Involvement of a membrane-associated serine/threonine kinase complex in cellular binding of visna virus

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, approximately 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 approximately 45- kDa protein, as well as a 30-kDa protein. Because the 30-kDa protein was readily detectable in TRANS[(35)S]-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 approximately 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 approximately 45-kDa protein as well as a novel approximately 56-kDa protein, only the approximately 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 [(32)P]orthophosphate-labeled visna-susceptible cells and report that the approximately 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.