Mature Amaranthus hypochondriacus seeds contain non-processed 11S precursors

Abstract

Amaranth is a dicotyledonous plant whose major seed storage proteins are globulins and glutelins. An unique feature of amaranth seeds is the presence of a fraction named albumin-2, that is extractable with water only after an exhaustive extraction of globulins and albumin-1. In this work, we tested the hypothesis that albumin-2 fraction could be constituted by a non-processed 11S globulin (proglobulin). To this end, the gene encoding the amaranth 11S subunit was cloned and expressed in Escherichia coli. Subsequently, the recombinant proglobulin and albumin-2 purified from seeds were treated with a sunflower vacuolar processing enzyme (VPE). A 55 kDa component of albumin-2 was specifically cleaved into 38 and 17–15 kDa polypeptides, as a consequence of this endoproteolytic cleavage a change of the oligomeric state from trimeric to hexameric was observed. Amaranth 11S globulin fraction was not modified under these proteolysis conditions. Using VPE-specific antibodies, it was shown that amaranth expresses a 57 kDa VPE, and that both developing and mature amaranth seeds have VPE activity, although the increase of this activity during amaranth seed development is higher than that observed for sunflower seeds. These results confirm the presence of unprocessed 11S precursors in mature amaranth seeds; this phenomenon cannot, however, be attributed to low VPE activity during developing of amaranth seeds.