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 repo rt, antiserum raised to one of these proteins, similar to 45 kDa, was shown to both label the surface of susceptible calls and block the binding of vi sna virus to cell membranes. In a recent study, we reported that the same a ntiserum, designated 2-23, significantly inhibited infection by visna virus and specifically immunoprecipitated a membrane-associated protein complex from susceptible cells, comprised of a similar to 45- kDa protein, as well as a 30-kDa protein. Because the 30-kDa protein was readily detectable in T RANS[S-35]-LABELed susceptible cells, we were able to characterize this pro tein biochemically, as a chondroitin sulfate proteoglycan. In the present s tudy, we sought to characterize the similar to 45-kDa protein and examined 2-23 immune complexes for the presence of kinase activity. Our data indicat e that although in vitro kinase assays of 2-23 immunoprecipitates specifica lly result in the phosphorylation of the similar to 45-kDa protein as well as a novel similar to 56-kDa protein, only the similar to 45-kDa protein ex hibits inherent serine/threonine kinase activity. In addition, the kinase a ctivity can be isolated in 2-23 immunoprecipitates of membranes prepared fr om visna virus-susceptible cells. Finally, in an effort to evaluate the bio logical relevance of our in vitro observations, we examined 2-23 immunoprec ipitates of [P-32]orthophosphate-labeled visna-susceptible cells and report that the similar to 56-kDa protein is phosphorylated constitutively on ser ine in vivo. Collectively, these data implicate a serine/threonine kinase c omplex in the binding/infection of visna virus. (C) 2000 Academic Press.