Parvoviral target cell specificity: Acquisition of fibrotropism by a mutant of the lymphotropic strain of minute virus of mice involves multiple amino acid substitutions within the capsid

Unlike the prototype strain of minute virus of mice, MVM(p), the lymphotropic strain, MVM(i), cannot form plaques on monolayers of mouse A9 fibroblasts. At very low frequency, mutants arise in MVM(i) stocks which are able to plaque on A9 cells, and we report here the isolation and mapping of such a mutant, designated hr101. Analysis of intratypic recombinants containing regions of the hr101 genome substituted into the infectious clone of its parent MVM(i) shows that the ability to form plaques on fibroblast monolayers maps to the same small region of the coat protein gene which we had previously shown, by constructing intrrtypic recombinants, to contain the fibrotropic determinant of MVM(p) (Gardiner and Tattersall, J. Virol 62, 2605-2613, 1988). DNA sequencing of the hr101 regions in virus stocks derived from these recombinants identified four single-base changes between the mutant coat protein gene and that of its parent. Each of these changes occurs in the same position as a similar change found between MVM(i) and MVM(p), and each of them change the amino acid encoded at that position. Three of the four changes substitute the same amino acid as found in MVM(p), and the fourth change substitutes an alanine in hrl01 for a glutamic acid residue in MVM(i), in a position occupied by glycine in MVM(p). Analysis of the recombinants within this region shows that plaque formation on A9 monolayers is dependent upon the latter change plus one adjacent, MVM(p)-like change. This observation was confirmed by recreating this double mutant in the infectious clone of MVM(i) via site-directed mutagenesis. In addition to extending the host range of MVM(i) into A9 fibroblasts, the hr101 mutations have a complex effect on the virus' ability to grow lytically in a series of different T-lymphocyte cell lines.