BJ Articles Ahead of Print

In cell membranes, the functional constituents, such as peptides, proteins, and polysaccharides, diffuse in a sea of lipids as single molecules and molecular aggregates. Thus, the fluidity of the heterogeneous multicomponent membrane is important to understand the roles of the membrane in cell functionality. Recently, Henle and Levine described the hydrodynamics of molecular diffusion in a spherical membrane. A tangential point force at the north pole induces a pair of vortices whose centers lie on a line perpendicular to the point force and are symmetrical with respect to the point force.

Metastasis of mesenchymal tumor cells is traditionally considered as a single cell process. Here we report an emergent collective phenomenon where the dissemination rate of mesenchymal breast cancer cells from 3D tumors depends on the tumor geometry. Combining experimental measurements and computational modeling we demonstrate that the collective dynamics is coordinated by the mechanical feedback between individual cells and their surrounding extracellular matrix (ECM). We find the tissue-like fibrous ECM supports long-range physical interactions between cells, which turn geometric cues into regulated cell dissemination dynamics.

Transcription factor (TF) proteins bind to DNA to regulate gene expression. Normally, accessibility to DNA is required for their function. However, in the nucleus the DNA is often inaccessible, wrapped around histone proteins in nucleosomes forming the chromatin. Pioneer TFs are thought to induce chromatin opening by recognizing their DNA binding sites on nucleosomes. For example, Oct4, a master regulator and inducer of stem cell pluripotency, binds to DNA in nucleosomes in a sequence specific manner.

The thermal unfolding of a recombinant monoclonal antibody IgG1 (mAb) was measured with differential scanning calorimetry (DSC). The DSC thermograms reveal a pre-transition at 72 °C with an unfolding enthalpy of ΔHcal ∼ 200-300 kcal/mol and a main transition at 85 °C with an enthalpy of ∼900 - 1000 kcal/mol. In contrast to small single-domain proteins, mAb unfolding is a complex reaction that is analysed with the multi-state Zimm-Bragg theory. For the investigated mAb, unfolding is characterised by a cooperativity parameter σ ∼6x10−5 and a Gibbs free energy of unfolding of gnu ∼100 cal/mol per amino acid.

Intercellular bridges are plasma continuities formed at the end of the cytokinesis process that facilitate intercellular mass transport between the two daughter cells. However, it remains largely unknown how the intercellular bridge mediates Ca2+ communication between post-mitotic cells. In the present work, we utilize BV-2 microglial cells planted on dumbbell-shaped micropatterned assemblies to resolve spatiotemporal characteristics of Ca2+ signal transfer over the intercellular bridges. With use of such micropatterns considerably longer and more regular intercellular bridges can be obtained than in conventional cell cultures.

Mechanical processes are involved at many stages of the development of living cells, and often external forces applied to a biomolecule result in its unfolding. Although our knowledge of the unfolding mechanisms and the magnitude of the forces involved has evolved, the role that water molecules play in the mechanical unfolding of biomolecules has not yet been fully elucidated. To this end, we investigated with steered molecular dynamics simulations the mechanical unfolding of dystrophin’s spectrin repeat 1, and related the changes in the protein’s structure to the ordering of the surrounding water molecules.

We consider a model lipid plasma membrane, one that describes the outer leaf as consisting of sphingomyelin, phosphatidylcholine, and cholesterol, and the inner leaf of phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine, and cholesterol. Their relative compositions are taken from experiment; the cholesterol freely interchanges between leaves. Fluctuations in local composition are coupled to fluctuations in the local membrane curvature, as in the Leibler-Andelman mechanism. Structure factors of components in both leaves display a peak at non-zero wavevector.

Highly charged, single α-helical (SAH) domains contain a high percentage of Arg, Lys, and Glu residues. Their dynamic salt bridge pairing creates exceptional stiffness of these helical rods, with a persistence length of more than 200 Å for the myosin VI SAH domain. With the aim of modulating the stiffness of the helical structure, we investigated by NMR spectroscopy the effect of substituting key charged Arg, Lys, Glu and Asp residues by Gly or His. Results indicate that such mutations result in transient breaking of the helix at the site of mutation, but with noticeable impact on amide hydrogen exchange rates extending as far as ±2 helical turns, pointing to a substantial degree of cooperativity in SAH helical stability.

Human glucokinase (GCK) is the prototypic example of an emerging class of proteins with allosteric-like behavior that originates from intrinsic polypeptide dynamics. High-resolution NMR investigations of GCK have elucidated millisecond timescale dynamics underlying allostery. In contrast, faster motions have remained underexplored, hindering the development of a comprehensive model of cooperativity. Here, we map nanosecond timescale dynamics and structural heterogeneity in GCK using a combination of unnatural amino acid incorporation, time-resolved fluorescence and 19F nuclear magnetic resonance spectroscopy.

(Biophysical Journal 116, 179–183; January 22, 2019)

In a contracting muscle, myosin cross-bridges extending from thick filaments pull the interdigitating thin (actin-containing) filaments, during cyclical ATP-driven interactions, toward the center of the sarcomere, the structural unit of striated muscle. Cross-bridge attachments in the sarcomere have been reported to exhibit a similar stiffness under both positive and negative forces. However, in vitro measurements on filaments with a sparse complement of heads detected a decrease of the cross-bridge stiffness at negative forces attributed to the buckling of the subfragment 2 tail portion.