Characterization of the R572T point mutant of a putative cleavage site in human foamy virus Env

A putative cleavage site of the human foamy virus (HFV) envelope glycoprote in (Env) was altered. Transient env expression revealed that the R572T muta nt Env was normally expressed and modified by asparagine-linked oligosaccha ride chains. However, this single-amino-acid substitution was sufficient to abolish all detectable cleavage of the gp130 precursor polyprotein. Cell s urface biotinylation demonstrated that the uncleaved mutant gp130 was trans ported to the plasma membrane. The uncleaved mutant protein was incapable o f syncytium formation. Glycoprotein-driven virion budding, a unique aspect of HFV assembly, occurred despite the absence of Env cleavage, We then subs tituted the R572T mutant env into a replication-competent HFV molecular clo ne. Transfection of the mutant viral DNA into BHK-21 cells followed by vira l titration with the FAB (foamy virus-activated beta-galactosidase expressi on) assay revealed that proteolysis of the HFV Env was essential for viral infectivity, Wild-type HFV Env partially complemented the defective virus p henotype, Taken together, these experimental results established the Locati on of the HFV Env proteolytic site; the effects of cleavage on Env transpor t, processing, and function; and the importance of Env proteolysis for viru s maturation and infectivity.