Abstract
The rod-shaped nanoparticles of the widespread plant pathogen tobacco mosaic virus (TMV) have been a matter of intense debates and cutting-edge research for more than a hundred years. During the late 19th century, their behavior in filtration tests applied to the agent causing the 'plant mosaic disease' eventually led to the discrimination of viruses from bacteria. Thereafter, they promoted the development of biophysical cornerstone techniques such as electron microscopy and ultracentrifugation. Since the 1950s, the robust, helically arranged nucleoprotein complexes consisting of a single RNA and more than 2100 identical coat protein subunits have enabled molecular studies which have pioneered the understanding of viral replication and self-assembly, and elucidated major aspects of virus–host interplay, which can lead to agronomically relevant diseases. However, during the last decades, TMV has acquired a new reputation as a well-defined high-yield nanotemplate with multivalent protein surfaces, allowing for an ordered high-density presentation of multiple active molecules or synthetic compounds. Amino acid side chains exposed on the viral coat may be tailored genetically or biochemically to meet the demands for selective conjugation reactions, or to directly engineer novel functionality on TMV-derived nanosticks. The natural TMV size (length: 300 nm) in combination with functional ligands such as peptides, enzymes, dyes, drugs or inorganic materials is advantageous for applications ranging from biomedical imaging and therapy approaches over surface enlargement of battery electrodes to the immobilization of enzymes. TMV building blocks are also amenable to external control of in vitro assembly and re-organization into technically expedient new shapes or arrays, which bears a unique potential for the development of 'smart' functional 3D structures. Among those, materials designed for enzyme-based biodetection layouts, which are routinely applied, e.g., for monitoring blood sugar concentrations, might profit particularly from the presence of TMV rods: Their surfaces were recently shown to stabilize enzymatic activities upon repeated consecutive uses and over several weeks. This review gives the reader a ride through strikingly diverse achievements obtained with TMV-based particles, compares them to the progress with related viruses, and focuses on latest results revealing special advantages for enzyme-based biosensing formats, which might be of high interest for diagnostics employing 'systems-on-a-chip'.
Highlights
In the early years of virology, viruses were primarily regarded as small infective agents sometimes causing fatal diseases
We report on trends in the gradually changing field of research on virus and virus-like particles, with special focus on the well-known tobacco mosaic virus (TMV) nanosticks and their use as nanostructured scaffolds for the efficient and advantageous display of biomolecules, namely of active enzymes
About 50% of the CPCys subunits could be decorated with biotin linkers, and virtually all thereof with enzymes (Figure 5B,C), resulting in complete coverage of the TMV sticks by more than a single layer: The volume of all immobilized enzyme molecules surpassed what was available directly around the TMV surface
Summary
In the early years of virology, viruses were primarily regarded as small infective agents sometimes causing fatal diseases. Making TMV an enzyme itself The conformation of the densely packed CP subunits of TMV rod and disk assemblies results in a surface-relief with regularly ordered nanometric grooves and cavities (see [46] for the most recent high-resolution cryo-EM reconstruction of TMV at 3.35 Å resolution) These have inspired a striking piece of investigation, as one of the repetitively arranged depressions resembles the substrate-binding site of glutathione peroxidase (GPx, EC 1.11.1.9), a natural selenoenzyme: Computer-aided design identified adjacent, adequately positioned amino acids of the viral CP which offered the chance to be modified into a glutathione-attracting pocket containing a catalytic selenocysteine (Sec) moiety [133]. Label-free biodetection systems exploiting the beneficial effects of TMV adapter scaffolds carrying sensor enzymes are in the pipeline of current research efforts as well, and might promote novel diagnostic layouts for smaller, non-antigenic but bioconvertible analytes
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