1
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Grupp B, Denkhaus L, Gerhardt S, Vögele M, Johnsson N, Gronemeyer T. The structure of a tetrameric septin complex reveals a hydrophobic element essential for NC-interface integrity. Commun Biol 2024; 7:48. [PMID: 38184752 PMCID: PMC10771490 DOI: 10.1038/s42003-023-05734-w] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2023] [Accepted: 12/20/2023] [Indexed: 01/08/2024] Open
Abstract
The septins of the yeast Saccharomyces cerevisiae assemble into hetero-octameric rods by alternating interactions between neighboring G-domains or N- and C-termini, respectively. These rods polymerize end to end into apolar filaments, forming a ring beneath the prospective new bud that expands during the cell cycle into an hourglass structure. The hourglass finally splits during cytokinesis into a double ring. Understanding these transitions as well as the plasticity of the higher order assemblies requires a detailed knowledge of the underlying structures. Here we present the first X-ray crystal structure of a tetrameric Shs1-Cdc12-Cdc3-Cdc10 complex at a resolution of 3.2 Å. Close inspection of the NC-interfaces of this and other septin structures reveals a conserved contact motif that is essential for NC-interface integrity of yeast and human septins in vivo and in vitro. Using the tetrameric structure in combination with AlphaFold-Multimer allowed us to propose a model of the octameric septin rod.
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Affiliation(s)
- Benjamin Grupp
- Institute of Molecular Genetics and Cell Biology, Ulm University, Ulm, Germany
| | - Lukas Denkhaus
- Institute of Biochemistry, Albert-Ludwigs University, Freiburg, Germany
| | - Stefan Gerhardt
- Institute of Biochemistry, Albert-Ludwigs University, Freiburg, Germany
| | - Matthis Vögele
- Institute of Molecular Genetics and Cell Biology, Ulm University, Ulm, Germany
| | - Nils Johnsson
- Institute of Molecular Genetics and Cell Biology, Ulm University, Ulm, Germany
| | - Thomas Gronemeyer
- Institute of Molecular Genetics and Cell Biology, Ulm University, Ulm, Germany.
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2
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Hussain A, Nguyen VT, Reigan P, McMurray M. Evolutionary degeneration of septins into pseudoGTPases: impacts on a hetero-oligomeric assembly interface. Front Cell Dev Biol 2023; 11:1296657. [PMID: 38125875 PMCID: PMC10731463 DOI: 10.3389/fcell.2023.1296657] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/18/2023] [Accepted: 11/17/2023] [Indexed: 12/23/2023] Open
Abstract
The septin family of eukaryotic proteins comprises distinct classes of sequence-related monomers that associate in a defined order into linear hetero-oligomers, which are capable of polymerizing into cytoskeletal filaments. Like actin and ⍺ and β tubulin, most septin monomers require binding of a nucleotide at a monomer-monomer interface (the septin "G" interface) for assembly into higher-order structures. Like ⍺ and β tubulin, where GTP is bound by both subunits but only the GTP at the ⍺-β interface is subject to hydrolysis, the capacity of certain septin monomers to hydrolyze their bound GTP has been lost during evolution. Thus, within septin hetero-oligomers and filaments, certain monomers remain permanently GTP-bound. Unlike tubulins, loss of septin GTPase activity-creating septin "pseudoGTPases"-occurred multiple times in independent evolutionary trajectories, accompanied in some cases by non-conservative substitutions in highly conserved residues in the nucleotide-binding pocket. Here, we used recent septin crystal structures, AlphaFold-generated models, phylogenetics and in silico nucleotide docking to investigate how in some organisms the septin G interface evolved to accommodate changes in nucleotide occupancy. Our analysis suggests that yeast septin monomers expressed only during meiosis and sporulation, when GTP is scarce, are evolving rapidly and might not bind GTP or GDP. Moreover, the G dimerization partners of these sporulation-specific septins appear to carry compensatory changes in residues that form contacts at the G interface to help retain stability despite the absence of bound GDP or GTP in the facing subunit. During septin evolution in nematodes, apparent loss of GTPase activity was also accompanied by changes in predicted G interface contacts. Overall, our observations support the conclusion that the primary function of nucleotide binding and hydrolysis by septins is to ensure formation of G interfaces that impose the proper subunit-subunit order within the hetero-oligomer.
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Affiliation(s)
- Alya Hussain
- Program in Structural Biology and Biochemistry, Department of Cell and Developmental Biology, University of Colorado Anschutz Medical Campus, Aurora, CO, United States
| | - Vu T. Nguyen
- Department of Pharmaceutical Sciences, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of Colorado Anschutz Medical Campus, Aurora, CO, United States
| | - Philip Reigan
- Department of Pharmaceutical Sciences, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of Colorado Anschutz Medical Campus, Aurora, CO, United States
| | - Michael McMurray
- Program in Structural Biology and Biochemistry, Department of Cell and Developmental Biology, University of Colorado Anschutz Medical Campus, Aurora, CO, United States
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3
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Nakazawa K, Chauvin B, Mangenot S, Bertin A. Reconstituted in vitro systems to reveal the roles and functions of septins. J Cell Sci 2023; 136:jcs259448. [PMID: 37815088 DOI: 10.1242/jcs.259448] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/11/2023] Open
Abstract
Septins are essential cytoskeletal proteins involved in key cellular processes and have also been implicated in diseases from cancers to neurodegenerative pathologies. However, they have not been as thoroughly studied as other cytoskeletal proteins. In vivo, septins interact with other cytoskeletal proteins and with the inner plasma membrane. Hence, bottom-up in vitro cell-free assays are well suited to dissect the roles and behavior of septins in a controlled environment. Specifically, in vitro studies have been invaluable in describing the self-assembly of septins into a large diversity of ultrastructures. Given that septins interact specifically with membrane, the details of these septin-membrane interactions have been analyzed using reconstituted lipid systems. In particular, at a membrane, septins are often localized at curvatures of micrometer scale. In that context, in vitro assays have been performed with substrates of varying curvatures (spheres, cylinders or undulated substrates) to probe the sensitivity of septins to membrane curvature. This Review will first present the structural properties of septins in solution and describe the interplay of septins with cytoskeletal partners. We will then discuss how septins interact with biomimetic membranes and induce their reshaping. Finally, we will highlight the curvature sensitivity of septins and how they alter the mechanical properties of membranes.
