Synchronization of elastically coupled processive molecular motors and regulation of cargo transport

Thermodynamic Efficiency of Somatic Exocytosis of Serotonin

Through somatic exocytosis neurons liberate immense amounts of transmitter molecules that modulate the functioning of the nervous system. A stream of action potentials triggers an ATP-dependent transport of transmitter-containing vesicles to the plasma membrane, that ends with a large-scale exocytosis. It is commonly assumed that biological processes use metabolic energy with a high thermodynamic efficiency, meaning that most energy generates work with minor dissipation. However, the intricate ultrastructure underlying the pathway for the vesicle flow necessary for somatic exocytosis challenges this possibility. To study this problem here we first applied thermodynamic theory to quantify the efficiency of somatic exocytosis of the vital transmitter serotonin. Then we correlated the efficiency to the ultrastructure of the transport pathway of the vesicles. Exocytosis was evoked in cultured Retzius neurons of the leech by trains of 10 impulses delivered at 20 Hz. The kinetics of exocytosis was quantified from the gradual fluorescence increase of FM1-43 dye as it became incorporated into vesicles that underwent their exo-endocytosis cycle. By fitting a model of the vesicle transport carried by motor forces to the kinetics of exocytosis, we calculated the thermodynamic efficiency of the ATP expenses per vesicle, as the power of the transport divided by total energy ideally produced by the hydrolysis of ATP during the process. The efficiency was remarkably low (0.1-6.4%) and the values formed a W-shape distribution with the transport distances of the vesicles. Electron micrographs and fluorescent staining of the actin cortex indicated that the slopes of the W chart could be explained by the interaction of vesicles with the actin cortex and the calcium-releasing endoplasmic reticulum. We showed that the application of thermodynamic theory permitted to predict aspects of the intracellular structure. Our results suggest that the distribution of subcellular structures that are essential for somatic exocytosis abates the thermodynamic efficiency of the transport by hampering vesicle mobilization. It is remarkable that the modulation of the nervous system occurs at the expenses of an efficient use of metabolic energy.

The Role of Supercoiling in the Motor Activity of RNA Polymerases

RNA polymerase (RNAP) is, in its elongation phase, an emblematic example of a molecular motor whose activity is highly sensitive to DNA supercoiling. After a review of DNA supercoiling basic features, we discuss how supercoiling controls polymerase velocity, while being itself modified by polymerase activity. This coupling is supported by single-molecule measurements. Physical modeling allows us to describe quantitatively how supercoiling and torsional constraints mediate a mechanical coupling between adjacent polymerases. On this basis, we obtain a description that may explain the existence and functioning of RNAP convoys.

Simultaneous nano-tracking of multiple motor proteins via spectral discrimination of quantum dots

Simultaneous nanometric tracking of multiple motor proteins was achieved by combining multicolor fluorescent labeling of target proteins and imaging spectroscopy, revealing dynamic behaviors of multiple motor proteins at the sub-diffraction-limit scale. Using quantum dot probes of distinct colors, we experimentally verified the localization precision to be a few nanometers at temporal resolution of 30 ms or faster. One-dimensional processive movement of two heads of a single myosin molecule and multiple myosin molecules was successfully traced. Furthermore, the system was modified for two-dimensional measurement and applied to tracking of multiple myosin molecules. Our approach is useful for investigating cooperative movement of proteins in supramolecular nanomachinery.

Surfing along Filopodia: A Particle Transport Revealed by Molecular-Scale Fluctuation Analyses

Filopodia perform cellular functions such as environmental sensing or cell motility, but they also grab for particles and withdraw them leading to an increased efficiency of phagocytic uptake. Remarkably, withdrawal of micron-sized particles is also possible without noticeable movements of the filopodia. Here, we demonstrate that polystyrene beads connected by optical tweezers to the ends of adherent filopodia of J774 macrophages, are transported discontinuously toward the cell body. After a typical resting time of 1-2 min, the cargo is moved with alternating velocities, force constants, and friction constants along the surface of the filopodia. This surfing-like behavior along the filopodium is recorded by feedback-controlled interferometric three-dimensional tracking of the bead motions at 10-100 kHz. We measured transport velocities of up to 120 nm/s and transport forces of ∼ 70 pN. Small changes in position, fluctuation width, and temporal correlation, which are invisible in conventional microscopy, indicate molecular reorganization of transport-relevant proteins in different phases of the entire transport process. A detailed analysis implicates a controlled particle transport with fingerprints of a nanoscale unbinding/binding behavior. The manipulation and analysis methods presented in our study may also be helpful in other fields of cellular biophysics.

Landauer's blowtorch effect as a thermodynamic cross process: Brownian cooling

The local heating of a selected region in a double-well potential alters the relative stability of the two wells and gives rise to an enhancement of population transfer to the cold well. We show that this Landauer's blowtorch effect may be considered in the spirit of a thermodynamic cross process linearly connecting the flux of particles and the thermodynamic force associated with the temperature difference and consequently ensuring the existence of a reverse cross effect. This reverse effect is realized by directing the thermalized particles in a double-well potential by application of an external bias from one well to the other, which suffers cooling.