TABLE 5.

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

Proteaseb	Mean cleavage efficiency (%) ± SD
	FIV DU/IN peptide (TGVF/SSWV)	P1′-S peptide (SGVF/SVNG)
Wild-type FIV	76.3 ± 11.9	80.4
Mutants
Q99V	67.9 ± 6.6	82.2
37/55/56/59	2.8 ± 0.9	29.9
55/56/59/99	1.8 ± 1.0	24.8
37/55/56/59/99	1.6 ± 0.8	31.9
37/55/56/59/98/99	2.3 ± 1.1	25.7
Wild-type HIV	1.5 ± 1.0	5.0

^aThe determinant was probed by assaying the cleavage activity of the panel of mutants as described in Table 4 on two peptides that contain Ser at the P1′ position. These peptides were cleaved cleaved efficiently by FIV PR but poorly by HIV-1 PR. All assays were done with 150 nM PR. The incubation time for the FIV dUTPase/integrase (DU/IN) junction peptide was 25 min and for phage peptide (SGVF/SVNG) was 30 min.

^bSee Table 1, footnote a.