TABLE 4.

Determinant of preference for Gln or Asn at the P1 position in a substrate^a

Proteaseb	Mean cleavage efficiencya (%) ± SD
	FIV CA/NC2 peptide (TKVQ/VVQS)	P1-Gln peptide (SRVQ/VVNG)	P1-Asn peptide (SRVN/VVNG)
Wild-type FIV	70.8 ± 6.2	76.2	87.8
Mutants
Q99V	37.1 ± 3.0	19.2	25.7
37/55/56/59	63.1 ± 4.9	78.6	87.0
55/56/59/99	16.9 ± 2.1	5.8	17.3
37/55/56/59/99	23.8 ± 4.6	14.3	26.8
37/55/56/59/98/99	27.6 ± 3.1	10.5	28.0
Wild-type HIV	15.8 ± 5.1	10.1	9.7

^aThe determinant was probed by assaying the cleavage efficiency of Q99⁸²V and a panel of chimeric mutants on three peptides. These peptides contained either Gln or Asn at P1 and were cleaved very efficiently by FIV PR but poorly by HIV-1 PR. The data from three different peptides showed similar results; the Q99⁸²V mutant and any mutant containing the Q99⁸²V substitution drastically decreased the cleavage efficiency on these three peptides. All assays were done with 150 nM PR. The incubation time was 45 min for both the FIV CA/NC2 junction peptide and the P1-Gln peptide and 30 min for the P1-Asn peptide.

^bSee Table 1, footnote a.