Athale, C. A. | bioRxiv Science

bioRxiv2017

‘Switch-like’ transition from random to directed motility of microtubules by a yeast dynein

Processive transport by multiple molecular motors that step stochastically, requires a form of mechanical coupling. In a quantitative microtubule (MT) gliding assay with yeast cytoplasmic dynein, we investigate the nature of this coupling by examining the effect of MT length and motor density on transport. We find speed and velocity have a length dependence for low motor numbers, but are independent of MT length for high motor densities. The dependence of speed, velocity and degree of randomness of MT transport is best understood when evaluated in terms of the numbers of motors bound to a filament. A model of collective transport of MTs, based on stochastic stepping and asymmetric detachment rates, reproduces the experimental trends of decreasing diffusivity with increasing number of motors. Additionally, the model predicts a switch-like increase in directionality of MT transport above a threshold number of motors. Such a rapid transition from random to directed motility with increasing numbers of yeast dyneins, could play a role in vivo during mitosis in the search and orientation of the Saccharomyces cerevisiae nucleus.

biophysics↗

bioRxiv2017

Mechanics of Hydra Detachment from Subrates: The Role of Substrate Rigidity and Starvation

Hydra is a fresh water hydrozoan living as a solitary polyp with a sedentary feeder lifestyle attached to a substrate. In times of food shortage they are reported to detach from their substrate and move either by drifting or somer-saulting. The attachment to the substrate is usually by the basal-body which secretes a mucosal adhesive. The mechanical strength of the adhesion of Hydra has not been quantified so far. Here, we measure the force required to detach Hydra vulgaris and Hydra magnipapillata from a surface and the role of physical and physiological factors. In order to do this, we have developed a flow chamber with a calibrated jet of water. We find H. vulgaris adhering to a hard substrate - a glass cover slip- requires more force to detach it as compared to a soft substrate- polyacrylamide gel. While H. vulgaris after one week of starvation detaches with very similar values of stress, H. magnipapillata detaches more readily when starved. These results suggest that the strength of adhesion is strongly affected by the stiffness of the substrate, while nutritional status dependence of detachment force appears to be species dependent. Given that Hydra detachment is required during locomotion, our measurements on the one hand suggest the magnitude of forces the animal must exert to detach itself. Additionally, our results suggest active detachment of the base might be required for Hydra to achieve movement, and only a small contribution coming from weakening adhesion.

biophysics↗