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Affiliation(s)
- Koyomi Nakazawa
- Physico Chimie Curie , Institut Curie, CNRS UMR 168, Sorbonne Université, 11 Rue Pierre et Paris Curie, 75005 Paris, France
| | - Brieuc Chauvin
- Physico Chimie Curie , Institut Curie, CNRS UMR 168, Sorbonne Université, 11 Rue Pierre et Paris Curie, 75005 Paris, France
| | - Stéphanie Mangenot
- Laboratoire Matière et Systèmes Complexes , Université de Paris Cité, CNRS UMR 7057, 45 Rue des Saint Pères, 75006 Paris, France
| | - Aurélie Bertin
- Physico Chimie Curie , Institut Curie, CNRS UMR 168, Sorbonne Université, 11 Rue Pierre et Paris Curie, 75005 Paris, France
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4
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Grupp B, Lemkul JA, Gronemeyer T. An in silico approach to determine inter-subunit affinities in human septin complexes. Cytoskeleton (Hoboken) 2023; 80:141-152. [PMID: 36843207 DOI: 10.1002/cm.21749] [Citation(s) in RCA: 5] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2022] [Revised: 02/21/2023] [Accepted: 02/22/2023] [Indexed: 02/28/2023]
Abstract
The septins are a conserved family of filament-forming guanine nucleotide binding proteins, often named the fourth component of the cytoskeleton. Correctly assembled septin structures are required for essential intracellular processes such as cytokinesis, vesicular transport, polarity establishment, and cellular adhesion. Structurally, septins belong to the P-Loop NTPases but they do not mediate signals to effectors through GTP binding and hydrolysis. GTP binding and hydrolysis are believed to contribute to septin complex integrity, but biochemical approaches addressing this topic are hampered by the stability of septin complexes after recombinant expression and the lack of nucleotide-depleted complexes. To overcome this limitation, we used a molecular dynamics-based approach to determine inter-subunit binding free energies in available human septin dimer structures and in their apo forms, which we generated in silico. The nucleotide in the GTPase active subunits SEPT2 and SEPT7, but not in SEPT6, was identified as a stabilizing element in the G interface. Removal of GDP from SEPT2 and SEPT7 results in flipping of a conserved Arg residue and disruption of an extensive hydrogen bond network in the septin unique element, concomitant with a decreased inter-subunit affinity. Based on these findings we propose a singular "lock-hydrolysis" mechanism stabilizing human septin filaments.
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Affiliation(s)
- Benjamin Grupp
- Institute of Molecular Genetics and Cell Biology, James Franck Ring N27, Ulm University, Ulm, Germany
| | - Justin A Lemkul
- Department of Biochemistry, Virginia Tech, Blacksburg, Virginia, USA
| | - Thomas Gronemeyer
- Institute of Molecular Genetics and Cell Biology, James Franck Ring N27, Ulm University, Ulm, Germany
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5
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Grupp B, Gronemeyer T. A biochemical view on the septins, a less known component of the cytoskeleton. Biol Chem 2023; 404:1-13. [PMID: 36423333 DOI: 10.1515/hsz-2022-0263] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/25/2022] [Accepted: 10/30/2022] [Indexed: 11/25/2022]
Abstract
The septins are a conserved family of guanine nucleotide binding proteins, often named the fourth component of the cytoskeleton. They self-assemble into non-polar filaments and further into higher ordered structures. Properly assembled septin structures are required for a wide range of indispensable intracellular processes such as cytokinesis, vesicular transport, polarity establishment and cellular adhesion. Septins belong structurally to the P-Loop NTPases. However, unlike the small GTPases like Ras, septins do not mediate signals to effectors through GTP binding and hydrolysis. The role of nucleotide binding and subsequent GTP hydrolysis by the septins is rather controversially debated. We compile here the structural features from the existing septin crystal- and cryo-EM structures regarding protofilament formation, inter-subunit interface architecture and nucleotide binding and hydrolysis. These findings are supplemented with a summary of available biochemical studies providing information regarding nucleotide binding and hydrolysis of fungal and mammalian septins.
