Protein based nanomaterials that respond to the biological environment have potential as drug delivery vehicles, biosensors and as scaffolds for nanowire-based power generation. The inherent biocompatilibity of proteins versus inorganic counterparts, such as carbon nanotubes, has led nanotechnologists to study the architecture of nanoscale protein structures that are found in viral coat proteins, bacterial flagella and pili.
A model of fibril-mediated pilin-derived PNT assembly. Image source: Audette et al, Journal of Nanobiotechnology, 2013, 11: 24
The type IV pilus of Psuedomonas aeruginosa is a protein nanotube (PNT) with natural functions in adherence and interactions with cellular receptors, making it an attractive system to adapt to new applications. In a recent publication in Journal of Nanobiotechnology, Gerald Audette and colleagues, from York University, Canada, present a detailed biochemical characterisation of pilin-derived PNT assembly.
The authors studied the ability of hydrophobic trigger molecules (commonly used in protein crystallisation) to initiate the polymerisation of the bacterial protein monomer subunit, ∆K122 pilin. An optimal trigger molecule, 2-methyl-3,4-pentanediol (MPD), was found and the quantity and molecular weight of the resulting polymer species formed were studied using multi-angle light scattering analysis and size exclusion chromatography. ∆K122 pilin exists as a monomer-dimer equilibrium in solution (pictured above, top). The trigger (MPD) initiates spontaneous assembly into intermediate polymer fibrils that gradually coalesce to form three-stranded helical structures, closely analogous to the bacterial pilus, and large enough to be visualised by transmission electron microscopy.
A second key observation was that PNTs did not interact with polyethylene glycol (PEG), a substance that is useful as a non-reactive coating for building implantable nanodevices. The wormlike, filamentous nature of the protein nanotube is also partly able to avoid an immune response, if used in biomedical applications, since the long structures are difficult for macrophages to engulf. In the future therefore, the protein coat and hollow cavity of PNTs may allow the transport of smaller molecules, such as drugs, to their targets on the cell surface.