Hyperstable stacked-disk structure of tobacco mosaic virus protein: electron cryomicroscopy image reconstruction related to atomic models

The stacked disk aggregate of tobacco mosaic virus protein is an intriguing object due to its high degree of stability, in spite of indications that the aggregate is held together to a great extent by water-mediated interactions between adjacent protein rings. Here, we present a set of models that were constructed using the atomic coordinates of the four-layer aggregate, and compare these with a three-dimensional reconstruction of the stacked disk obtained by means of cryoelectron microscopy and helical image processing. The comparison of the four possible models of the stacked disk with the data shows that there is a better correlation of the data with the left-handed model built from the A-A ring pair coordinates than with the two models involving the A-B ring pair, or with the right-handed model of the A-A ring pair. This establishes that the packing of the protein subunits in the stacked disk is different from that previously believed. We also note some differences between the observed structure and A-A ring pair model in the region of the flexible loop at small radius that might be an indication of conformational differences that give rise to the stability of the aggregate.