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Open Access Publications from the University of California

Scholarly Works (12 results)

  • Article
  • Peer Reviewed
UC San Francisco Previously Published Works (2018)
The Hsp90 molecular chaperones are ATP-dependent enzymes that maintain protein homeostasis and regulate many essential cellular processes. Higher eukaryotes have organelle-specific Hsp90 paralogs that are adapted to each subcellular environment. The mitochondrial Hsp90, TNF receptor-associated protein 1 (TRAP1), supports the folding and activity of electron transport components and is increasingly appreciated as a critical player in mitochondrial signaling. Calcium plays a well-known and important regulatory role in mitochondria where it can accumulate to much higher concentrations than in the cytoplasm. Surprisingly, we found here that calcium can replace magnesium, the essential enzymatic cofactor, to support TRAP1 ATPase activity. Anomalous X-ray diffraction experiments revealed a calcium-binding site within the TRAP1 nucleotide-binding pocket located near the ATP α-phosphate and completely distinct from the magnesium-binding site adjacent to the β- and γ-phosphates. In the presence of magnesium, ATP hydrolysis by TRAP1, as with other Hsp90s, was noncooperative, whereas calcium binding resulted in cooperative hydrolysis by the two protomers within the Hsp90 dimer. The structural data suggested a mechanism for this cooperative behavior. Because of the cooperativity, at high ATP concentrations, ATPase activity was higher with calcium, whereas the converse was observed at low ATP concentrations. Integrating these observations, we propose a model in which the divalent cation choice can control switching between noncooperative and cooperative TRAP1 ATPase mechanisms in response to varying ATP concentrations. This switching may facilitate coordination between cellular energetics, mitochondrial signaling, and protein homeostasis via alterations in the TRAP1 ATP-driven cycle and its consequent effects on different mitochondrial clients.
    Cover page: Calcium binding to a remote site can replace magnesium as cofactor for mitochondrial Hsp90 (TRAP1) ATPase activity
    • Thesis
    • Peer Reviewed
    UC San Francisco Electronic Theses and Dissertations (2017)

    Hsp90 is a highly conserved, ATP-dependent molecular chaperone that is essential for maintaining the functions of its client proteins. It has been estimated that about 10% of the proteome of a eukaryotic cell interacts with Hsp90. A large subset of this portion consists of protein kinases and steroid hormone receptors, putting Hsp90 as the master regulator of many essential cellular functions. The mechanism of how Hsp90 uses ATP hydrolysis to carry out its function remains unclear. Structural studies of Hsp90 revealed that Hsp90 is a V-shaped homodimer with each protomer composed of three well-folded domains: an ATP-binding N-terminal Domain (NTD), a Middle Domain (MD), and a C-terminal dimerization Domain (CTD). Efficient ATP hydrolysis by Hsp90 requires that the dimer forms a closed state where NTDs are dimerized, forming a closed ”clamp” conformation that is stabilized by ATP binding. The kinetics of forming this stably closed state is not driven by ATP binding, as there are other rate-limiting steps that need to occur within the protein to allow the NTDs to be dimerized. So how does Hsp90 actually use the energy from ATP to remodel its client protein? the focus of this thesis examines the other possibility of this happening during ATP hydrolysis. In the first chapter, I followed up an observation made by a previous graduate student Laura Lavery. She observed that the ATP-bound closed state of a mitochondrial Hsp90 (TRAP1) is asymmetric. The asymmetry is most prominent at the juncture between the MD:CTD interface—one protomer is buckled while the other remains straight, resembling the same conformation previously observed in the symmetric closed ”clamp” state. This buckling happens precisely where conserved binding sites have been mapped for client proteins. This suggests that if conformational changes were to occur due to ATP hydrolysis, subsequent rearrangements of the asymmetric MD:CTD interfaces back to the previously observed symmetric closed state

