Table 1.

Quantification of immunoprecipitated proteins using gp27

	Count	Count, (−background)	Methione (mature protein)	Count, (−background)/Met	%
gp27 (0)	20,913	20,885	2	10,443	41.7
gp27 (−1)	7,651	7,623	2	3,812	15.2
gp27 (0,−1)	28,564	28,508	2	14,255	56.9
p23	18,140	18,112	5	3,622	14.5
p24	29,720	29,692	10	2,969	11.9
GMP25	13,531	13,503	4	3,376	13.5
p26	1,656	1,628	2	814	3.3
Background	28	0
T1/ST2R-BP	10,000		4	2,500	10
atp20	17,500		7	2,500	10

[³⁵S]Methionine was used for labeling HeLa cells overnight, and immunoprecipitates were analysed by 2D gel electrophoresis. The PhosphorImager system was used for quantification. Counts for both forms of gp27 were combined [gp27(0) = uncharged; gp27(−1) = negatively charged]. The background area was chosen to lie in the middle of the p24 family proteins. After subtraction of background, the count value was divided by the number of methionine residues present in the respective mature (without signal sequence) p24 family proteins. This enabled comparison on a molar basis, assuming that incorporation of methionine was equally efficient at every position. Of all the immunoprecipitated p24 family proteins, gp27 constituted 56.9%. Values for p23, p24, and GMP25 ranged from 11.9 to 14.5%, but the amount of p26 was significantly lower (3.25%). The rows for T1/ST2R-BP and atp20 show a back-calculation assuming they would be present in an amount similar to p23, p24 or GMP25. Spots corresponding to the counts predicted by this calculation were not observed, although these two p24 family members would be well within the analysed molecular weight and isoelectric point.