Fig. 1.

Theoretical enrichment factor (EF) of envelope proteins, from crude cell extract to purified envelope fractions. Chloroplasts contain 40% of plant cell proteins. Thus, an enrichment factor of chloroplast proteins in Percoll-purified chloroplast fraction compared with total extract would be 2.5. Because the chloroplast envelope represents a minor chloroplast component (i.e. only 1–2% of the chloroplast proteins), envelope proteins should be around 250 times more abundant in the purified envelope fraction when compared with whole cell extract. This enrichment factor (EF) could be used to discriminate genuine envelope proteins from contaminants.