A SOLUBLE FORM OF A RECEPTOR FOR SUBGROUP-A AVIAN-LEUKOSIS AND SARCOMA-VIRUSES (ALSV-A) BLOCKS INFECTION AND BINDS DIRECTLY TO ALSV-A

A receptor that confers susceptibility to infection by subgroup A avia n leukosis and sarcoma viruses (ALSV-A) has been described (P. Bates, J. A. T. Young, and H. E. Varmus, Cell 74:1043-1051, 1993). A soluble form of the receptor was generated to determine whether this protein i nteracts directly with virus particles in the absence of other cell su rface factors. The soluble protein comprised the extracellular region of the ALSV-A receptor fused to an antibody epitope tag and six histid ine residues. Preincubating this protein with virus led to an efficien t block to infection of avian cells by ALSV-A but had no effect on inf ection by ALSV-B, ALSV-C, or ALSV-D. Furthermore, an antibody directed against the introduced epitope tag immunoprecipitated ALSV-A particle s bound to the soluble receptor. In contrast, other ALSV subgroups wer e not immunoprecipitated by this procedure. These data demonstrate tha t the cloned receptor interacts directly with ALSV-A and discriminates between different ALSV subgroups at the level of virus binding.