Rational design of elastin-like polypeptide fusion proteins to tune self-assembly and properties of protein vesicles

Understanding the Phase Behavior of a Multistimuli-Responsive Elastin-like Polymer: Insights from Dynamic Light Scattering Analysis

Elastin-like polymers are a class of stimuli-responsive protein polymers that hold immense promise in applications such as drug delivery, hydrogels, and biosensors. Yet, understanding the intricate interplay of factors influencing their stimuli-responsive behavior remains a challenging frontier. Using temperature-controlled dynamic light scattering and zeta potential measurements, we investigate the interactions between buffer, pH, salt, water, and protein using an elastin-like polymer containing ionizable lysine residues. We observed the elevation of transition temperature in the presence of the common buffering agent HEPES at low concentrations, suggesting a "salting-in" effect of HEPES as a cosolute through weak association with the protein. Our findings motivate a more comprehensive investigation of the influence of buffer and other cosolute molecules on elastin-like polymer behavior.

Self-Assembled Recombinant Elastin and Globular Protein Vesicles with Tunable Properties for Diverse Applications

Vesicles are self-assembled structures comprised of a membrane-like exterior surrounding a hollow lumen with applications in drug delivery, artificial cells, and micro-bioreactors. Lipid or polymer vesicles are the most common and are made of lipids or polymers, respectively. They are highly useful structures for many applications but it can be challenging to decorate them with proteins or encapsulate proteins in them, owing to the use of organic solvent in their formation and the large size of proteins relative to lipid or polymer molecules. By utilization of recombinant fusion proteins to make vesicles, specific protein domains can be directly incorporated while also imparting tunability and stability. Protein vesicle assembly relies on the design and use of self-assembling amphiphilic proteins. A specific protein vesicle platform made in purely aqueous conditions of a globular, functional protein fused to a glutamate-rich leucine zipper (ZE) and a thermoresponsive elastin-like polypeptide (ELP) fused to an arginine-rich leucine zipper (ZR) is discussed here. The hydrophobic conformational change of the ELP above its transition temperature drives assembly, and strong ZE/ZR binding enables incorporation of the desired functional protein. Mixing the soluble proteins on ice induces zipper binding, and then warming above the ELP transition temperature (Tt) triggers the transition to and growth of protein-rich coacervates and, finally, reorganization of proteins into vesicles. Vesicle size is tunable based on salt concentration, rate of heating, protein concentration, size of the globular protein, molar ratio of the proteins, and the ELP sequence. Increasing the salt concentration decreases vesicle size by decreasing the Tt, resulting in a shorter coacervation transition stage. Likewise, directly changing the heating rate also changes this time and increasing protein concentration increases coalescence. Increasing globular protein size decreases the size of the vesicle due to steric hindrance. By changing the ELP sequence, which consists of (VPGXG)n, through the guest residue (X) or number of repeats (n), Tt is changed, affecting size. Additionally, the chemical nature of X variation has endowed vesicles with stimuli responsiveness and stability at physiological conditions.Protein vesicles have been used for biocatalysis, biomacromolecular drug delivery, and vaccine applications. Photo-cross-linkable vesicles were used to deliver small molecule cargo to cancer cells in vitro and antigen to immune cells in vivo. pH-responsive vesicles effectively delivered functional protein cargo, including cytochrome C, to the cytosol of cancer cells in vitro, using hydrophobic ion pairing to improve cargo distribution in the vesicles and release. The globular protein used to make the vesicles can be varied to achieve different functions. For example, enzyme vesicles exhibit biocatalysis, and antigen vesicles induce antibody and cellular immune responses after vaccination in mice. Collectively, the development and engineering of the protein vesicle platform has employed amphiphilic self-assembly strategies and rational protein engineering to control physical, chemical, and biological properties for biotechnology and nanomedicine applications.

Genetically Fused Resilin-like Polypeptide-Coiled Coil Bundlemer Conjugates Exhibit Tunable Multistimuli-Responsiveness and Undergo Nanofibrillar Assembly

Peptide-based materials are diverse candidates for self-assembly into modularly designed and stimuli-responsive nanostructures with precisely tunable compositions. Here, we genetically fused computationally designed coiled coil-forming peptides to the N- and C-termini of compositionally distinct multistimuli-responsive resilin-like polypeptides (RLPs) of various lengths. The successful expression of these hybrid polypeptides in bacterial hosts was confirmed through techniques such as gel electrophoresis, mass spectrometry, and amino acid analysis. Circular dichroism spectroscopy and ultraviolet-visible turbidimetry demonstrated that despite the fusion of disparate structural and responsive units, the coiled coils remained stable in the hybrid polypeptides, and the sequence-encoded differences in thermoresponsive phase separation of the RLPs were preserved. Cryogenic transmission electron microscopy and coarse-grained modeling showed that after thermal annealing in solution, the hybrid polypeptides adopted a closed loop conformation and assembled into nanofibrils capable of further hierarchically organizing into cluster structures and ribbon-like structures mediated by the self-association tendency of the RLPs.

