M.S.T. Koay

Genetically encoded FRET sensors to monitor intracellular Zn2+ homeostasis

We developed genetically encoded fluorescence resonance energy transfer (FRET)-based sensors that display a large ratiometric change upon Zn2+ binding, have affinities that span the pico- to nanomolar range and can readily be targeted to subcellular organelles. Using this sensor toolbox we found that cytosolic Zn2+ was buffered at 0.4 nM in pancreatic β cells, and we found substantially higher Zn2+ concentrations in insulin-containing secretory vesicles.

Self-sorting of foreign proteins in a bacterial nanocompartment

Nature uses bottom-up approaches for the controlled assembly of highly ordered hierarchical structures with defined functionality, such as organelles, molecular motors, and transmembrane pumps. The field of bionanotechnology draws inspiration from nature by utilizing biomolecular building blocks such as DNA, proteins, and lipids, for the (self-) assembly of new structures for applications in biomedicine, optics, or electronics. Among the toolbox of available building blocks, proteins that form cage-like structures, such as viruses and virus-like particles, have been of particular interest since they form highly symmetrical assemblies and can be readily modified genetically or chemically both on the outer or inner surface. Bacterial encapsulins are a class of nonviral protein cages that self-assemble in vivo into stable icosahedral structures. Using teal fluorescent proteins (TFP) engineered with a specific native C-terminal docking sequence, we report the molecular self-sorting and selective packaging of TFP cargo into bacterial encapsulins during in vivo assembly. Using native mass spectrometry techniques, we show that loading of either monomeric or dimeric TFP cargo occurs with unprecedented high fidelity and exceptional loading accuracy. Such self-assembling systems equipped with self-sorting capabilities would open up exciting opportunities in nanotechnology, for example, as artificial (molecular storage or detoxification) organelles or as artificial cell factories for in situ biocatalysis.

Synthesis and characterization of de novo designed peptides modelling the binding sites of [4Fe-4S] clusters in photosystem I.

Photosystem I (PS I) converts the energy of light into chemical energy via transmembrane charge separation. The terminal electron transfer cofactors in PS I are three low-potential [4Fe-4S] clusters named F(X), F(A) and F(B), the last two are bound by the PsaC subunit. We have modelled the F(A) and F(B) binding sites by preparing two apo-peptides (maquettes), sixteen amino acids each. These model peptides incorporate the consensus [4Fe-4S] binding motif along with amino acids from the immediate environment of the iron-sulfur clusters F(A) and F(B). The [4Fe-4S] clusters were successfully incorporated into these model peptides, as shown by optical absorbance, EPR and Mössbauer spectroscopies. The oxidation-reduction potential of the iron-sulfur cluster in the F(A)-maquette is -0.44+/-0.03 V and in the F(B)-maquette is -0.47+/-0.03 V. Both are close to that of F(A) and F(B) in PS I and are considerably more negative than that observed for other [4Fe-4S] model systems described earlier (Gibney, B. R., Mulholland, S. E., Rabanal, F., and Dutton, P. L. Proc. Natl. Acad. Sci. U.S.A. 93 (1996) 15041-15046). Our optical data show that both maquettes can irreversibly bind to PS I complexes, where PsaC-bound F(A) and F(B) were removed, and possibly participate in the light-induced electron transfer reaction in PS I.

Role of Electrostatics in the Assembly Pathway of a Single-Stranded RNA Virus

ABSTRACT We have recently discovered (R. D. Cadena-Nava et al., J. Virol. 86:3318–3326, 2012, doi:10.1128/JVI.06566-11) that the in vitro packaging of RNA by the capsid protein (CP) of cowpea chlorotic mottle virus is optimal when there is a significant excess of CP, specifically that complete packaging of all of the RNA in solution requires sufficient CP to provide charge matching of the N-terminal positively charged arginine-rich motifs (ARMS) of the CPs with the negatively charged phosphate backbone of the RNA. We show here that packaging results from the initial formation of a charge-matched protocapsid consisting of RNA decorated by a disordered arrangement of CPs. This protocapsid reorganizes into the final, icosahedrally symmetric nucleocapsid by displacing the excess CPs from the RNA to the exterior surface of the emerging capsid through electrostatic attraction between the ARMs of the excess CP and the negative charge density of the capsid exterior. As a test of this scenario, we prepare CP mutants with extra and missing (relative to the wild type) cationic residues and show that a correspondingly smaller and larger excess, respectively, of CP is needed for complete packaging of RNA. IMPORTANCE Cowpea chlorotic mottle virus (CCMV) has long been studied as a model system for the assembly of single-stranded RNA viruses. While much is known about the electrostatic interactions within the CCMV virion, relatively little is known about these interactions during assembly, i.e., within intermediate states preceding the final nucleocapsid structure. Theoretical models and coarse-grained molecular dynamics simulations suggest that viruses like CCMV assemble by the bulk adsorption of CPs onto the RNA driven by electrostatic attraction, followed by structural reorganization into the final capsid. Such a mechanism facilitates assembly by condensing the RNA for packaging while simultaneously concentrating the local density of CP for capsid nucleation. We provide experimental evidence of such a mechanism by demonstrating that efficient assembly is initiated by the formation of a disordered protocapsid complex whose stoichiometry is governed by electrostatics (charge matching of the anionic RNA and the cationic N termini of the CP).

