Table 2.

Mass of virion-associated SIV env

Fraction no.	Mass, kDa (number of subunits)
	Sedimentation equilibrium (weight average)	STEM (no. average)
50	396.9 (2.98)
51	430.2 (3.23)	387 (2.91)
52	406.3 (3.05)
53	405.1 (3.05)

The mass of SIV SU (shed from the virion surface after incubation at 60°C for 1 h) was determined by mass spectrometry (103 kDa, D. Sheeley and R.J.C., unpublished data), allowing the average mass for each potential N-linked glycosylation site to be calculated. This value was then multiplied by the number of potential N-linked glycosylation sites (26 sites) and added to the protein molar mass (known from the amino acid sequence data) of virion-associated SIV env to give an estimate of the monomer mass (133 kDa).