IDENTIFICATION AND CHARACTERIZATION OF THE BEL-3 PROTEIN OF HUMAN FOAMY VIRUS

The human foamy virus (HFV) is a complex retrovirus that contains seve ral regulatory and auxiliary bel genes besides the gag, pol, and env g enes. In contrast to the gene products of bel 1 and bel 2/bet that wer e identified previously, the Bel 3 protein has not been described to d ate. Here we report the identification of Bel 3 in HFV-infected cells by immunoprecipitation, indirect immunofluorescence, and expression cl oning under the control of a strong heterologous promoter. Bel 3 was i mmunoprecipitated with an antiserum directed against a bacterially exp ressed and purified form of recombinant Bel 3 antigen. Bel 3 was found to be expressed in low amounts in the cytoplasm of HFV-infected cells and to migrate with an apparent molecular mass of 19.4 kDa on electro phoresis in SDS-polyacrylamide gels, consistent with the calculated va lue of 18.2 kDa. Radioimmunoprecipitation of HFV-infected cell lysates with the hyperimmune serum against Bel 3 revealed at least two additi onal immunoreactive bands of 15.5 and 10.6 kDa. The results indicate t hat Bel 3 was labile, because it was partially degraded even at early time points after infection. On transfection and expression in transfe cted COS cells, recombinant Bel 3 was immunoprecipitated and migrated in three polypeptide bands of 18.7, 14.8, and 9.3 kDa under denaturing conditions. In the absence of reducing agents, the bacterially expres sed and purified recombinant Bel 3 protein of 16.1 kDa can form homodi mers of 30 kDa.