Calcium-binding protein in sieve tube exudate.

A calcium-binding macromolecule, with an estimated molecular weight greater than 100,000, was detected in phloem exudate from Cucurbita maxima and related species. The macromolecule was a component of sieve tube sap, rather than a contaminant leached from cell walls or cut parenchyma cells during exudate collection. The protein nature of this macromolecule was deduced from its size, lability, susceptibility to proteolytic digestion, and by the dependence of calcium-binding activity on thiol-protecting agents. This protein is distinct from the major proteins of exudate and does not appear to be related to calmodulin.