Calcium transport across plasma membrane vesicles isolated from shoots of Stellaria media and Avena sativa

Abstract

1993. Calcium transport across plasma membrane vesicles isolated from shoots of Stellaria media and Avena sativa. -Physiol. Plant. 89: 731-740.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^^ occurred (0.77 and 0.62 nmol Ca-+ mg' protein, for ATP and GTP, respectively, in oat, and 0.53 and 0.51 nmol Ca-+ mg"' protein, for ATP and GTP respectively, in chickweed). Nucleotide-dependent Ca-^-transport was sensitive to 1 [xM 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 Ca^^-transport is independent of the proton pump. In oat, kinetic data indicate a discontinuity m the absorption isotherm at 10 | JLM free calcium.