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    could be used to drive client protein remodeling. Using a combination of biophysical methods (crystallography, Double Electron-Electron Resonance (DEER), and FRET), we observed that TRAP1 hydrolyzes the 2 bound ATPs sequentially. The buckled protomer hydrolyzes the first ATP, which is then followed by a flip in the asymmetry (the buckled conformation becomes straight and vice versa, on each side), which primes the second ATP for hydrolysis by a buckled protomer. In this model, the MD:CTD interface is guaranteed to undergo remodeling with each ATP hydrolysis and would make efficient use of energy from ATP. The implications for this asymmetric ATP hydrolysis mechanism may also be relevant to other Hsp90s. While we have not observed any other asymmetric Hsp90 structures by itself, several functional Hsp90 complexes seen so far seem to have asymmetric composition/arrangements of their components. In the second chapter, we explore how TRAP1 ATPase activity can be modulated by different divalent cations as co-factors. Despite having two ATP binding sites, the ATPase activity of most Hsp90 homologs appears to be non-cooperative (each site behaves independently from one another). However, we saw that ATPase activity of TRAP1 can be cooperative in presence of calcium, and the activity in presence of magnesium appears to be bi-phasic, with higher activity at low ATP concentrations. This unique behavior of TRAP1 may yet be another adaptation of the Hsp90 machine that has evolved within the mitochondrial matrix environment. Using crystallography, we also discovered that calcium binds to the NTD of TRAP1 unlike previously observed chaperone/calcium/ATP complexes. While the exact biological role for this phenomena is not yet clear, these findings provide a clear molecular basis for the regulation of TRAP1 by calcium. Taken together, the work described in this thesis provide insights into the mechanism of ATP hydrolysis by Hsp90, and a potential role that TRAP1 plays in calcium/magnesium-regulated mitochondrial physiology.