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Affiliation(s)
- Benjamin Grupp
- Institute of Molecular Genetics and Cell Biology, Ulm University, James Franck Ring N27, 89081 Ulm, Germany
| | - Thomas Gronemeyer
- Institute of Molecular Genetics and Cell Biology, Ulm University, James Franck Ring N27, 89081 Ulm, Germany
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6
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Hassell D, Denney A, Singer E, Benson A, Roth A, Ceglowski J, Steingesser M, McMurray M. Chaperone Requirements for De Novo Folding of Saccharomyces cerevisiae Septins. Mol Biol Cell 2022; 33:ar111. [PMID: 35947497 DOI: 10.1091/mbc.e22-07-0262] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
Polymers of septin protein complexes play cytoskeletal roles in eukaryotic cells. The specific subunit composition within complexes controls functions and higher-order structural properties. All septins have globular GTPase domains. The other eukaryotic cytoskeletal NTPases strictly require assistance from molecular chaperones of the cytosol, particularly the cage-like chaperonins, to fold into oligomerization-competent conformations. We previously identified cytosolic chaperones that bind septins and influence the oligomerization ability of septins carrying mutations linked to human disease, but it was unknown to what extent wild-type septins require chaperone assistance for their native folding. Here we use a combination of in vivo and in vitro approaches to demonstrate chaperone requirements for de novo folding and complex assembly by budding yeast septins. Individually purified septins adopted non-native conformations and formed non-native homodimers. In chaperonin- or Hsp70-deficient cells, septins folded slower and were unable to assemble post-translationally into native complexes. One septin, Cdc12, was so dependent on co-translational chaperonin assistance that translation failed without it. Our findings point to distinct translation elongation rates for different septins as a possible mechanism to direct a stepwise, co-translational assembly pathway in which general cytosolic chaperones act as key intermediaries.
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Affiliation(s)
- Daniel Hassell
- University of Colorado Anschutz Medical Campus, Aurora, CO 80045
| | - Ashley Denney
- University of Colorado Anschutz Medical Campus, Aurora, CO 80045
| | - Emily Singer
- University of Colorado Anschutz Medical Campus, Aurora, CO 80045
| | - Aleyna Benson
- University of Colorado Anschutz Medical Campus, Aurora, CO 80045
| | - Andrew Roth
- University of Colorado Anschutz Medical Campus, Aurora, CO 80045
| | - Julia Ceglowski
- University of Colorado Anschutz Medical Campus, Aurora, CO 80045
| | - Marc Steingesser
- University of Colorado Anschutz Medical Campus, Aurora, CO 80045
| | - Michael McMurray
- University of Colorado Anschutz Medical Campus, Aurora, CO 80045
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7
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Fischer M, Frank D, Rösler R, Johnsson N, Gronemeyer T. Biochemical Characterization of a Human Septin Octamer. Front Cell Dev Biol 2022; 10:771388. [PMID: 35309913 PMCID: PMC8928218 DOI: 10.3389/fcell.2022.771388] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2021] [Accepted: 02/14/2022] [Indexed: 11/25/2022] Open
Abstract
Septins are part of the cytoskeleton and polymerize into non-polar filaments of heteromeric hexamers or octamers. They belong to the class of P-loop GTPases but the roles of GTP binding and hydrolysis on filament formation and dynamics are not well understood. The basic human septin building block is the septin rod, a hetero-octamer composed of SEPT2, SEPT6, SEPT7, and SEPT9 with a stoichiometry of 2:2:2:2 (2-6-7-9-9-7-6-2). Septin rods polymerize by end-to-end and lateral joining into linear filaments and higher ordered structures such as rings, sheets, and gauzes. We purified a recombinant human septin octamer from E. coli for in vitro experimentation that is able to polymerize into filaments. We could show that the C-terminal region of the central SEPT9 subunit contributes to filament formation and that the human septin rod decreases the rate of in vitro actin polymerization. We provide further first kinetic data on the nucleotide uptake- and exchange properties of human hexameric and octameric septin rods. We could show that nucleotide uptake prior to hydrolysis is a dynamic process and that a bound nucleotide is exchangeable. However, the hydrolyzed γ-phosphate is not released from the native protein complex. We consequently propose that GTP hydrolysis in human septins does not follow the typical mechanism known from other small GTPases.
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Affiliation(s)
- Martin Fischer
- Institute of Molecular Genetics and Cell Biology, Ulm University, Ulm, Germany
| | - Dominik Frank
- Institute of Molecular Genetics and Cell Biology, Ulm University, Ulm, Germany
| | - Reinhild Rösler
- Core Unit Mass Spectrometry and Proteomics, Ulm University, Ulm, Germany
| | - Nils Johnsson
- Institute of Molecular Genetics and Cell Biology, Ulm University, Ulm, Germany
| | - Thomas Gronemeyer
- Institute of Molecular Genetics and Cell Biology, Ulm University, Ulm, Germany
- *Correspondence: Thomas Gronemeyer,
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8
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Synergistic role of nucleotides and lipids for the self-assembly of Shs1 septin oligomers. Biochem J 2021; 477:2697-2714. [PMID: 32726433 DOI: 10.1042/bcj20200199] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/09/2020] [Revised: 07/07/2020] [Accepted: 07/09/2020] [Indexed: 12/25/2022]
Abstract
Budding yeast septins are essential for cell division and polarity. Septins assemble as palindromic linear octameric complexes. The function and ultra-structural organization of septins are finely governed by their molecular polymorphism. In particular, in budding yeast, the end subunit can stand either as Shs1 or Cdc11. We have dissected, here, for the first time, the behavior of the Shs1 protomer bound to membranes at nanometer resolution, in complex with the other septins. Using electron microscopy, we have shown that on membranes, Shs1 protomers self-assemble into rings, bundles, filaments or two-dimensional gauzes. Using a set of specific mutants we have demonstrated a synergistic role of both nucleotides and lipids for the organization and oligomerization of budding yeast septins. Besides, cryo-electron tomography assays show that vesicles are deformed by the interaction between Shs1 oligomers and lipids. The Shs1-Shs1 interface is stabilized by the presence of phosphoinositides, allowing the visualization of micrometric long filaments formed by Shs1 protomers. In addition, molecular modeling experiments have revealed a potential molecular mechanism regarding the selectivity of septin subunits for phosphoinositide lipids.
