M. Reichert et al., Role of the proline-rich motif of bovine leukemia virus transmembrane protein gp30 in viral load and pathogenicity in sheep, J VIROLOGY, 75(17), 2001, pp. 8082-8089
The cytoplasmic tail of bovine leukemia virus (BLV) transmembrane protein g
p30 has multiple amino acid motifs that mimic those present in signaling pr
oteins associated with B-cell and T-cell receptors. The proline-rich motif
of gp30, PX,PX SP, is analogous to the recognition site of Src homology 3 (
SH3) domains of signaling molecules. Using site-directed mutagenesis of an
infectious molecular clone of BLV, point mutations were introduced which ch
anged three of the prolines of the motif to alanines. The influence of thes
e mutations on the pathogenicity of BLV was studied in sheep which received
either (i) plasmid DNA with provirus containing proline-to-alanine mutatio
ns (pppBLV), (ii) plasmid DNA with wild-type provirus (wtBLV), or (iii) tra
nsfection reagent alone. Although all of the BLV-injected animals seroconve
rted at approximately the same time, viral loads at later time points were
high in five of five of the wtBLV group and two of five of the pppBLV group
but low in three of five of the pppBLV group, as determined by semiquantit
ative PCR. Viral expression was lower in the pppBLV-transfected sheep, as m
easured by p24 antigen enzyme-linked immunosorbent assay in cultured cells,
and serologic titers were lower. Thirty-one months after transfection, fou
r of four wtBLV-transfected sheep had died of leukemia and lymphoma, and al
l five of the pppBLV-transfected sheep were clinically healthy and had norm
al peripheral blood lymphocyte counts. These data indicate that the proline
-rich motif of gp30 is not required for viral infectivity but is important
for high viral load in vivo, suggesting that SH3-mediated gp30 interactions
are critical for viral pathogenesis following infection. Absence of intera
ctions with the proline-rich motif may prevent or delay tumorigenesis in sh
eep.