      Cover page: Asymmetric mechanism of molecular chaperone Hsp90
      • Article
      • Peer Reviewed
      UC San Francisco Previously Published Works (2014)
      While structural symmetry is a prevailing feature of homo-oligomeric proteins, asymmetry provides unique mechanistic opportunities. We present the crystal structure of full-length TRAP1, the mitochondrial Hsp90 molecular chaperone, in a catalytically active closed state. The TRAP1 homodimer adopts a distinct, asymmetric conformation, where one protomer is reconfigured via a helix swap at the middle:C-terminal domain (MD:CTD) interface. This interface plays a critical role in client binding. Solution methods validate the asymmetry and show extension to Hsp90 homologs. Point mutations that disrupt unique contacts at each MD:CTD interface reduce catalytic activity and substrate binding and demonstrate that each protomer needs access to both conformations. Crystallographic data on a dimeric NTD:MD fragment suggests that asymmetry arises from strain induced by simultaneous NTD and CTD dimerization. The observed asymmetry provides the potential for an additional step in the ATPase cycle, allowing sequential ATP hydrolysis steps to drive both client remodeling and client release.
        Cover page: Structural Asymmetry in the Closed State of Mitochondrial Hsp90 (TRAP1) Supports a Two-Step ATP Hydrolysis Mechanism
        • Article
        • Peer Reviewed
        UC San Francisco Previously Published Works (2014)
        Hsp90 is a conserved chaperone that facilitates protein homeostasis. Our crystal structure of the mitochondrial Hsp90, TRAP1, revealed an extension of the N-terminal β-strand previously shown to cross between protomers in the closed state. In this study, we address the regulatory function of this extension or 'strap' and demonstrate its responsibility for an unusual temperature dependence in ATPase rates. This dependence is a consequence of a thermally sensitive kinetic barrier between the apo 'open' and ATP-bound 'closed' conformations. The strap stabilizes the closed state through trans-protomer interactions. Displacement of cis-protomer contacts from the apo state is rate-limiting for closure and ATP hydrolysis. Strap release is coupled to rotation of the N-terminal domain and dynamics of the nucleotide binding pocket lid. The strap is conserved in higher eukaryotes but absent from yeast and prokaryotes suggesting its role as a thermal and kinetic regulator, adapting Hsp90s to the demands of unique cellular and organismal environments.
          Cover page: A novel N-terminal extension in mitochondrial TRAP1 serves as a thermal regulator of chaperone activityCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC San Francisco Previously Published Works (2017)
          Colloidal aggregates of small molecules are the most common artifact in early drug discovery, sequestering and inhibiting target proteins without specificity. Understanding their structure and mechanism has been crucial to developing tools to control for, and occasionally even exploit, these particles. Unfortunately, their polydispersity and transient stability have prevented exploration of certain elementary properties, such as how they pack. Dye-stabilized colloidal aggregates exhibit enhanced homogeneity and stability when compared to conventional colloidal aggregates, enabling investigation of some of these properties. By small-angle X-ray scattering and multiangle light scattering, pair distance distribution functions suggest that the dye-stabilized colloids are filled, not hollow, spheres. Stability of the coformulated colloids enabled investigation of their preference for binding DNA, peptides, or folded proteins, and their ability to purify one from the other. The coformulated colloids showed little ability to bind DNA. Correspondingly, the colloids preferentially sequestered protein from even a 1600-fold excess of peptides that are themselves the result of a digest of the same protein. This may reflect the avidity advantage that a protein has in a surface-to-surface interaction with the colloids. For the first time, colloids could be shown to have preferences of up to 90-fold for particular proteins over others. Loaded onto the colloids, bound enzyme could be spun down, resuspended, and released back into buffer, regaining most of its activity. Implications of these observations for colloid mechanisms and utility will be considered.
            Cover page: Internal Structure and Preferential Protein Binding of Colloidal Aggregates
            • Article
            • Peer Reviewed
            UC Davis Previously Published Works (2022)
            SignificanceEssential for sexual reproduction, meiosis is a specialized cell division required for the production of haploid gametes. Critical to this process are the pairing, recombination, and segregation of homologous chromosomes (homologs). While pairing and recombination are linked, it is not known how many linkages are sufficient to hold homologs in proximity. Here, we reveal that random diffusion and the placement of a small number of linkages are sufficient to establish the apparent "pairing" of homologs. We also show that colocalization between any two loci is more dynamic than anticipated. Our study provides observations of live interchromosomal dynamics during meiosis and illustrates the power of combining single-cell measurements with theoretical polymer modeling.
              • Article
              • Peer Reviewed
              UC San Francisco Previously Published Works (2017)
              A strategy for acquiring structural information from sparsely isotopically labeled large proteins is illustrated with an application to the E. coli heat-shock protein, HtpG (high temperature protein G), a 145 kDa dimer. It uses 13C-alanine methyl labeling in a perdeuterated background to take advantage of the sensitivity and resolution of Methyl-TROSY spectra, as well as the backbone-centered structural information from 1H-13C residual dipolar couplings (RDCs) of alanine methyl groups. In all, 40 of the 47 expected crosspeaks were resolved and 36 gave RDC data. Assignments of crosspeaks were partially achieved by transferring assignments from those made on individual domains using triple resonance methods. However, these were incomplete and in many cases the transfer was ambiguous. A genetic algorithm search for consistency between predictions based on domain structures and measurements for chemical shifts and RDCs allowed 60% of the 40 resolved crosspeaks to be assigned with confidence. Chemical shift changes of these crosspeaks on adding an ATP analog to the apo-protein are shown to be consistent with structural changes expected on comparing previous crystal structures for apo- and complex- structures. RDCs collected on the assigned alanine methyl peaks are used to generate a new solution model for the apo-protein structure.