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9
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Takagi J, Cho C, Duvalyan A, Yan Y, Halloran M, Hanson-Smith V, Thorner J, Finnigan GC. Reconstructed evolutionary history of the yeast septins Cdc11 and Shs1. G3-GENES GENOMES GENETICS 2021; 11:6025175. [PMID: 33561226 PMCID: PMC7849910 DOI: 10.1093/g3journal/jkaa006] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 08/11/2020] [Accepted: 11/13/2020] [Indexed: 11/21/2022]
Abstract
Septins are GTP-binding proteins conserved across metazoans. They can polymerize into extended filaments and, hence, are considered a component of the cytoskeleton. The number of individual septins varies across the tree of life—yeast (Saccharomyces cerevisiae) has seven distinct subunits, a nematode (Caenorhabditis elegans) has two, and humans have 13. However, the overall geometric unit (an apolar hetero-octameric protomer and filaments assembled there from) has been conserved. To understand septin evolutionary variation, we focused on a related pair of yeast subunits (Cdc11 and Shs1) that appear to have arisen from gene duplication within the fungal clade. Either Cdc11 or Shs1 occupies the terminal position within a hetero-octamer, yet Cdc11 is essential for septin function and cell viability, whereas Shs1 is not. To discern the molecular basis of this divergence, we utilized ancestral gene reconstruction to predict, synthesize, and experimentally examine the most recent common ancestor (“Anc.11-S”) of Cdc11 and Shs1. Anc.11-S was able to occupy the terminal position within an octamer, just like the modern subunits. Although Anc.11-S supplied many of the known functions of Cdc11, it was unable to replace the distinct function(s) of Shs1. To further evaluate the history of Shs1, additional intermediates along a proposed trajectory from Anc.11-S to yeast Shs1 were generated and tested. We demonstrate that multiple events contributed to the current properties of Shs1: (1) loss of Shs1–Shs1 self-association early after duplication, (2) co-evolution of heterotypic Cdc11–Shs1 interaction between neighboring hetero-octamers, and (3) eventual repurposing and acquisition of novel function(s) for its C-terminal extension domain. Thus, a pair of duplicated proteins, despite constraints imposed by assembly into a highly conserved multi-subunit structure, could evolve new functionality via a complex evolutionary pathway.
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Affiliation(s)
- Julie Takagi
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720-3202, USA
| | - Christina Cho
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720-3202, USA
| | - Angela Duvalyan
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720-3202, USA
| | - Yao Yan
- Department of Biochemistry and Molecular Biophysics, Kansas State University, Manhattan, KS 66506, USA
| | - Megan Halloran
- Department of Biochemistry and Molecular Biophysics, Kansas State University, Manhattan, KS 66506, USA
| | - Victor Hanson-Smith
- Department of Microbiology and Immunology, University of California, San Francisco, CA 94158, USA
| | - Jeremy Thorner
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720-3202, USA
| | - Gregory C Finnigan
- Department of Biochemistry and Molecular Biophysics, Kansas State University, Manhattan, KS 66506, USA
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10
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Jiao F, Cannon KS, Lin YC, Gladfelter AS, Scheuring S. The hierarchical assembly of septins revealed by high-speed AFM. Nat Commun 2020; 11:5062. [PMID: 33033254 PMCID: PMC7545167 DOI: 10.1038/s41467-020-18778-x] [Citation(s) in RCA: 18] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/24/2020] [Accepted: 09/15/2020] [Indexed: 12/15/2022] Open
Abstract
Septins are GTP-binding proteins involved in diverse cellular processes including division and membrane remodeling. Septins form linear, palindromic heteromeric complexes that can assemble in filaments and higher-order structures. Structural studies revealed various septin architectures, but questions concerning assembly-dynamics and -pathways persist. Here we used high-speed atomic force microscopy (HS-AFM) and kinetic modeling which allowed us to determine that septin filament assembly was a diffusion-driven process, while formation of higher-order structures was complex and involved self-templating. Slightly acidic pH and increased monovalent ion concentrations favor filament-assembly, -alignment and -pairing. Filament-alignment and -pairing further favored diffusion-driven assembly. Pairing is mediated by the septin N-termini face, and may occur symmetrically or staggered, likely important for the formation of higher-order structures of different shapes. Multilayered structures are templated by the morphology of the underlying layers. The septin C-termini face, namely the C-terminal extension of Cdc12, may be involved in membrane binding.
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Affiliation(s)
- Fang Jiao
- Department of Anesthesiology, Weill Cornell Medicine, New York, NY, 10065, USA
- Department of Physiology and Biophysics, Weill Cornell Medicine, New York, NY, 10065, USA
| | - Kevin S Cannon
- Department of Biology, University of North Carolina and Chapel Hill, Chapel Hill, NC, 27599, USA
| | - Yi-Chih Lin
- Department of Anesthesiology, Weill Cornell Medicine, New York, NY, 10065, USA
- Department of Physiology and Biophysics, Weill Cornell Medicine, New York, NY, 10065, USA
| | - Amy S Gladfelter
- Department of Biology, University of North Carolina and Chapel Hill, Chapel Hill, NC, 27599, USA
- Marine Biological Laboratory, Woods Hole, MA, 02543, USA
| | - Simon Scheuring
- Department of Anesthesiology, Weill Cornell Medicine, New York, NY, 10065, USA.
- Department of Physiology and Biophysics, Weill Cornell Medicine, New York, NY, 10065, USA.
