Ordabayev et al. developed an open-source analysis software for colocalization single-molecule fluorescence experiments

Tapqir analysis

Yerdos Ordabayev et al. in the Department of Biochemistry use Bayesian probabilistic programming to implement computer software “Tapqir” for analysis of colocalization single-molecule spectroscopy (CoSMoS) image data. CoSMoS is a tool widely used in vitro to study the biochemical and physical mechanisms of the protein and nucleic acid macromolecular “machines” that perform essential biological functions. In this method, formation and/or dissociation of molecular complexes is observed by single-molecule fluorescence microscopy as the colocalization of binder and target macromolecules each labeled with a different color of fluorescent dye. Despite the use of the method for over twenty years, reliable analysis of CoSMoS data remains a significant challenge to the effective and more widespread use of the technique.

This work describes a holistic causal probabilistic model of CoSMoS image data formation. This model is physics-based and includes realistic shot noise in fluorescent spots, camera noise, the size and shape of spots, and the presence of both specific and nonspecific binder molecules in the images. Most importantly, instead of yielding a binary spot-/no-spot determination, the algorithm calculates the probability of a colocalization event. Unlike alternative approaches, Tapqir does not require subjective threshold settings of parameters so they can be used effectively and accurately by non-expert analysts. The program is implemented in the state-of-the-art Python-based probabilistic programming language Pyro (open-sourced by Uber AI Labs in 2017), which enables efficient use of graphics processing unit (GPU)-based hardware for rapid parallel processing of data and facilitates future modifications to the model. Tapqir is free, open-source software. We envision that Tapqir program is likely to be adopted by researchers who use single-molecule colocalization methods to study a wide range of different biological systems.

Yerdos A Ordabayev, Larry J Friedman, Jeff Gelles, Douglas L Theobald. Bayesian machine learning analysis of single-molecule fluorescence colocalization images. eLife 2022;11:e73860.
Publication Date: March 23, 2022.

Goode, Gelles and Kondev labs synergize in discovery of a new synergistic actin depolymerization mechanism

Shashank Shekhar, Jane Kondev, Jeff Gelles and Bruce Goode

Shashank Shekhar, Jane Kondev, Jeff Gelles and Bruce Goode

All animal and plant cells contain a highly elaborate system of filamentous protein polymers called the actin cytoskeleton, a scaffold that can be rapidly transformed to alter a cell’s shape and function. A critical step in reconfiguring this scaffold is the rapid disassembly (or turnover) of the actin filaments. But how is this achieved? It has long been known that the protein Cofilin plays a central role in this process, but it has been unclear how Cofilin achieves this feat. Cofilin can sever actin filaments into smaller fragments to promote their disassembly, but whether it also catalyzes subunit dissociation from filament ends has remained uncertain and controversial. Until now, this problem has been difficult to address because of limitations in directly observing Cofilin’s biochemical effects at filament ends. However, a new study published in Nature Communications led by postdoctoral associate Dr. Shashank Shekhar, jointly mentored by Bruce Goode, Jeff Gelles and Jane Kondev, uses microfluidics-assisted single molecule TIRF imaging to tackle the problem.

The new study shows that Cofilin and one other protein (Srv2/CAP) intimately collaborate at one end of the actin filament to accelerate subunit dissociation by over 300-fold! These are the fastest rates of actin depolymerization ever observed. Further, these results establish a new paradigm in which a protein that decorates filament sides (Cofilin) works in concert with a protein that binds to filament ends (Srv2/CAP) to produce an activity that is orders of magnitude stronger than the that of either protein alone.

Video of cofilin and Srv2/CAP collaborating

The work was funded by National Institutes of Health, National Science Foundation MRSEC and Simons Foundation grant.

Jeff Gelles elected to American Academy of Arts and Sciences

Jeff Gelles, 2019 AAAS recipient

credit: Heratch Ekmekjian

Jeff Gelles, the Aron and Imre Tauber Professor of Biochemistry and Molecular Pharmacology, has been elected to the American Academy of Arts and Sciences. He was among the  more than 200 outstanding individuals that were elected to the Academy in 2019 and announced on April 17.

