirical free energy function. The free energy function combined a Poisson-Boltzmann (PB) continuum electrostatic term, a van der Waals interaction energy, and a surface area term. Each term was empirically weighted, giving a Linear Interaction Energy or "PB/LIE" free energy. The model yielded a mean unsigned deviation of 0.43 kcal/mol and a Pearson correlation of 0.64 between experimental and computed free energies, which was superior to a Null model that assumes all complexes have the same affinity. Analyses of the models support several experimental observations that indicate the orientation of the ?2 helix is a critical determinant for peptide specificity. The models were also used to predict binding free energies for nine new variants, corresponding to point mutants of the Syndecan1 and Caspr4 peptides. The predictions did not reveal improved binding; however, they suggest that an unnatural amino acid could be used to increase protease resistance and peptide lifetimes in vivo. The overall performance of the model should allow its use in the design of new PDZ ligands in the future.

    View details for DOI 10.3389/fmolb.2017.00065

    View details for PubMedID 29018806

    View details for PubMedCentralID PMC5623046

  • Distinct Roles for Conformational Dynamics in Protein-Ligand Interactions. Structure (London, England : 1993) Liu, X., Speckhard, D. C., Shepherd, T. R., Sun, Y. J., Hengel, S. R., Yu, L., Fowler, C. A., Gakhar, L., Fuentes, E. J. 2016; 24 (12): 2053?66


    Conformational dynamics has an established role in enzyme catalysis, but its contribution to ligand binding and specificity is largely unexplored. Here we used the Tiam1 PDZ domain and an engineered variant (QM PDZ) with broadened specificity to investigate the role of structure and conformational dynamics in molecular recognition. Crystal structures of the QM PDZ domain both free and bound to ligands showed structural features central to binding (enthalpy), while nuclear-magnetic-resonance-based methyl relaxation experiments and isothermal titration calorimetry revealed that conformational entropy contributes to affinity. In addition to motions relevant to thermodynamics, slower microsecond to millisecond switching was prevalent in the QM PDZ ligand-binding site consistent with a role in ligand specificity. Our data indicate that conformational dynamics plays distinct and fundamental roles in tuning the affinity (conformational entropy) and specificity (excited-state conformations) of molecular interactions. More broadly, our results have important implications for the evolution, regulation, and design of protein-ligand interactions.

    View details for DOI 10.1016/j.str.2016.08.019

    View details for PubMedID 27998539

    View details for PubMedCentralID PMC5488749

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