TABLE 1.
Receptor binding and infectivity of cysteine mutantsa
| Mutant | Assay
|
|
|---|---|---|
| Receptor binding (Kd) | Infectivity (% wild type) | |
| Wild type | 1.7 × 10−8 | 100.00 |
| C9S | 1.6 × 10−8 | 0.16 |
| C45S | 0.7 × 10−8 | 0.09 |
| C9,45S | 1.6 × 10−8 | 0.09 |
| Aclb | NDc | 0.02 |
The apparent Kd for binding of s47 to EnvA-expressing cells and infectivity mediated by EnvA pseudotyped MLV virions were measured as previously described (3). For the infectivity assay, the results from four independent experiments, normalized to wild-type infectivity within each experiment, were averaged. The standard deviation for the infectivity results ranged from ±0.01% for C9,45S to ±0.09% for C9S. ND, not determined.
Acl is a mutant EnvA in which the cleavage site has been mutated (8). Although it forms trimers, is expressed at high levels at the cell surface, and is well incorporated into virions, this EnvA is poorly cleaved and does not support infection.
Although the Kd valve for the Acl mutant was not determined, immunoprecipitation studies indicate that Acl retains wild-type receptor binding (data not shown).