Non-canonical lipoproteins with programmable assembly and architecture

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.

Lipidation Alters the Structure and Hydration of Myristoylated Intrinsically Disordered Proteins

Lipidated proteins are an emerging class of hybrid biomaterials that can integrate the functional capabilities of proteins into precisely engineered nano-biomaterials with potential applications in biotechnology, nanoscience, and biomedical engineering. For instance, fatty-acid-modified elastin-like polypeptides (FAMEs) combine the hierarchical assembly of lipids with the thermoresponsive character of elastin-like polypeptides (ELPs) to form nanocarriers with emergent temperature-dependent structural (shape or size) characteristics. Here, we report the biophysical underpinnings of thermoresponsive behavior of FAMEs using computational nanoscopy, spectroscopy, scattering, and microscopy. This integrated approach revealed that temperature and molecular syntax alter the structure, contact, and hydration of lipid, lipidation site, and protein, aligning with the changes in the nanomorphology of FAMEs. These findings enable a better understanding of the biophysical consequence of lipidation in biology and the rational design of the biomaterials and therapeutics that rival the exquisite hierarchy and capabilities of biological systems.

Solution behavior and encapsulation properties of fatty acid-elastin-like polypeptide conjugates

Developing new biomaterials is an active research area owing to their applications in regenerative medicine, tissue engineering and drug delivery. Elastin-like polypeptides (ELPs) are good candidates for these applications because they are biosourced, biocompatible and biodegradable. With the aim of developing ELP-based micelles for drug delivery applications we have synthesized 15 acyl-ELP compounds by conjugating myristic, palmitic, stearic, oleic or linoleic acid to the N-terminus of three ELPs differing in molar mass. The ELP-fatty acid conjugates have interesting solution behavior. They form micelles at low temperatures and aggregate above the cloud point temperature (Tcp). The critical micelle concentration depends on the fatty acid nature while the micelle size is mainly determined by the ELP block length. We were able to show that ELPs were better hydrated in the micelles than in their individual state in solution. The micelles are stable in phosphate-buffered saline at temperatures below the Tcp, which can vary between 20 °C and 38 °C depending on the length or hydrophilicity of the ELP. Acyl-ELP micelles were loaded with the small hydrophobic molecule Nile red. The encapsulation efficiency and release kinetics showed that the best loading conditions were achieved with the largest ELP conjugated to stearic acid.

Expanding the chemical repertoire of protein-based polymers for drug-delivery applications

Expanding the chemical repertoire of natural and artificial protein-based polymers (PBPs) can enable the production of sequence-defined, yet chemically diverse, biopolymers with customized or new properties that cannot be accessed in PBPs composed of only natural amino acids. Various approaches can enable the expansion of the chemical repertoire of PBPs, including chemical and enzymatic treatments or the incorporation of unnatural amino acids. These techniques are employed to install a wide variety of chemical groups-such as bio-orthogonally reactive, cross-linkable, post-translation modifications, and environmentally responsive groups-which, in turn, can facilitate the design of customized PBP-based drug-delivery systems with modified, fine-tuned, or entirely new properties and functions. Here, we detail the existing and emerging technologies for expanding the chemical repertoire of PBPs and review several chemical groups that either demonstrate or are anticipated to show potential in the design of PBP-based drug delivery systems. Finally, we provide our perspective on the remaining challenges and future directions in this field.

Temperature-Responsive Nano-Biomaterials from Genetically Encoded Farnesylated Disordered Proteins

Despite broad interest in understanding the biological implications of protein farnesylation in regulating different facets of cell biology, the use of this post-translational modification to develop protein-based materials and therapies remains underexplored. The progress has been slow due to the lack of accessible methodologies to generate farnesylated proteins with broad physicochemical diversities rapidly. This limitation, in turn, has hindered the empirical elucidation of farnesylated proteins' sequence-structure-function rules. To address this gap, we genetically engineered prokaryotes to develop operationally simple, high-yield biosynthetic routes to produce farnesylated proteins and revealed determinants of their emergent material properties (nano-aggregation and phase-behavior) using scattering, calorimetry, and microscopy. These outcomes foster the development of farnesylated proteins as recombinant therapeutics or biomaterials with molecularly programmable assembly.

Adaptive Recombinant Nanoworms from Genetically Encodable Star Amphiphiles

Recombinant nanoworms are promising candidates for materials and biomedical applications ranging from the templated synthesis of nanomaterials to multivalent display of bioactive peptides and targeted delivery of theranostic agents. However, molecular design principles to synthesize these assemblies (which are thermodynamically favorable only in a narrow region of the phase diagram) remain unclear. To advance the identification of design principles for the programmable assembly of proteins into well-defined nanoworms and to broaden their stability regimes, we were inspired by the ability of topologically engineered synthetic macromolecules to acess rare mesophases. To test this design principle in biomacromolecular assemblies, we used post-translational modifications (PTMs) to generate lipidated proteins with precise topological and compositional asymmetry. Using an integrated experimental and computational approach, we show that the material properties (thermoresponse and nanoscale assembly) of these hybrid amphiphiles are modulated by their amphiphilic architecture. Importantly, we demonstrate that the judicious choice of amphiphilic architecture can be used to program the assembly of proteins into adaptive nanoworms, which undergo a morphological transition (sphere-to-nanoworms) in response to temperature stimuli.

Thermosensitive Vesicles from Chemically Encoded Lipid-Grafted Elastin-like Polypeptides

Biomimetic design to afford smart functional biomaterials with exquisite properties represents synthetic challenges and provides unique perspectives. In this context, elastin-like polypeptides (ELPs) recently became highly attractive building blocks in the development of lipoprotein-based membranes. In addition to the bioengineered post-translational modifications of genetically encoded recombinant ELPs developed so far, we report here a simple and versatile method to design biohybrid brush-like lipid-grafted-ELPs using chemical post-modification reactions. We have explored a combination of methionine alkylation and click chemistry to create a new class of hybrid lipoprotein mimics. Our design allowed the formation of biomimetic vesicles with controlled permeability, correlated to the temperature-responsiveness of ELPs.

Folding and self-assembly of short intrinsically disordered peptides and protein regions

Proteins and peptide fragments are highly relevant building blocks in self-assembly for nanostructures with plenty of applications. Intrinsically disordered proteins (IDPs) and protein regions (IDRs) are defined by the absence of a well-defined secondary structure, yet IDPs/IDRs show a significant biological activity. Experimental techniques and computational modelling procedures for the characterization of IDPs/IDRs are discussed. Directed self-assembly of IDPs/IDRs allows reaching a large variety of nanostructures. Hybrid materials based on the derivatives of IDPs/IDRs show a promising performance as alternative biocides and nanodrugs. Cell mimicking, in vivo compartmentalization, and bone regeneration are demonstrated for IDPs/IDRs in biotechnological applications. The exciting possibilities of IDPs/IDRs in nanotechnology with relevant biological applications are shown.