Cell-free sorting of plastoquinone and ubiquinone in spinach cells

Transfer of ubiquinone (UQ) and plastoquinone (PQ) between endoplasmic reticulum (ER) and mitochondria or chloroplast, respectively, was reconstituted in a cell-free system using membrane fractions isolated from dark-grown spinach seedlings (Spinacia oleracea cv. Medania). UQ transfer was time and temperature dependent, moderately stimulated by ATP and significantly increased (4-fold) by cytosolic proteins. In contrast, PQ transfer was neither reduced by lowered temperature, nor markedly elevated by ATP but similarly stimulated by cytosol. The nature of proteins involved in these processes has not yet been studied. These results suggest that AT dependent UQ transport partly occurs by mechanisms involving the vesicular pathway. In contrast, the postulated vesicular mechanism for PQ translocation was found to be ATP independent. Differences in mechanisms of PQ and UQ translocation could reflect high selectivity of transport of both quinones found in vivo.