Harnessing the Potential of Self-Assembled Peptide Hydrogels for Neural Regeneration and Tissue Engineering

Spinal cord injury, traumatic brain injury, and neurosurgery procedures usually lead to neural tissue damage. Self-assembled peptide (SAP) hydrogels, a type of innovative hierarchical nanofiber-forming peptide sequences serving as hydrogelators, have emerged as a promising solution for repairing tissue defects and promoting neural tissue regeneration. SAPs possess numerous features, such as adaptable morphologies, biocompatibility, injectability, tunable mechanical stability, and mimicking of the native extracellular matrix. This review explores the capacity of neural cell regeneration and examines the critical aspects of SAPs in neuroregeneration, including their biochemical composition, topology, mechanical behavior, conductivity, and degradability. Additionally, it delves into the latest strategies involving SAPs for central or peripheral neural tissue engineering. Finally, the prospects of SAP hydrogel design and development in the realm of neuroregeneration are discussed.

Encoding Structure in Intrinsically Disordered Protein Biomaterials

In nature, proteins range from those with highly ordered secondary and tertiary structures to those that completely lack a well-defined three-dimensional structure, termed intrinsically disordered proteins (IDPs). IDPs are generally characterized by one or more segments that have a compositional bias toward small hydrophilic amino acids and proline residues that promote structural disorder and are called intrinsically disordered regions (IDRs). The combination of IDRs with ordered regions and the interactions between the two determine the phase behavior, structure, and function of IDPs. Nature also diversifies the structure of proteins and thereby their functions by hybridization of the proteins with other moieties such as glycans and lipids; for instance, post-translationally glycosylated and lipidated proteins are important cell membrane components. Additionally, diversity in protein structure and function is achieved in nature through cross-linking proteins within themselves or with other domains to create various topologies. For example, an essential characteristic of the extracellular matrix (ECM) is the cross-linking of its network components, including proteins such as collagen and elastin, as well as polysaccharides such as hyaluronic acid (HA). Inspired by nature, synthetic IDP (SynIDP)-based biomaterials can be designed by employing similar strategies with the goal of introducing structural diversity and hence unique physiochemical properties. This Account describes such materials produced over the past decade and following one or more of the following approaches: (1) incorporating highly ordered domains into SynIDPs, (2) conjugating SynIDPs to other moieties through either genetically encoded post-translational modification or chemical conjugation, and (3) engineering the topology of SynIDPs via chemical modification. These approaches introduce modifications to the primary structure of SynIDPs, which are then translated to unique three-dimensional secondary and tertiary structures. Beginning with completely disordered SynIDPs as the point of origin, structure may be introduced into SynIDPs by each of these three unique approaches individually along orthogonal axes or by combinations of the three, enabling bioinspired designs to theoretically span the entire range of three-dimensional structural possibilities. Furthermore, the resultant structures span a wide range of length scales, from nano- to meso- to micro- and even macrostructures. In this Account, emphasis is placed on the physiochemical properties and structural features of the described materials. Conjugates of SynIDPs to synthetic polymers and materials achieved by simple mixing of components are outside the scope of this Account. Related biomedical applications are described briefly. Finally, we note future directions for the design of functional SynIDP-based biomaterials.

Self-assembly of temperature-responsive di-block polypeptides functionalized with unnatural amino acids

The incorporation of unnatural amino acids (uAAs) into protein-based polymers has emerged as a powerful methodology to expand their chemical repertoire. Recently, we demonstrated that incorporating uAAs into two temperature-responsive protein-based polymers-namely resilin- and elastin-like polypeptides (RLPs and ELPs, respectively)-can alter their properties. In this study, we incorporated aromatic uAAs into the protein sequence of RLP-ELP diblocks to yield new and diverse assemblies from a single DNA template. Specifically, we show that incorporating aromatic uAAs can modulate the phase-transition behaviors and self-assembly of the diblocks into various morphologies, including spherical and cylindrical micelles and single- and double-layered vesicles, with some constructs also demonstrating a temperature-responsive shape-shifting behavior. Next, we evaluated the ability of the RLP-ELP assemblies to encapsulate a chemotherapeutic drug, doxorubicin, and show how the identity of the incorporated uAAs and the morphology of the nanostructure affect the encapsulation efficiency. Taken together, our findings demonstrate that the multi-site incorporation of uAAs into temperature-responsive, amphiphilic protein-based diblock copolymers is a promising approach for the functionalization and tuning of self-assembled nanostructures.