Bio

Honors & Awards


  • Stanford/SJSU IRACDA Postdoctoral Fellowship, NIH (2013-present)

Professional Education


  • PhD, University of California, Berkeley, Molecular & Cell Biology (2013)
  • BA, Pomona College, Molecular Biology (2007)

Stanford Advisors


Research & Scholarship

Lab Affiliations


Publications

Journal Articles


  • Hijacking Multivesicular Bodies Enables Long-Term and Exosome-Mediated Long-Distance Action of Anthrax Toxin CELL REPORTS Abrami, L., Brandi, L., Moayeri, M., Brown, M. J., Krantz, B. A., Leppla, S. H., van der Goot, F. G. 2013; 5 (4): 986-996

    Abstract

    Anthrax lethal toxin is a classical AB toxin comprised of two components: protective antigen (PA) and lethal factor (LF). Here, we show that following assembly and endocytosis, PA forms a channel that translocates LF, not only into the cytosol, but also into the lumen of endosomal intraluminal vesicles (ILVs). These ILVs can fuse and release LF into the cytosol, where LF can proteolyze and disable host targets. We find that LF can persist in ILVs for days, fully sheltered from proteolytic degradation, both in vitro and in vivo. During this time, ILV-localized LF can be transmitted to daughter cells upon cell division. In addition, LF-containing ILVs can be delivered to the extracellular medium as exosomes. These can deliver LF to the cytosol of naive cells in a manner that is independent of the typical anthrax toxin receptor-mediated trafficking pathway, while being sheltered from neutralizing extracellular factors of the immune system.

    View details for DOI 10.1016/j.celrep.2013.10.019

    View details for Web of Science ID 000328266000014

    View details for PubMedID 24239351

  • Electrostatic Ratchet in the Protective Antigen Channel Promotes Anthrax Toxin Translocation JOURNAL OF BIOLOGICAL CHEMISTRY Wynia-Smith, S. L., Brown, M. J., Chirichella, G., Kemalyan, G., Krantz, B. A. 2012; 287 (52): 43753-43764

    Abstract

    Central to the power-stroke and brownian-ratchet mechanisms of protein translocation is the process through which nonequilibrium fluctuations are rectified or ratcheted by the molecular motor to transport substrate proteins along a specific axis. We investigated the ratchet mechanism using anthrax toxin as a model. Anthrax toxin is a tripartite toxin comprised of the protective antigen (PA) component, a homooligomeric transmembrane translocase, which translocates two other enzyme components, lethal factor (LF) and edema factor (EF), into the cytosol of the host cell under the proton motive force (PMF). The PA-binding domains of LF and EF (LF(N) and EF(N)) possess identical folds and similar solution stabilities; however, EF(N) translocates ∼10-200-fold slower than LF(N), depending on the electrical potential (Δψ) and chemical potential (ΔpH) compositions of the PMF. From an analysis of LF(N)/EF(N) chimera proteins, we identified two 10-residue cassettes comprised of charged sequence that were responsible for the impaired translocation kinetics of EF(N). These cassettes have nonspecific electrostatic requirements: one surprisingly prefers acidic residues when driven by either a Δψ or a ΔpH; the second requires basic residues only when driven by a Δψ. Through modeling and experiment, we identified a charged surface in the PA channel responsible for charge selectivity. The charged surface latches the substrate and promotes PMF-driven transport. We propose an electrostatic ratchet in the channel, comprised of opposing rings of charged residues, enforces directionality by interacting with charged cassettes in the substrate, thereby generating forces sufficient to drive unfolding.

    View details for DOI 10.1074/jbc.M112.419598

    View details for Web of Science ID 000312940800054

    View details for PubMedID 23115233

  • Anthrax toxin protective antigen integrates poly-gamma-D-glutamate and pH signals to sense the optimal environment for channel formation PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Kintzer, A. F., Tang, I. I., Schawel, A. K., Brown, M. J., Krantz, B. A. 2012; 109 (45): 18378-18383

    Abstract

    Many toxins assemble into oligomers on the surface of cells. Local chemical cues signal and trigger critical rearrangements of the oligomer, inducing the formation of a membrane-fused or channel state. Bacillus anthracis secretes two virulence factors: a tripartite toxin and a poly-γ-d-glutamic acid capsule (γ-DPGA). The toxin's channel-forming component, protective antigen (PA), oligomerizes to create a prechannel that forms toxic complexes upon binding the two other enzyme components, lethal factor (LF) and edema factor (EF). Following endocytosis into host cells, acidic pH signals the prechannel to form the channel state, which translocates LF and EF into the host cytosol. We report γ-DPGA binds to PA, LF, and EF, exhibiting nanomolar avidity for the PA prechannel oligomer. We show PA channel formation requires the pH-dependent disruption of the intra-PA domain-2-domain-4 (D2-D4) interface. γ-DPGA stabilizes the D2-D4 interface, preventing channel formation both in model membranes and cultured mammalian cells. A 1.9-Å resolution X-ray crystal structure of a D2-D4-interface mutant and corresponding functional studies reveal how stability at the intra-PA interface governs channel formation. We also pinpoint the kinetic pH trigger for channel formation to a residue within PA's membrane-insertion loop at the inter-PA D2-D4 interface. Thus, γ-DPGA may function as a chemical cue, signaling that the local environment is appropriate for toxin assembly but inappropriate for channel formation.

