Am. Ramon et al., CALCIUM-TRANSPORT ACROSS PLASMA-MEMBRANE VESICLES ISOLATED FROM SHOOTS OF STELLARIA-MEDIA AND AVENA-SATIVA, Physiologia Plantarum, 89(4), 1993, pp. 731-740
Plasma membrane vesicles (ca 40% inside-out, after one freeze-thaw cyc
le) were extracted and purified from the shoots of oat (Avena sativa L
.) and chickweed (Stellaria media L.) using the two-phase aqueous poly
mer 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+-t
ransport 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 calciu
m 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 o
at and chickweed, respectively). The results obtained suggest that Ca2
+-transport is independent of the proton pump. In oat, kinetic data in
dicate a discontinuity in the absorption isotherm at 10 mu M free calc
ium.