Intra-cellular active transport by native cargos is ubiquitous. We investigate the motion of spherical nano-particles (NPs) grafted with flexible polymers that end with a nuclear localization signal peptide. This peptide allows the recruitment of several mammalian dynein motors from cytoplasmic extracts. To determine how motor–motor interactions influenced motility on the single microtubule level, we conducted bead-motility assays incorporating surface adsorbed microtubules and combined them with model simulations that were based on the properties of a single dynein. The experimental and simulation results revealed long time trajectories: when the number of NP-ligated motors Nm increased, run-times and run-lengths were enhanced and mean velocities were somewhat decreased. Moreover, the dependence of the velocity on run-time followed a universal curve, regardless of the system composition. Model simulations also demonstrated left- and right-handed helical motion and revealed self-regulation of the number of microtubule-bound, actively transporting dynein motors. This number was stochastic along trajectories and was distributed mainly between one, two, and three motors, regardless of Nm. We propose that this self-regulation allows our synthetic NPs to achieve persistent motion that is associated with major helicity. Such a helical motion might affect obstacle bypassing, which can influence active transport efficiency when facing the crowded environment of the cell.

We deduce the directional step distribution of yeast dynein motor protein on the microtubule surface by combing intrinsic features of the dynein and microtubule. These include the probability distribution of the separation vector between the two microtubule binding domains (MTBDs), the angular probability distribution of a single MTBD translation, the existence of a microtubule seam defect, microtubule binding sites, and theoretical extension that accounts for a load force on the motor. Our predictions are in excellent accord with the measured longitudinal step size distributions at various load forces. Moreover, we predict the side-step distribution and its dependence on longitudinal load forces, which shows a few surprising features. First, the distribution is broad. Second, in the absence of load, we find a small right-hand bias. Third, the side-step bias is susceptible to the longitudinal load force; it vanishes at a load equal to the motor stalling force and changes to a left-hand bias above that value. Fourth, our results are sensitive to the ability of the motor to explore the seam several times during its walk. While available measurements of side-way distribution are limited, our findings are amenable to experimental check and, moreover, suggest a diversity of results depending on whether the microtubule seam is viable to motor sampling. Significance StatementThe function of microtubule (MT) associated protein motors, kinesin and dynein, is essential for a myriad of intracellular processes. Different measurements on yeast cytoplasmic-dynein stepping characteristics appear to be unrelated to each other. We provide a unified physical-statistical model that combines these seemingly independent features with a theoretical expression that accounts for the exertion of a longitudinal load force, to yield the longitudinal step distribution at various load forces. The latter is in excellent accord with the measured distributions. Moreover, we deduce the side-step distribution, which surprisingly is susceptible to longitudinal load forces and comprises a right or left bias. This side-way bias is consistent with observations of helical motion of a nanoparticle carried by a number of motors.

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Native cargos achieve efficient intra-cellular active transport by recruiting multiple motors proteins from the cytoplasm. Here we investigate the motion of spherical nano-particles (NPs), acting as model cargos, grafted with flexible polymers, each ending with a nuclear localization signal (NLS) peptide, thereby allowing recruitment of mammalian cytoplasmic dynein. Bead-motility assays show several unique features. As the number of motors increases, the run-time and run-length increase, the longitudinal velocity mean decreases, whereas its width shows a non-monotonous behavior. Moreover, both single and multi-motor NPs perform angular (i.e., projected transverse) motion. Strikingly, the directions of angular and longitudinal motions are correlated, such that retrograde steps are combined with right-handed motion. We formulate a theoretical model to simulate the multi-dynein transported NP. The model builds on a recently theoretical description of single yeast dynein stepping on the curved microtubule surface, modified to account for mammalian dynein, and generalized to include motor-motor elastic and excluded-volume coupling. The simulation results are majorly in agreement with our experimental results. Moreover, long time trajectories exhibit both left- and right- handed helical motion around the MT symmetry axis, consistent with the measured and simulated angular velocity. The model gives further insight into the mechanism of the NP motion and suggests that the NPs are self-regulating the number of participating motors, optimizing between single motor, for obstacle bypassing, and multi-motor behavior, for persistent directional motion. We suggest that native cargos could use a similar mechanism to achieve efficient transport in the crowded cellular environment.

Targeting the cell nucleus remains a challenge for drug delivery. Here we present a universal platform for smart design of nano-particles (NPs) decoration that allows recruitment of multiple dynein motors to drive their active motion towards the nucleus. The uniqueness of our approach is based on using: (i) a spacer polymer, commonly Biotin-Polyethylene-glycol-thiol (B-PEG-SH), whose grafting density and molecular weight can be tuned thereby allowing NP transport optimization, and (ii) protein binding peptides, like cell penetrating, NLS, or cancer targeting, peptides. Universal chemistry is employed to link peptides to the PEG free-end. To manifest our platform, we use a SV40T large antigen-originating NLS peptide. Our modular design allows tuning the number of recruited motors, and to replace the NLS by a variety of other localization signal molecules. Our control of the NP decoration scheme, and the modularity of our platform, carries great advantage for nano-carrier design for drug delivery applications.