                Cover page: NMR characterization of HtpG, the E. coli Hsp90, using sparse labeling with 13C-methyl alanine
                • Article
                • Peer Reviewed
                UC San Francisco Previously Published Works (2013)
                Four-dimensional fluorescence microscopy--which records 3D image information as a function of time--provides an unbiased way of tracking dynamic behavior of subcellular components in living samples and capturing key events in complex macromolecular processes. Unfortunately, the combination of phototoxicity and photobleaching can severely limit the density or duration of sampling, thereby limiting the biological information that can be obtained. Although widefield microscopy provides a very light-efficient way of imaging, obtaining high-quality reconstructions requires deconvolution to remove optical aberrations. Unfortunately, most deconvolution methods perform very poorly at low signal-to-noise ratios, thereby requiring moderate photon doses to obtain acceptable resolution. We present a unique deconvolution method that combines an entropy-based regularization function with kernels that can exploit general spatial characteristics of the fluorescence image to push the required dose to extreme low levels, resulting in an enabling technology for high-resolution in vivo biological imaging.
                  Cover page: High-resolution restoration of 3D structures from widefield images with extreme low signal-to-noise-ratio
                  • Article
                  • Peer Reviewed
                  UC San Francisco Previously Published Works (2017)
                  Gene silencing by heterochromatin is proposed to occur in part as a result of the ability of heterochromatin protein 1 (HP1) proteins to spread across large regions of the genome, compact the underlying chromatin and recruit diverse ligands. Here we identify a new property of the human HP1α protein: the ability to form phase-separated droplets. While unmodified HP1α is soluble, either phosphorylation of its N-terminal extension or DNA binding promotes the formation of phase-separated droplets. Phosphorylation-driven phase separation can be promoted or reversed by specific HP1α ligands. Known components of heterochromatin such as nucleosomes and DNA preferentially partition into the HP1α droplets, but molecules such as the transcription factor TFIIB show no preference. Using a single-molecule DNA curtain assay, we find that both unmodified and phosphorylated HP1α induce rapid compaction of DNA strands into puncta, although with different characteristics. We show by direct protein delivery into mammalian cells that an HP1α mutant incapable of phase separation in vitro forms smaller and fewer nuclear puncta than phosphorylated HP1α. These findings suggest that heterochromatin-mediated gene silencing may occur in part through sequestration of compacted chromatin in phase-separated HP1 droplets, which are dissolved or formed by specific ligands on the basis of nuclear context.
                    Cover page: Liquid droplet formation by HP1α suggests a role for phase separation in heterochromatin
                    • Article
                    • Peer Reviewed
                    UC Davis Previously Published Works (2021)
                    The pairing of homologous chromosomes (homologs) in meiosis is essential for distributing the correct numbers of chromosomes into haploid gametes. In budding yeast, pairing depends on the formation of 150-200 Spo11-mediated double-strand breaks (DSBs) that are distributed among 16 homolog pairs, but it is not known if all, or only a subset of these DSBs, contribute to the close juxtaposition of homologs. Having established a system to measure the position of fluorescently tagged chromosomal loci in 3D space over time, we analyzed locus trajectories to determine how frequently, and how long, loci spend colocalized or apart. Continuous imaging revealed highly heterogeneous cell-to-cell behavior of foci, with the majority of cells exhibiting a “mixed” phenotype where foci move into and out of proximity, even at late stages of prophase, suggesting that the axial structures of the synaptonemal complex may be more dynamic than anticipated. The observed plateaus of the mean-squared change in distance (MSCD) between foci informed the development of a biophysical model of two diffusing polymers that captures the loss of centromere linkages as cells enter meiosis, nuclear confinement, and the formation of Spo11-dependent linkages. The predicted number of linkages per chromosome in our theoretical model closely approximates the small number (~2-4) of estimated synapsis-initiation sites, suggesting that excess DSBs have negligible effects on the overall juxtaposition of homologs. These insights into the dynamic in-terchromosomal behavior displayed during homolog pairing demonstrate the power of combining time-resolved in vivo analysis with modeling at the granular level.

                    Significance Statement

                    Essential for sexual reproduction, meiosis is a specialized cell division required for the production of haploid gametes. Critical to this process is the pairing, recombination, and segregation of homologous chromosomes (homologs). While pairing and recombination are linked, it is not known how many linkages are sufficient to hold homologs in proximity. Here, we reveal that random diffusion and the placement of a small number of linkages are sufficient to establish the apparent “pairing” of homologs. We also show that colocalization between any two loci is more dynamic than anticipated. Our study is the first to provide observations of live interchromosomal dynamics during meiosis and illustrates the power of combining single-cell measurements with theoretical polymer modeling.
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