ACTIVE FOAMY VIRUS PROTEINASE IS ESSENTIAL FOR VIRUS INFECTIVITY BUT NOT FOR FORMATION OF A POL POLYPROTEIN

To analyze proteolytic processing of foamy (spuma) retroviruses, two m utations were generated in the presumed active-site triplet Asp-Ser-Gl y in the predicted proteinase (PR) region of the human foamy virus (HS RV). The mutations changed either the presumed catalytic aspartic acid residue to a catalytically incompetent alanine or the adjacent serine to a threonine found in most cellular and retroviral proteases at thi s position. Both mutations were cloned into the full-length infectious HSRV DNA clone. Wild-type and SIT mutant genomes directed the synthes is of particles with similar infectious titers, while the HSRV DIA PR mutant was noninfectious. Immunoblot analysis of transfected cells rev ealed identical patterns for the wild-type and for the S/T PR mutant. HSRV DIA mutant-transfected cells expressed only a single Gag polyprot ein of 78 kDa instead of the 78-kDa-74-kDa doublet found in HSRV-infec ted or wild-type-transfected cells. Analysis with pol-specific antiser a yielded a protein of approximately 120 kDa reactive viith antisera a gainst pol- but not gag-specific domains. No Gag-Pol polyprotein was d etected in this study. Electron microscopy analysis of transfected cel ls showed heterogeneous particle morphology in the case of the D/A mut ant, with particles of normal appearance and particles of aberrant siz e and shape. These results indicate that foamy viruses have an asparti c PR that is essential for infectivity but not for formation of the 12 0-kDa Pol polyprotein.