TABLE 2.

Fusion activity of cleavage site mutants

F protein	Relative cell surface expressiona	% (±SD) cell-cell fusionb with
		SV41 HN		CH5-41 (30 min)	No HN (8 h)
		1 h	4 h
Se-WR F	1.0	-	-	31.3 ± 3.6	-
Se-M(1+2)	1.2 ± 0.1	10.2 ± 1.0	14.9 ± 3.0	27.7 ± 3.3	-
Se-M(2+4)	1.1 ± 0.2	0.3 ± 0.4	1.7 ± 1.6	30.4 ± 2.2	-

^aThe relative amount of the F proteins on the cell surface of the HeLa cells coexpressing each cleavage site mutant and the SV41 HN protein was measured by cell surface biotinylation as described in the legend to Fig. 3C at 22 h posttransfection.

^bAfter treatment with 5 μg/ml of trypsin (at 37°C for 15 min) at 22 h posttransfection, cell-cell fusion (%) of HeLa cells coexpressing each cleavage site mutant and one of the HN proteins was morphometrically quantified at the times indicated. -, cell-cell fusion was not found even when all of the well was observed.