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11
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Farkašovský M. Septin architecture and function in budding yeast. Biol Chem 2020; 401:903-919. [PMID: 31913844 DOI: 10.1515/hsz-2019-0401] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2019] [Accepted: 12/28/2019] [Indexed: 01/22/2023]
Abstract
The septins constitute a conserved family of guanosine phosphate-binding and filament-forming proteins widespread across eukaryotic species. Septins appear to have two principal functions. One is to form a cortical diffusion barrier, like the septin collar at the bud neck of Saccharomyces cerevisiae, which prevents movement of membrane-associated proteins between the mother and daughter cells. The second is to serve as a polymeric scaffold for recruiting the proteins required for critical cellular processes to particular subcellular areas. In the last decade, structural information about the different levels of septin organization has appeared, but crucial structural determinants and factors responsible for septin assembly remain largely unknown. This review highlights recent findings on the architecture and function of septins and their remodeling with an emphasis on mitotically dividing budding yeasts.
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Affiliation(s)
- Marian Farkašovský
- Department of Biochemistry and Protein Structure, Institute of Molecular Biology SAS, Dubravska cesta 21, 84551 Bratislava, Slovak Republic
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12
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Chollet J, Dünkler A, Bäuerle A, Vivero-Pol L, Mulaw MA, Gronemeyer T, Johnsson N. Cdc24 interacts with septins to create a positive feedback loop during bud site assembly in yeast. J Cell Sci 2020; 133:jcs240283. [PMID: 32327559 DOI: 10.1242/jcs.240283] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/09/2019] [Accepted: 04/08/2020] [Indexed: 01/04/2023] Open
Abstract
Yeast cells select the position of their new bud at the beginning of each cell cycle. The recruitment of septins to this prospective bud site is one of the critical events in a complex assembly pathway that culminates in the outgrowth of a new daughter cell. During recruitment, septin rods follow the high concentration of Cdc42GTP that is generated by the focused localization of the Cdc42 guanine-nucleotide-exchange factor Cdc24. We show that, shortly before budding, Cdc24 not only activates Cdc42 but also transiently interacts with Cdc11, the septin subunit that caps both ends of the septin rods. Mutations in Cdc24 that reduce affinity to Cdc11 impair septin recruitment and decrease the stability of the polarity patch. The interaction between septins and Cdc24 thus reinforces bud assembly at sites where septin structures are formed. Once the septins polymerize to form the septin ring, Cdc24 is found at the cortex of the bud and directs further outgrowth from this position.
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Affiliation(s)
- Julian Chollet
- Institute of Molecular Genetics and Cell Biology, Department of Biology, Ulm University, James-Franck-Ring N27, D-89081 Ulm, Germany
| | - Alexander Dünkler
- Institute of Molecular Genetics and Cell Biology, Department of Biology, Ulm University, James-Franck-Ring N27, D-89081 Ulm, Germany
| | - Anne Bäuerle
- Institute of Molecular Genetics and Cell Biology, Department of Biology, Ulm University, James-Franck-Ring N27, D-89081 Ulm, Germany
| | - Laura Vivero-Pol
- Institute of Molecular Genetics and Cell Biology, Department of Biology, Ulm University, James-Franck-Ring N27, D-89081 Ulm, Germany
| | - Medhanie A Mulaw
- Comprehensive Cancer Center Ulm, Institute of Experimental Cancer Research, Ulm University, James-Franck-Ring N27, D-89081 Ulm, Germany
| | - Thomas Gronemeyer
- Institute of Molecular Genetics and Cell Biology, Department of Biology, Ulm University, James-Franck-Ring N27, D-89081 Ulm, Germany
| | - Nils Johnsson
- Institute of Molecular Genetics and Cell Biology, Department of Biology, Ulm University, James-Franck-Ring N27, D-89081 Ulm, Germany
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13
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Auxier B, Dee J, Berbee ML, Momany M. Diversity of opisthokont septin proteins reveals structural constraints and conserved motifs. BMC Evol Biol 2019; 19:4. [PMID: 30616529 PMCID: PMC6323724 DOI: 10.1186/s12862-018-1297-8] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2018] [Accepted: 11/19/2018] [Indexed: 01/22/2023] Open
Abstract
BACKGROUND Septins are cytoskeletal proteins important in cell division and in establishing and maintaining cell polarity. Although septins are found in various eukaryotes, septin genes had the richest history of duplication and diversification in the animals, fungi and protists that comprise opisthokonts. Opisthokont septin paralogs encode modular proteins that assemble into heteropolymeric higher order structures. The heteropolymers can create physical barriers to diffusion or serve as scaffolds organizing other morphogenetic proteins. How the paralogous septin modules interact to form heteropolymers is still unclear. Through comparative analyses, we hoped to clarify the evolutionary origin of septin diversity and to suggest which amino acid residues were responsible for subunit binding specificity. RESULTS Here we take advantage of newly sequenced genomes to reconcile septin gene trees with a species phylogeny from 22 animals, fungi and protists. Our phylogenetic analysis divided 120 septins representing the 22 taxa into seven clades (Groups) of paralogs. Suggesting that septin genes duplicated early in opisthokont evolution, animal and fungal lineages share septin Groups 1A, 4 and possibly also 1B and 2. Group 5 septins were present in fungi but not in animals and whether they were present in the opisthokont ancestor was unclear. Protein homology folding showed that previously identified conserved septin motifs were all located near interface regions between the adjacent septin monomers. We found specific interface residues associated with each septin Group that are candidates for providing subunit binding specificity. CONCLUSIONS This work reveals that duplication of septin genes began in an ancestral opisthokont more than a billion years ago and continued through the diversification of animals and fungi. Evidence for evolutionary conservation of ~ 49 interface residues will inform mutagenesis experiments and lead to improved understanding of the rules guiding septin heteropolymer formation and from there, to improved understanding of development of form in animals and fungi.