Predicting the loading of virus-like particles with fluorescent proteins

The virus-like particle (VLP) of the Cowpea Chlorotic Mottle Virus (CCMV) has often been used to encapsulate foreign cargo. Here we show two different rational design approaches, covalent and noncovalent, for loading teal fluorescent proteins (TFP) into the VLP. The covalent loading approach allows us to gain control over capsid loading on a molecular level. The achieved loading control is used to accurately predict the loading of cargo into CCMV VLP. The effects of molecular confinement were compared for the differently loaded VLPs created with the covalent method. We see that the loading of more than 10 fluorescent proteins in the 18 nm internal cavity of the CCMV capsid gives rise to a maximum efficiency of homo-FRET between the loaded proteins, as measured by fluorescence anisotropy. This shows that already at low levels of VLP loading molecular crowding starts to play a role.

Self-assembly and characterization of small and monodisperse dye nanospheres in a protein cage

Phthalocyanines (Pc) are dyes in widespread use in materials science and nanotechnology, with numerous applications in medicine, photonics, electronics and energy conversion. With the aim to construct biohybrid materials, we here prepared and analyzed the structure of two Pc-loaded virus-like particles (VLP) with diameters of 20 and 28 nm (i.e., T = 1 and T = 3 icosahedral symmetries, respectively). Our cryo-electron microscopy (cryo-EM) studies show an unprecedented, very high level of Pc molecule organization within both VLP. We found that 10 nm diameter nanospheres form inside the T = 1 VLP by self-assembly of supramolecular Pc stacks. Monodisperse, self-assembled organic dye nanospheres were not previously known, and are a consequence of capsid-imposed symmetry and size constraints. The Pc cargo also produces major changes in the protein cage structure and in the mechanical properties of the VLP. Pc-loaded VLP are potential photosensitizer/carrier systems in photodynamic therapy (PDT), for which their mechanical behaviour must be characterized. Many optoelectronic applications of Pc dyes, on the other hand, are dependent on dye organization at the nanoscale level. Our multidisciplinary study thus opens the way towards nanomedical and nanotechnological uses of these functional molecules.

Assembling Enzymatic Cascade Pathways inside Virus-Based Nanocages Using Dual-Tasking Nucleic Acid Tags

The packaging of proteins into discrete compartments is an essential feature for cellular efficiency. Inspired by Nature, we harness virus-like assemblies as artificial nanocompartments for enzyme-catalyzed cascade reactions. Using the negative charges of nucleic acid tags, we develop a versatile strategy to promote an efficient noncovalent co-encapsulation of enzymes within a single protein cage of cowpea chlorotic mottle virus (CCMV) at neutral pH. The encapsulation results in stable 21–22 nm sized CCMV-like particles, which is characteristic of an icosahedral T = 1 symmetry. Cryo-EM reconstruction was used to demonstrate the structure of T = 1 assemblies templated by biological soft materials as well as the extra-swelling capacity of these T = 1 capsids. Furthermore, the specific sequence of the DNA tag is capable of operating as a secondary biocatalyst as well as bridging two enzymes for co-encapsulation in a single capsid while maintaining their enzymatic activity. Using CCMV-like particles to mimic nanocompartments can provide valuable insight on the role of biological compartments in enhancing metabolic efficiency.

Self‐Assembled Cage‐Like Protein Structures

Proteins and protein-based assemblies represent the most structurally and functionally diverse molecules found in nature. Protein cages, viruses and bacterial microcompartments are highly organized structures that are composed primarily of protein building blocks and play important roles in molecular ion storage, nucleic acid packaging and catalysis. The outer and inner surface of protein cages can be modified, either chemically or genetically, and the internal cavity can be used to template, store and arrange molecular cargo within a defined space. Owing to their structural, morphological, chemical and thermal diversity, protein cages have been investigated extensively for applications in nanotechnology, nanomedicine and materials science. Here we provide a concise overview of the most common icosahedral viral and nonviral assemblies, their role in nature, and why they are highly attractive scaffolds for the encapsulation of functional materials.

Using viruses as nanomedicines

The field of nanomedicine involves the design and fabrication of novel nanocarriers for the intracellular delivery of therapeutic cargo or for use in molecular diagnostics. Although traditionally recognized for their ability to invade and infect host cells, viruses and bacteriophages have been engineered over the past decade as highly promising molecular platforms for the targeted delivery and treatment of many human diseases. Inherently biodegradable, the outer capsids of viruses are composed entirely of protein building blocks, which can be genetically or chemically engineered with molecular imaging reagents, targeting ligands and therapeutic molecules. While there are several examples of viruses as in vitro molecular cargo carriers, their potential for applications in nanomedicine has only recently emerged. Here we highlight recent developments towards the design and engineering of viruses for the treatment of cancer, bacterial infections and immune system‐related diseases.

Self-assembly triggered by self-assembly: optically active, paramagnetic micelles encapsulated in protein cage nanoparticles

In this contribution, optically active and paramagnetic micelles of the ligand 1,4,7,10-tetraaza-1-(1-carboxymethylundecane)-4,7,10-triacetic acid cyclododecane (DOTAC10) have been incorporated inside capsids of the cowpea chlorotic mottle virus (CCMV) protein through a hierarchical process of self-assembly triggered by self-assembly. The DOTAC10 ligand was used to complex Gd(III), in order to form paramagnetic micelles, as well as to encapsulate an amphiphilic Zn(II) phthalocyanine (ZnPc) dye that optically confirmed the encapsulation of the micelles. The incorporation of ZnPc molecules in the paramagnetic micelles led to high capsid loading of both Gd(III) and ZnPc, as the micelles were stabilized by the amphiphilic dye encapsulation. The resulting protein cage nanoparticles (PCNs) show an improved r1 relaxivity, suggesting the possible use of these nanostructures as contrast agents (CAs) for magnetic resonance imaging (MRI). Since the encapsulated ZnPc dye also has a potential therapeutic value, the present results represent a first step towards the consecution of fully self-assembled PCNs for multimodal imaging and therapy.