The LID strain of polyomavirus differs from other laboratory strains i
n causing a rapidly lethal infection of newborn C3H/Bi mice, This viru
lent behavior of LID was attenuated by dilution, yet at sublethal dose
s LID was able to induce tumors at a high frequency, like its parent v
irus PTA, By constructing and assaying LID-PTA recombinant viruses and
by DNA sequencing, the determinant of virulence in LID was mapped to
the major viral capsid protein, VP1, The VP1s of LID and PTA differed
at two positions: at 185, LID has phenylalanine and PTA has tyrosine,
and at 296, LID has alanine and PTA has valine, Results obtained with
viruses constructed by site-directed mutagenesis showed that alanine a
t position 296 is sufficient to confer a fully virulent phenotype rega
rdless of which amino acid is at position 185, However, with valine at
position 296, an effect of phenylalanine at position 185 is apparent,
as this virus possesses an intermediate level of virulence, A crystal
structure of polyomavirus complexed with 3'-sialyl lactose previously
indicated van der Waals contacts between the side chain of valine 296
and the sialic acid ring (T. Stehle, Y, Yan, T, L, Benjamin, and S, C
, Harrison, Nature [London] 369:160-163, 1994), When this interaction
was modeled with alanine, these contacts were greatly reduced, Direct
confirmation that the substitutions in VP1 affected receptor binding w
as obtained by studying virus hemagglutination behavior. The ensemble
of results are discussed in terms of the idea that a lower affinity of
the virus for its receptor can result in more rapid spread and increa
sed pathogenicity.