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Affiliation(s)
- Benjamin Auxier
- Department of Botany, University of British Columbia, Vancouver, Canada
- current address: Laboratory of Genetics, Wageningen University and Research, P.O. Box 16, 6700AA, Wageningen, The Netherlands
| | - Jaclyn Dee
- Department of Botany, University of British Columbia, Vancouver, Canada
| | - Mary L. Berbee
- Department of Botany, University of British Columbia, Vancouver, Canada
| | - Michelle Momany
- Fungal Biology Group and Plant Biology Department, University of Georgia, Athens, USA
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14
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Mela A, Momany M. Septin mutations and phenotypes in S. cerevisiae. Cytoskeleton (Hoboken) 2018; 76:33-44. [PMID: 30171672 DOI: 10.1002/cm.21492] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/02/2018] [Revised: 08/09/2018] [Accepted: 08/22/2018] [Indexed: 11/07/2022]
Abstract
Septins are highly conserved guanosine triphosphate (GTP)-binding proteins that are a component of the cytoskeletal systems of virtually all eukaryotes (except higher plants). Septins play important roles in a multitude of cellular processes, including cytokinesis, establishment of cell polarity, and cellular partitioning. The ease of genetic screens and a fully sequenced genome have made Saccharomyces cerevisiae one of the most extensively studied and well-annotated model organisms in eukaryotic biology. Here, we present a synopsis of the known point mutations in the seven S. cerevisiae septin genes: CDC3, CDC10, CDC11, CDC12, SHS1, SPR3, and SPR28. We map these mutations onto septin protein structures, highlighting important conserved motifs, and relating the functional consequences of mutations in each domain.
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Affiliation(s)
- Alexander Mela
- Department of Plant Biology, University of Georgia, Athens, GA 30602
| | - Michelle Momany
- Department of Plant Biology, University of Georgia, Athens, GA 30602
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15
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Baur JD, Rösler R, Wiese S, Johnsson N, Gronemeyer T. Dissecting the nucleotide binding properties of the septins from S. cerevisiae. Cytoskeleton (Hoboken) 2018; 76:45-54. [PMID: 30091242 DOI: 10.1002/cm.21484] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2018] [Revised: 07/24/2018] [Accepted: 07/30/2018] [Indexed: 11/11/2022]
Abstract
Septins are a conserved family of guanosine triphosphate (GTP)-binding proteins that assemble into an ordered array of filaments at the mother bud neck in Saccharomyces cerevisiae cells. They are present in all higher eukaryotes except plants. Septins belong structurally to the P-Loop nucleoside triphosphatase (NTPases) like Rab and Ras. However, unlike other small guanosine triphosphatase (GTPases) septins are supposed to act as scaffolds rather than signalling mediators. This is why they are considered as the fourth class of cytoskeletal proteins. It is assumed that septins fulfil their functions independently of the bound nucleotide. The role of guanosine diphosphosphate (GDP) and GTP binding and subsequent hydrolysis was controversial debated in the last couple of years. Lack of crystal structures of yeast septin subunits or rods and difficulties to isolate single monomeric septin subunits often hindered the correlation of results obtained from in vivo studies with biochemical data. Recently, nucleotide binding and hydrolysis was connected to the formation of septin rods from its subunits. However, the evidence was only indirectly obtained through the use of septin mutants in the context of intact cells. We provide here mechanistic insight into the nucleotide binding of the yeast septins by in vitro assays using purified septin rods and building blocks, thereby adding further insights to the already available models on septin filament formation.
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Affiliation(s)
- Julian David Baur
- Institute of Molecular Genetics and Cell Biology, Ulm University, Ulm, Germany
| | - Reinhild Rösler
- Core Unit Mass Spectrometry and Proteomics, Ulm University, Ulm, Germany
| | - Sebastian Wiese
- Core Unit Mass Spectrometry and Proteomics, Ulm University, Ulm, Germany
| | - Nils Johnsson
- Institute of Molecular Genetics and Cell Biology, Ulm University, Ulm, Germany
| | - Thomas Gronemeyer
- Institute of Molecular Genetics and Cell Biology, Ulm University, Ulm, Germany
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16
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Abbey M, Gaestel M, Menon MB. Septins: Active GTPases or just GTP-binding proteins? Cytoskeleton (Hoboken) 2018; 76:55-62. [PMID: 29747238 DOI: 10.1002/cm.21451] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2018] [Revised: 04/24/2018] [Accepted: 05/05/2018] [Indexed: 01/19/2023]
Abstract
Septins are conserved cytoskeletal proteins with unique filament forming capabilities and roles in cytokinesis and cell morphogenesis. Septins undergo hetero-oligomerization and assemble into higher order structures including filaments, rings, and cages. Hetero- and homotypic interactions of septin isoforms involve alternating GTPase (G)-domain interfaces and those mediated by N- and C-terminal extensions. While most septins bind GTP, display weak GTP-hydrolysis activity and incorporate guanine nucleotides in their interaction interfaces, studies using GTPase-inactivating mutations have failed to conclusively establish a crucial role for GTPase activity in mediating septin functions. In this mini-review, we will critically assess the role of GTP-binding and -hydrolysis on septin assembly and function. The relevance of G-domain activity will also be discussed in the context of human septin mutations as well as the development of specific small-molecules targeting septin polymerization. As structural determinants of septin oligomer interfaces, G-domains are attractive targets for ligand-based inhibition of septin assembly. Whether such an intervention can predictably alter septin function is a major question for future research.
