TABLE 6.

Determinant of preference for Ser at P1′ position in a substrate^a

Protease	Mean cleavage efficiency (%) ± SD with the FIV DU/IN peptide (TGVF/SSWV)
Wild-type FIV	63.3 ± 8.3
Mutants
I37V	68.5 ± 7.1
I37V/N55M	57.4 ± 7.7
I37V/M56I	31.5 ± 5.1
I37V/V59I	41.5 ± 5.2
Wild-type HIV	3.1 ± 1.0

^aThe determinant was further probed with another panel of mutants that included I37³²V, I37³²V/N55⁴⁶M, I37³²V/M56⁴⁷I, and I37³²V/V59⁵⁰I. The cleavage efficiency of these mutants on the FIV dUTPase/integrase (DU/IN) junction peptide was assayed. The data showed that the mutants containing the M56⁴⁷I or V59⁵⁰I substitution decreased the cleavage efficiency significantly.