Morphological Characterization of the Self-Assembly of Virus Movement Proteins into Nanotubes in the Absence of Virus Particles.

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One infection mechanism of plant viruses is the generation of nanotubes by viral movement proteins, allowing cell-to-cell virus particle transport. Previously, it was assumed that viral nanotubes extend directly from the host-cell plasma membrane. In virus-infected plants, these nanotubes reach an extraordinary diameter:length ratio (≈100 nm:µm or mm range). Here, viral nanotubes are produced in a transient protoplast system; the coding sequence for alfalfa mosaic virus movement protein is translationally fused to green fluorescent protein. The maximum extension of viral nanotubes into the culture medium is achieved 24-48 h posttransfection, with lengths in the micro- and millimeter ranges. Scanning electron microscopy and transmission electron microscopy show that strong inhomogeneous viral nanotubes are formed compared to particle-filled systems. The nanotubes have similar length, but fluctuating wall thickness and diameter and are susceptible to entanglement and recombination. Indirect methods demonstrate that movement proteins assemble independently at the top of the nanotube. These viral nanotubes grow distinctly from previously known natural particle-filled systems and are a unique biological tubular nanomaterial that has the potential for micro- or nanoapplications as a mechanically stable structural component.