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Affiliation(s)
- Megha Abbey
- Hannover Medical School, Institute of Cell Biochemistry, Hannover, 30625, Germany
| | - Matthias Gaestel
- Hannover Medical School, Institute of Cell Biochemistry, Hannover, 30625, Germany
| | - Manoj B Menon
- Hannover Medical School, Institute of Cell Biochemistry, Hannover, 30625, Germany
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17
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Marquardt J, Chen X, Bi E. Architecture, remodeling, and functions of the septin cytoskeleton. Cytoskeleton (Hoboken) 2018; 76:7-14. [PMID: 29979831 DOI: 10.1002/cm.21475] [Citation(s) in RCA: 43] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2018] [Revised: 06/05/2018] [Accepted: 06/22/2018] [Indexed: 01/22/2023]
Abstract
The septin family of proteins has fascinated cell biologists for decades due to the elaborate architecture they adopt in different eukaryotic cells. Whether they exist as rings, collars, or gauzes in different cell types and at different times in the cell cycle illustrates a complex series of regulation in structure. While the organization of different septin structures at the cortex of different cell types during the cell cycle has been described to various degrees, the exact structure and regulation at the filament level are still largely unknown. Recent advances in fluorescent and electron microscopy, as well as work in septin biochemistry, have allowed new insights into the aspects of septin architecture, remodeling, and function in many cell types. This mini-review highlights many of the recent findings with an emphasis on the budding yeast model.
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Affiliation(s)
- Joseph Marquardt
- Department of Cell and Developmental Biology, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, Pennsylvania
| | - Xi Chen
- Department of Cell and Developmental Biology, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, Pennsylvania
| | - Erfei Bi
- Department of Cell and Developmental Biology, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, Pennsylvania
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18
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Valadares NF, d' Muniz Pereira H, Ulian Araujo AP, Garratt RC. Septin structure and filament assembly. Biophys Rev 2017; 9:481-500. [PMID: 28905266 DOI: 10.1007/s12551-017-0320-4] [Citation(s) in RCA: 59] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2017] [Accepted: 08/17/2017] [Indexed: 12/13/2022] Open
Abstract
Septins are able to polymerize into long apolar filaments and have long been considered to be a component of the cytoskeleton alongside intermediate filaments (which are also apolar in nature), microtubules and actin filaments (which are not). Their central guanosine triphosphate (GTP)-binding domain, which is essential for stabilizing the filament itself, is flanked by N- and C-terminal domains for which no direct structural information is yet available. In most cases, physiological filaments are built from a number of different septin monomers, and in the case of mammalian septins this is most commonly either three or four. Comprehending the structural basis for the spontaneous assembly of such filaments requires a deeper understanding of the interfaces between individual GTP-binding domains than is currently available. Nevertheless, in this review we will summarize the considerable progress which has been made over the course of the last 10 years. We will provide a brief description of each structure determined to date and comment on how it has added to the body of knowledge which is rapidly growing. Rather than simply repeat data which have already been described in the literature, as far as is possible we will try to take advantage of the full set of information now available (mostly derived from human septins) and draw the reader's attention to some of the details of the structures themselves and the filaments they form which have not be commented on previously. An additional aim is to clarify some misconceptions.
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Affiliation(s)
| | - Humberto d' Muniz Pereira
- Instituto de Física de São Carlos, Universidade de São Paulo, Av. Trabalhador Sancarlense, 400, São Carlos, SP, 13560-590, Brazil
| | - Ana Paula Ulian Araujo
- Instituto de Física de São Carlos, Universidade de São Paulo, Av. Trabalhador Sancarlense, 400, São Carlos, SP, 13560-590, Brazil
| | - Richard Charles Garratt
- Instituto de Física de São Carlos, Universidade de São Paulo, Av. Trabalhador Sancarlense, 400, São Carlos, SP, 13560-590, Brazil.
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19
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Weems A, McMurray M. The step-wise pathway of septin hetero-octamer assembly in budding yeast. eLife 2017; 6. [PMID: 28541184 PMCID: PMC5461111 DOI: 10.7554/elife.23689] [Citation(s) in RCA: 45] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2017] [Accepted: 05/24/2017] [Indexed: 01/22/2023] Open
Abstract
Septin proteins bind guanine nucleotides and form rod-shaped hetero-oligomers. Cells choose from a variety of available septins to assemble distinct hetero-oligomers, but the underlying mechanism was unknown. Using a new in vivo assay, we find that a stepwise assembly pathway produces the two species of budding yeast septin hetero-octamers: Cdc11/Shs1–Cdc12–Cdc3–Cdc10–Cdc10–Cdc3–Cdc12–Cdc11/Shs1. Rapid GTP hydrolysis by monomeric Cdc10 drives assembly of the core Cdc10 homodimer. The extended Cdc3 N terminus autoinhibits Cdc3 association with Cdc10 homodimers until prior Cdc3–Cdc12 interaction. Slow hydrolysis by monomeric Cdc12 and specific affinity of Cdc11 for transient Cdc12•GTP drive assembly of distinct trimers, Cdc11–Cdc12–Cdc3 or Shs1–Cdc12–Cdc3. Decreasing the cytosolic GTP:GDP ratio increases the incorporation of Shs1 vs Cdc11, which alters the curvature of filamentous septin rings. Our findings explain how GTP hydrolysis controls septin assembly, and uncover mechanisms by which cells construct defined septin complexes. DOI:http://dx.doi.org/10.7554/eLife.23689.001
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Affiliation(s)
- Andrew Weems
- Department of Cell and Developmental Biology, University of Colorado Anschutz Medical Campus, Aurora, United States
| | - Michael McMurray
- Department of Cell and Developmental Biology, University of Colorado Anschutz Medical Campus, Aurora, United States
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20
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Booth EA, Sterling SM, Dovala D, Nogales E, Thorner J. Effects of Bni5 Binding on Septin Filament Organization. J Mol Biol 2016; 428:4962-4980. [PMID: 27806918 DOI: 10.1016/j.jmb.2016.10.024] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/10/2016] [Revised: 10/17/2016] [Accepted: 10/24/2016] [Indexed: 12/31/2022]
Abstract
Septins are a protein family found in all eukaryotes (except higher plants) that have roles in membrane remodeling and formation of diffusion barriers and as a scaffold to recruit other proteins. In budding yeast, proper execution of cytokinesis and cell division requires the formation of a collar of circumferential filaments at the bud neck. These filaments are assembled from apolar septin hetero-octamers. Currently, little is known about the mechanisms that control the arrangement and dynamics of septin structures. In this study, we utilized both Förster resonance energy transfer and electron microscopy to analyze the biophysical properties of the septin-binding protein Bni5 and how its association with septin filaments affects their organization. We found that the interaction of Bni5 with the terminal subunit (Cdc11) at the junctions between adjacent hetero-octamers in paired filaments is highly cooperative. Both the C-terminal end of Bni5 and the C-terminal extension of Cdc11 make important contributions to their interaction. Moreover, this binding may stabilize the dimerization of Bni5, which, in turn, forms cross-filament braces that significantly narrow, and impose much more uniform spacing on, the gap between paired filaments.
