Metastable Aggregates in the Polymerisation of Tobacco‐Mosaic‐Virus Protein

Abstract

Metastable ‘overshoot’ aggregates are formed when tobacco mosaic virus A‐protein (4–8 S) polymerises to disks (20–22 S) and stacks of disks (27–30 S) at pH 6.8–7.6, 20–25 °C. Two cases are discernible. First, at ionic strength 0.1 M, below pH 7.1, a transient, definite 26‐S peak appears: it transforms in a matter of hours into stable 22‐S (disk) aggregates, at a rate that decreases with decreasing pH and increasing temperature; above pH 7.1, there are only 4–8‐S and 22‐S aggregates. Second, after a rapid increase of ionic strength from 0.1 to 0.75 M, below pH 7.6 a broad distribution of very large overshoot aggregates forms (maximum at 60 S, at pH 6.9). With decreasing pH, the maximum size of the overshoot aggregates increases and reequilibration to stacks of two disks (28 S) becomes slower. The 4–8‐S and 26‐S aggregates can both form the very large overshoot aggregates, but disks and stacks of disks alone cannot. Under these conditions, disks undergo only limited stacking (27–30 S) and a very slow conformational change. A similar conformational change parallels the decomposition of the very large overshoot aggregates. At pH 8, overshoot does not occur, probably because a higher pH inhibits the growth of rods. Though the structure of the overshoot‐forming aggregates is still unknown, three‐layer aggregates (8 S and 26 S) are one possibility. We present a scheme including all reversible polymerisations so far observed for TMV protein and discuss their implications for the reconstitution of the virus in vitro. Copyright © 1977, Wiley Blackwell. All rights reserved