Fragments generated by pH dissociation of ME-virus and their relation to the structure of the virion

Abstract

ME-virus, in the presence of 0.1 m-chloride ions at pH 5.7, can be thermally dissociated into 14 s subunits, free RNA and an insoluble precipitate (I-protein). The 14 s is a pentamer that can be further dissociated with 1 to 2 m-urea into five 5 s subunits. By sedimentation equilibrium measurements the 14 s subunits weighed about 425,000 daltons; the 5 s subunit weighed about 86,000 daltons. The latter subunit is a protein molecule (protomer) composed of one each of three non-identical polypeptide chains α, β and γ and is believed to be generated by dissociation of a δ chain from each of 60 structural subunits in the capsid. The I-protein contains the two minor components δ and ε which are always found in virus preparations; it also contains a definite proportion of the α, β and γ chains. The stoichiometry of the non-δ portion of the I-protein is consistent with that expected for a structure (ε-pentamer) analogous to a 14 s pentamer containing one ε chain in place of one of the five β chains; the amount of precipitate implies that there are two such ε-pentamers per average virion. Thus I-protein is believed to consist of two separate components (a) ε-pentamers and (b) δ chains. The requirement for 12 fivefold vertices in an icosahedral particle and the five-co-ordinated nature of the pentamers suggest that the ME-virion is composed of 12 × 5 or 60 protomers; the composition of the I-protein is consistent with the hypothesis that there are 58 (α, β, γ, δ) protomers for every 2 (α, ε, γ) protomers. The ME-virion behaves as if its 60 subunits were held together by two sets of bonding sites; (a) type 1 sites binding 12 pentamers together to form the 60 subunit shell and (b) type 2 sites binding 5 protomers together to form pentamers. It is proposed that mild acid dissociation of ME-virus is due to specific rupture of type 1 bonding sites and that dissociation of pentamers into protomers by molar concentrations of amide is due to dissociation of type 2 bonding sites.