    View details for DOI 10.1073/pnas.1208280109

    View details for Web of Science ID 000311156700039

    View details for PubMedID 23100533

  • Ratcheting up protein translocation with anthrax toxin PROTEIN SCIENCE Feld, G. K., Brown, M. J., Krantz, B. A. 2012; 21 (5): 606-624

    Abstract

    Energy-consuming nanomachines catalyze the directed movement of biopolymers in the cell. They are found both dissolved in the aqueous cytosol as well as embedded in lipid bilayers. Inquiries into the molecular mechanism of nanomachine-catalyzed biopolymer transport have revealed that these machines are equipped with molecular parts, including adjustable clamps, levers, and adaptors, which interact favorably with substrate polypeptides. Biological nanomachines that catalyze protein transport, known as translocases, often require that their substrate proteins unfold before translocation. An unstructured protein chain is likely entropically challenging to bind, push, or pull in a directional manner, especially in a way that produces an unfolding force. A number of ingenious solutions to this problem are now evident in the anthrax toxin system, a model used to study protein translocation. Here we highlight molecular ratchets and current research on anthrax toxin translocation. A picture is emerging of proton-gradient-driven anthrax toxin translocation, and its associated ratchet mechanism likely applies broadly to other systems. We suggest a cyclical thermodynamic order-to-disorder mechanism (akin to a heat-engine cycle) is central to underlying protein translocation: peptide substrates nonspecifically bind to molecular clamps, which possess adjustable affinities; polypeptide substrates compress into helical structures; these clamps undergo proton-gated switching; and the substrate subsequently expands regaining its unfolded state conformational entropy upon translocation.

    View details for DOI 10.1002/pro.2052

    View details for Web of Science ID 000302620600002

    View details for PubMedID 22374876

  • Charge Requirements for Proton Gradient-driven Translocation of Anthrax Toxin JOURNAL OF BIOLOGICAL CHEMISTRY Brown, M. J., Thoren, K. L., Krantz, B. A. 2011; 286 (26): 23189-23199

    Abstract

    Anthrax lethal toxin is used as a model system to study protein translocation. The toxin is composed of a translocase channel, called protective antigen (PA), and an enzyme, called lethal factor (LF). A proton gradient (ΔpH) can drive LF unfolding and translocation through PA channels; however, the mechanism of ΔpH-mediated force generation, substrate unfolding, and establishment of directionality are poorly understood. One recent hypothesis suggests that the ΔpH may act through changes in the protonation state of residues in the substrate. Here we report the charge requirements of LF's amino-terminal binding domain (LF(N)) using planar lipid bilayer electrophysiology. We found that acidic residues are required in LF(N) to utilize a proton gradient for translocation. Constructs lacking negative charges in the unstructured presequence of LF(N) translocate independently of the ΔpH driving force. Acidic residues markedly increase the rate of ΔpH-driven translocation, and the presequence is optimized in its natural acidic residue content for efficient ΔpH-driven unfolding and translocation. We discuss a ΔpH-driven charge state Brownian ratchet mechanism for translocation, where glutamic and aspartic acid residues in the substrate are the "molecular teeth" of the ratchet. Our Brownian ratchet model includes a mechanism for unfolding and a novel role for positive charges, which we propose chaperone negative charges through the PA channel during ΔpH translocation.

    View details for DOI 10.1074/jbc.M111.231167

    View details for Web of Science ID 000292025000053

    View details for PubMedID 21507946

  • Homing endonuclease I-CreI derivatives with novel DNA target specificities NUCLEIC ACIDS RESEARCH Rosen, L. E., Morrison, H. A., Masri, S., Brown, M. J., Springstubb, B., Sussman, D., Stoddard, B. L., Seligman, L. M. 2006; 34 (17): 4791-4800

    Abstract

    Homing endonucleases are highly specific enzymes, capable of recognizing and cleaving unique DNA sequences in complex genomes. Since such DNA cleavage events can result in targeted allele-inactivation and/or allele-replacement in vivo, the ability to engineer homing endonucleases matched to specific DNA sequences of interest would enable powerful and precise genome manipulations. We have taken a step-wise genetic approach in analyzing individual homing endonuclease I-CreI protein/DNA contacts, and describe here novel interactions at four distinct target site positions. Crystal structures of two mutant endonucleases reveal the molecular interactions responsible for their altered DNA target specificities. We also combine novel contacts to create an endonuclease with the predicted target specificity. These studies provide important insights into engineering homing endonucleases with novel target specificities, as well as into the evolution of DNA recognition by this fascinating family of proteins.

    View details for DOI 10.1093/nar/gkl645

    View details for Web of Science ID 000241427300019

    View details for PubMedID 16971456

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