Study of active self-assembly using biomolecular motors

Protein Based Biomaterials for Therapeutic and Diagnostic Applications

Proteins are some of the most versatile and studied macromolecules with extensive biomedical applications. The natural and biological origin of proteins offer such materials several advantages over their synthetic counterparts, such as innate bioactivity, recognition by cells and reduced immunogenic potential. Furthermore, proteins can be easily functionalized by altering their primary amino acid sequence and can often be further self-assembled into higher order structures either spontaneously or under specific environmental conditions. This review will feature the recent advances in protein-based biomaterials in the delivery of therapeutic cargo such as small molecules, genetic material, proteins, and cells. First, we will discuss the ways in which secondary structural motifs, the building blocks of more complex proteins, have unique properties that enable them to be useful for therapeutic delivery. Next, supramolecular assemblies, such as fibers, nanoparticles, and hydrogels, made from these building blocks that are engineered to behave in a cohesive manner, are discussed. Finally, we will cover additional modifications to protein materials that impart environmental responsiveness to materials. This includes the emerging field of protein molecular robots, and relatedly, protein-based theranostic materials that combine therapeutic potential with modern imaging modalities, including near-infrared fluorescence spectroscopy (NIRF), single-photo emission computed tomography/computed tomography (SPECT/CT), positron emission tomography (PET), magnetic resonance imaging (MRI), and ultrasound/photoacoustic imaging (US/PAI).

Kinetics of actin networks formation measured by time resolved particle-tracking microrheology

Actin is one of the most studied cytoskeleton proteins showing a very rich span of structures and functions. For example, adenosine triphosphate (ATP)-assisted polymerization of actin is used to push protrusions forward in a mechanism that enables cells to crawl on a substrate. In this process, the chemical energy released from the hydrolysis of ATP is what enables force generation. We study a minimal model system comprised of actin monomers in an excess of ATP concentration. In such a system polymerization proceeds in three stages: nucleation of actin filaments, elongation, and network formation. While the kinetics of filament growth was characterized previously, not much is known about the kinetics of network formation and the evolution of networks towards a steady-state structure. In particular, it is not clear how the non-equilibrium nature of this ATP-assisted polymerization manifests itself in the kinetics of self-assembly. Here, we use time-resolved microrheology to follow the kinetics of the three stages of self-assembly as a function of initial actin monomer concentration. Surprisingly, we find that at high enough initial monomer concentrations the effective elastic modulus of the forming actin networks overshoots and then relaxes with a -2/5 power law. We attribute the overshoot to the non-equilibrium nature of the polymerization and the relaxation to rearrangements of the network into a steady-state structure.

Magnetic Force-Induced Alignment of Microtubules by Encapsulation of CoPt Nanoparticles Using a Tau-Derived Peptide

Construction of magnetotactic materials is a significant challenge in nanotechnology applications such as nanodevices and nanotransportation. Artificial magnetotactic materials can be designed from magnetotactic bacteria because these bacteria use magnetic nanoparticles for aligning with and moving within magnetic fields. Microtubules are attractive scaffolds to construct magnetotactic materials because of their intrinsic motility. Nonetheless, it is challenging to magnetically control their orientation while retaining their motility by conjugating magnetic nanoparticles on their outer surface. Here we solve the issue by encapsulating magnetic cobalt-platinum nanoparticles inside microtubules using our developed Tau-derived peptide that binds to their internal pockets. The in situ growth of cobalt-platinum nanoparticles resulted in the formation of a linear-chain assembly of nanoparticles inside the microtubules. The magnetic microtubules significantly aligned with a high order parameter (0.71) along the weak magnetic field (0.37 T) and showed increased motility. This work provides a new concept for designing magnetotactic materials.

Fluorescent Tau-derived Peptide for Monitoring Microtubules in Living Cells

Microtubules (MTs) are key cytoskeletal components that modulate various cellular activities with their dynamic structural changes, including polymerization and depolymerization. To monitor the dynamics of MTs in living cells, many drug-based fluorescent probes have been developed; however, these also potentially disturb the polymerization/depolymerization of MTs. Here, we report nondrug, peptide-based fluorescent probes to monitor MTs in living cells. We employed a Tau-derived peptide (TP) that has been shown to bind MTs without inhibiting polymerization/depolymerization in vitro. We show that a tetramethylrhodamine (TMR)-labeled TP (TP-TMR) is internalized into HepG2 cells and binds to intracellular MTs, enabling visualization of MTs as clear, fibrous structures. The binding of TP-TMR shows no apparent effects on polymerization/depolymerization of MTs induced by MT-targeted drugs and temperature change. The main uptake mechanism of TP-TMR was elucidated as endocytosis, and partial endosomal escape resulted in the binding of TP-TMR to MTs. TP-TMR exhibited no cytotoxicity compared with MT-targeted drug scaffolds. These results indicate that TP scaffolds can be exploited as useful MT-targeted tools in living cells, such as in long-term imaging of MTs.

Morphological transitions of axially-driven microfilaments

The interactions of microtubules with motor proteins are ubiquitous in cellular and sub-cellular processes that involve motility and cargo transport. In vitro motility assays have demonstrated that motor-driven microtubules exhibit rich dynamical behaviors from straight to curved configurations. Here, we theoretically investigate the dynamic instabilities of elastic filaments, with free-ends, driven by single follower forces that emulate the action of molecular motors. Using the resistive force theory at low Reynolds number, and a combination of numerical techniques with linear stability analysis, we show the existence of four distinct regimes of filament behavior, including a novel buckled state with locked curvature. These successive instabilities recapitulate the full range of experimentally-observed microtubule behavior, implying that neither structural nor actuation asymmetry are needed to elicit this rich repertoire of motion.

How non-bonding domains affect the active assembly of microtubule spools

Structural defects can determine and influence various properties of materials, and many technologies rely on the manipulation of defects (e.g., semiconductor industries). In biological systems, management of defects/errors (e.g. DNA repair) is critical to an organism's survival, which has inspired the design of artificial nanomachines that mimic nature's ability to detect defects and repair damage. Biological motors have captured considerable attention in developing such capabilities due to their ability to convert energy into directed motion in response to environmental stimuli, which maximizes their ability for detection and repair. The objective of the present study was to develop an understanding of how the presence of non-bonding domains, here considered as a "defect", in microtubule (MT) building blocks affect the kinesin-driven, active assembly of MT spools. The assembly/joining of micron-scale bonding (i.e., biotin-containing) and non-bonding (i.e., no biotin) MTs resulted in segmented MT building blocks consisting of alternating bonding and non-bonding domains. Here, the introduction of these MT building blocks into a kinesin gliding motility assay along with streptavidin-coated quantum dots resulted in the active assembly of spools with altered morphology but retained functionality. Moreover, it was noted that non-bonding domains were autonomously and preferentially released from the spools over time, representing a mechanism by which defects may be removed from these structures. Overall, our findings demonstrate that this active assembly system has an intrinsic ability for quality control, which can be potentially expanded to a wide range of applications such as self-regulation and healing of active materials.

Stabilization of microtubules by encapsulation of the GFP using a Tau-derived peptide

Encapsulation of the GFP inside microtubules by using a Tau-derived peptide increased the stability, rigidity, and velocity of microtubules.