Media highlights in the January issues of Biophysical Journal

Bethesda, MD – The January 1st and January 15th issues of Biophysical Journal, published by the Biophysical Society, is now available online. Topics of interest include ciliates, force, spasmoneme, H2A/H2B, nucleosome, AFM, aquaporin, elasticity, and epithelial cells.

Volume 94, Issue 1, January 1, 2008

Power-limited contraction dynamics of Vorticella convallaria: an ultra-fast biological spring
Arpita Upadhyaya, Massachusetts Institute of Technology Mara Baraban, Massachusetts Institute of Technology Jacqueline Wong, Massachusetts Institute of Technology Paul T. Matsudaira, Whitehead Institute, MIT Alexander van Oudenaarden, Massachusetts Institute of Technology L.. Mahadevan, Harvard University
Keywords: ciliates; force; spasmoneme; velocity; viscosity

Speed is not characteristic of the microscopic movements common in biology where even fast cells typically move only about a body length every second. However, speed is what singles out the ultra-fast motion of a single-celled organism, Vorticella, as the stalk by which it is attached to a substrate contracts by coiling into a helix. The cell body is but a few microns in size, and yet can move at velocities of a few cm/s, i.e. it travels thousands of body lengths in a second. Our work quantifies the dynamics of this unusual spring-like engine and shows that this rapid movement is ultimately limited by the maximum power available when the stalk contracts, just as in a simple mechanical spring that is compressed and then suddenly released. However, unlike in a mechanical spring which stores and releases mechanical energy passively, in the contracting stalk of Vorticella, there is a dynamical conversion of one form of energy (electrostatic) into another (conformational) which process sets the ultimate internal physical limits on the performance of this mechano-chemical engine.

Sequence-Dependent Variations Associated with H2A/H2B Depletion of Nucleosomes

Laimonas Kelbauskas, The Biodesign Institute, Arizona State University
Nam Chan, Department of Chemistry and Biochemistry, Arizona State University Ralph Bash, The Biodesign Institute, Arizona State University Peter DeBartolo, Department of Chemistry and Biochemistry, Arizona State University Jenny Sun, The Biodesign Institute, Arizona State University Neal Woodbury, The Biodesign Institute, Arizona State University Dennis Lohr, Department of Chemistry and Biochemistry, Arizona State University
Keywords: 5S; FCS; FRET; H2A/H2B; Nucleosome; Sequence-dependent

Gaining access to regulatory DNA sequences buried in nucleosomes is a major impediment to the action of eukaryotic regulatory factors and to the operation of genomic processes. An understanding of how regulators find their target sites and allow transcriptional machinery to access DNA is essential for a fundamental understanding of gene regulation as well as for drug design and new treatments for gene-related diseases. H2A-H2B are released from nucleosomes during the action of several transcription-associated factors; such release enhances the exposure of the remaining histone-bound DNA. We have found that the effect of H2A-H2B release on nucleosomes varies with their DNA sequence; significant, sequence-dependent variations in DNA accessibility between a non-promoter and two promoter nucleosomes were noted in the H2A-H2B depleted complexes. The ability of the DNA sequence itself to contribute significantly to nucleosome stability and DNA accessibility provides a mechanism for differential recognition of specific nucleosomes, such as those on key promoter elements. Significant sequence-dependence nucleosome stability is not widely-recognized but is potentially of fundamental importance to gene expression.

Volume 94, Issue 2, January 15, 2008

Translocation of Aquaporin-Containing Vesicles to the Plasma Membrane is Facilitated by Actomyosin Relaxation
Christoph P Riethmuller, University of Munster Hans Oberleithner, University of Munster Marianne Wilhelmi, University of Munster Jonas Franz, University of Munster Eberhard Schlatter, University of Munster Jens Klokkers, University of Munster Bayram Edemir, University of Munster
Keywords: AFM; Aquaporin; Elasticity; Epithelial cells; Force Mapping; Kidney

The study adds a biomechanical perspective to the knowledge of vesicle transport regulation. Shuttling vesicle-embedded proteins from cytosol to the plasma membrane is an essential process for living cells. Biochemists have discovered a high number of proteins that contribute to this intricate regulation mostly by monitoring their phosphorylation status. Likewise numerous biophysical studies have explored the mechanics of single motor proteins of the myosin, dynein and kinesin families involved.

Here, a biomechanical approach is taken, where in living cells the tension of the cytoskeletal network is monitored by atomic force microscopy. This network spans all cells and thereby constitutes a supramolecular (mesoscopic) stiffness of the cell body.

The idea of the authors was that vesicles, driven by motor proteins along a fiber, face a resistance of crossing fibers, which varies with the tension of this network. The experiments performed on rat kidney cells favor this view. The vesicle transport is accelerated when the network fibers relax. In simple words, a physically relaxed cell is doing much better in vesicle transport compared to a cell under tension. This finding indicates that intracellular vesicle trafficking has a strong biomechanical component.

Abstracts for these articles can be found at http://www.biophysj.org/. Please contact the authors before publishing a story based on their work. In addition, please refer to the Biophysical Journal in any articles you write. Contact Ellen Weiss at eweiss@biophysics.org for more information or assistance.