Previous studies from our laboratory identified cellular membrane proteins that mediate binding of visna virus to susceptible cells. In the pilot report, antiserum raised to one of these proteins, ~45 kDa, was shown to both label the surface of susceptible cells and block the binding of visna virus to cell membranes. In a recent study, we reported that the same antiserum, designated 2-23, significantly inhibited infection by visna virus and specifically immunoprecipitated a membrane-associated protein complex from susceptible cells, comprised of a ~45- kDa protein, as well as a 30-kDa protein. Because the 30-kDa protein was readily detectable in TRANS[35S]-LABELed susceptible cells, we were able to characterize this protein biochemically, as a chondroitin sulfate proteoglycan. In the present study, we sought to characterize the ~45-kDa protein and examined 2-23 immune complexes for the presence of kinase activity. Our data indicate that although in vitro kinase assays of 2-23 immunoprecipitates specifically result in the phosphorylation of the ~45-kDa protein as well as a novel ~56-kDa protein, only the ~45-kDa protein exhibits inherent serine/threonine kinase activity. In addition, the kinase activity can be isolated in 2-23 immunoprecipitates of membranes prepared from visna virus-susceptible cells. Finally, in an effort to evaluate the biological relevance of our in vitro observations, we examined 2-23 immunoprecipitates of [32P]orthophosphate-labeled visna-susceptible cells and report that the ~56-kDa protein is phosphorylated constitutively on serine in vivo. Collectively, these data implicate a serine/threonine kinase complex in the binding/infection of visna virus. (C) 2000 Academic Press.
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