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Affiliation(s)
- Elizabeth A Booth
- Division of Biochemistry, Biophysics, and Structural Biology, Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720-3202, USA.
| | - Sarah M Sterling
- Division of Biochemistry, Biophysics, and Structural Biology, Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720-3202, USA.
| | - Dustin Dovala
- Program in Microbial Pathogenesis and Host Defense, Department of Microbiology and Immunology, University of California School of Medicine, San Francisco, CA 94143, USA.
| | - Eva Nogales
- Division of Biochemistry, Biophysics, and Structural Biology, Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720-3202, USA; Life Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA; Howard Hughes Medical Institute, Chevy Chase, MD 20815, USA.
| | - Jeremy Thorner
- Division of Biochemistry, Biophysics, and Structural Biology, Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720-3202, USA.
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21
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Glomb O, Gronemeyer T. Septin Organization and Functions in Budding Yeast. Front Cell Dev Biol 2016; 4:123. [PMID: 27857941 PMCID: PMC5093138 DOI: 10.3389/fcell.2016.00123] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2016] [Accepted: 10/19/2016] [Indexed: 12/14/2022] Open
Abstract
The septins are a conserved family of GTP-binding proteins present in all eukaryotic cells except plants. They were originally discovered in the baker's yeast Saccharomyces cerevisiae that serves until today as an important model organism for septin research. In yeast, the septins assemble into a highly ordered array of filaments at the mother bud neck. The septins are regulators of spatial compartmentalization in yeast and act as key players in cytokinesis. This minireview summarizes the recent findings about structural features and cell biology of the yeast septins.
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Affiliation(s)
- Oliver Glomb
- Department of Molecular Genetics and Cell Biology, Ulm University Ulm, Germany
| | - Thomas Gronemeyer
- Department of Molecular Genetics and Cell Biology, Ulm University Ulm, Germany
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22
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Finnigan GC, Duvalyan A, Liao EN, Sargsyan A, Thorner J. Detection of protein-protein interactions at the septin collar in Saccharomyces cerevisiae using a tripartite split-GFP system. Mol Biol Cell 2016; 27:2708-25. [PMID: 27385335 PMCID: PMC5007091 DOI: 10.1091/mbc.e16-05-0337] [Citation(s) in RCA: 35] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2016] [Accepted: 06/30/2016] [Indexed: 01/22/2023] Open
Abstract
A tripartite split-GFP system faithfully reports the order of the subunits in septin hetero-octamers (and thus can serve as a “molecular ruler”), conversely yields little or no false signal even with very highly expressed cytosolic proteins, and detects authentic interactions of other cellular proteins that are bona fide septin-binding proteins. Various methods can provide a readout of the physical interaction between two biomolecules. A recently described tripartite split-GFP system has the potential to report by direct visualization via a fluorescence signal the intimate association of minimally tagged proteins expressed at their endogenous level in their native cellular milieu and can capture transient or weak interactions. Here we document the utility of this tripartite split-GFP system to assess in living cells protein–protein interactions in a dynamic cytoskeletal structure—the septin collar at the yeast bud neck. We show, first, that for septin–septin interactions, this method yields a robust signal whose strength reflects the known spacing between the subunits in septin filaments and thus serves as a “molecular ruler.” Second, the method yields little or no spurious signal even with highly abundant cytosolic proteins readily accessible to the bud neck (including molecular chaperone Hsp82 and glycolytic enzyme Pgk1). Third, using two proteins (Bni5 and Hsl1) that have been shown by other means to bind directly to septins at the bud neck in vivo, we validate that the tripartite split-GFP method yields the same conclusions and further insights about specificity. Finally, we demonstrate the capacity of this approach to uncover additional new information by examining whether three other proteins reported to localize to the bud neck (Nis1, Bud4, and Hof1) are able to interact physically with any of the subunits in the septin collar and, if so, with which ones.
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Affiliation(s)
- Gregory C Finnigan
- Division of Biochemistry, Biophysics and Structural Biology, Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720-3202
| | - Angela Duvalyan
- Division of Biochemistry, Biophysics and Structural Biology, Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720-3202
| | - Elizabeth N Liao
- Division of Biochemistry, Biophysics and Structural Biology, Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720-3202
| | - Aspram Sargsyan
- Division of Biochemistry, Biophysics and Structural Biology, Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720-3202
| | - Jeremy Thorner
- Division of Biochemistry, Biophysics and Structural Biology, Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720-3202
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