The Gelles lab studies “little engines” or the nanometer-sized machines made of protein, RNA, and DNA molecules that carry out the essential processes in living cells.  The lab uses single-molecule light microscopy methods to study the functional mechanisms of these macromolecular complexes in cytoskeletal function, transcription and transcription regulation, and RNA processing.

Founded in 17890, the Academy recognizes the outstanding achievements of individuals in academia, the arts, business, government, and public affairs.

Read more: Amacad.org, BrandeisNow





Jeff Gelles to Receive 2019 BPS Kazuhito Kinosita Award in Single-Molecule Biophysics

Congratulations to Jeff Gelles, Aron and Imre Tauber Professor of Biochemistry and Molecular Pharmacology. He will receive the 2019 Kazuhito Kinosita Award in Single-Molecule Biophysics from the Biophysical Society (BPS). He will be honored at the Society’s 63rd Annual Meeting at the Baltimore Convention Center on March 5, 2019, during the annual Awards Symposium.

The award, named for Professor Kazuhiko Kinosita, seeks to advance cross-disciplinary research and cultivate an appreciation of single-molecule studies. BPS President Angela Gronenborn, University of Pittsburgh, said “Jeff has conducted single-molecule studies at the highest level and continues to spark interests in engaging others in single-molecule studies.” (BPS Press Release)

Dynamics of GreB-RNA polymerase interaction

Larry Tetone, Larry Friedman, and Melissa Osborne, and collaborators from the Gelles lab (Brandeis University) and the Landick lab (University of Wisconsin-Madison) used multi-wavelength single-molecule fluorescence methods to for the first time directly observe the dynamic binding and dissociation of an accessory protein with an RNAP during active transcript elongation.

Their findings are detailed in the recent paper “Dynamics of GreB-RNA polymerase interaction.” (PNAS, published online 1/30/2017).

Read more at The Little Engine Shop blog

Visualizing a protein decision complex in actin filament length control

Seen at the Gelles Lab Little Engine Shop blog this week, commentary on a new paper in Nature Communicationspublished in collaboration with the Goode Lab and researchers from New England Biolabs.

“Single-molecule visualization of a formin-capping protein ‘decision complex’ at the actin filament barbed end”

Regulation of actin filament length is a central process by which eukaryotic cells control the shape, architecture, and dynamics of their actin networks. This regulation plays a fundamental role in cell motility, morphogenesis, and a host of processes specific to particular cell types. This paper by recently graduated [Biophysics and Structural Biology] Ph.D. student Jeffrey Bombardier and collaborators resolves the long-standing mystery of how formins and capping protein work in concert and antagonistically to control actin filament length. Bombardier used the CoSMoS multi-wavelength single-molecule fluorescence microscopy technique to to discover and characterize a novel tripartite complex formed by a formin, capping protein, and the actin filament barbed end. Quantitative analysis of the kinetic mechanism showed that this complex is the essential intermediate and decision point in converting a growing formin-bound filament into a static capping protein-bound filament, and the reverse. Interestingly, the authors show that “mDia1 displaced from the barbed end by CP can randomly slide along the filament and later return to the barbed end to re-form the complex.” The results define the essential features of the molecular mechanism of filament length regulation by formin and capping protein; this mechanism predicts several new ways by which cells are likely to couple upstream regulatory inputs to filament length control.

Single-molecule visualization of a formin-capping protein ‘decision complex’ at the actin filament barbed end
Jeffrey P. Bombardier, Julian A. Eskin, Richa Jaiswal, Ivan R. Corrêa, Jr., Ming-Qun Xu, Bruce L. Goode, and Jeff Gelles
Nature Communications  6:8707 (2015)

The capping protein expression plasmid described in this article is available from Addgene.

Readers interested in this subject should also see a related article by Shekhar et al published simultaneously in the same journal.  We are grateful to the authors of that article for coordinating submission so that the two articles were published together.

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