Localization and synthesis of prenylquinones in isolated outer and inner envelope membranes from spinach chloroplasts

The prenylquinone content and biosynthetic capabilities of membrane fractions enriched in outer and inner envelope membranes from spinach chloroplasts were analyzed. Both envelope membranes contain prenylquinones, and in almost similar amounts (on a protein basis). However, the outer envelope membrane contains more alpha-tocopherol than the inner one although this prenylquinone is the major one in both fractions. On the contrary, plastoquinone-9 is present in higher amounts in the inner envelope membrane than in the outer one. In addition, it has been demonstrated that all the enzymes involved in the last steps of alpha-tocopherol and plastoquinone-9 biosynthesis, i.e., homogentisate decarboxylase polyprenyltransferase, S-adenosyl-methionine:methyl-6-phytylquinol methyltransferase, S-adenosyl-methionine: alpha-tocopherol methyltransferase, homogentisate decarboxylase solanesyltransferase, S-adenosyl-methionine:methyl-6-solanesylquinol methyltransferase, and possibly 2,3-dimethylphytylquinol cyclase, are localized on the inner envelope membrane. These results demonstrate that the inner membrane of the chloroplast envelope plays a key role in chloroplast biogenesis, and especially for the synthesis of the two major plastid prenylquinones.