CALCIUM-TRANSPORT ACROSS PLASMA-MEMBRANE VESICLES ISOLATED FROM SHOOTS OF STELLARIA-MEDIA AND AVENA-SATIVA

Plasma membrane vesicles (ca 40% inside-out, after one freeze-thaw cycle) were extracted and purified from the shoots of oat (Avena sativa L.) and chickweed (Stellaria media L.) using the two-phase aqueous polymer technique. In the presence of ATP or GTP, a rapid uptake of Ca-45(2+) occurred (0.77 and 0.62 nmol Ca2+ mg(-1) protein, for ATP and GTP, respectively, in oat, and 0.53 and 0.51 nmol Ca2+ mg(-1) protein, for ATP and GTP, respectively, in chickweed). Nucleotide-dependent Ca2+-transport was sensitive to mu M Erythrosin B (with ATP, inhibited by 52% in oat and in chickweed by 72%; with GTP, inhibition was similar in both species at ca 67%); ATP-dependent uptake was greater in oat than in chickweed, but not stimulated by calmodulin. Addition of the calcium ionophore A-23187 resulted in the release of label from the vesicles (41% and 63% release with ATP, and 24% and 52% release with GTP, in oat and chickweed, respectively). The results obtained suggest that Ca2+-transport is independent of the proton pump. In oat, kinetic data indicate a discontinuity in the absorption isotherm at 10 mu M free calcium.