We previously reported that a host protein, β microglobulin (βm) inhibited the detection of human cytomegalovirus (CMV) in urine specimens by enzyme immunoassay and postulated that βm bound to the virus particle and masked the viral antigenic determinants. We report here that CMV strain AD169 grown in cell culture bound human βm when this protein was added to cell culture fluids or when the virus was added to urine. Such binding was not seen with herpes simplex virus. CMV could also bind bovine βm from foetal calf serum in cell culture fluids. The use of radiolabelled βm in other experiments showed that CMV bound βm after release from cells and that the bound βm did not represent acquisition of class I HLA molecules during budding from host cell membranes. Immunoprecipitation studies showed that βm was bound by two viral envelope proteins βm BP1 (βm-binding protein 1) and βm BP2 of molecular masses 36000 and 65000 daltons respectively. βm could not bind to separated viral proteins under reducing or non-reducing conditions. We propose that interaction of these two proteins on the viral surface is required to enable CMV to bind βm.


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