On the Composition, Deposition and Mobilization of Proteins in the Cotyledons of Castor Bean (Ricinus communisL. cv. Hale) Seeds: Their Role as Storage Proteins

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Abstract
Proteins in the soluble and insoluble fractions, extracted from mature castor bean cv. Hale seed cotyledons, differ quantitatively and qualitatively from their counterparts extracted from the endosperm. The soluble fraction contains no glycoproteins, and the lectins RCA1 and ricin D are absent. While the insoluble proteins are electrophoretically and immunologically similar to those in the endosperm, they do not form the 100 kD subunit dimers which characterize some of the endosperm insoluble crystalloid proteins. Rapid rates of deposition of all of the soluble and insoluble proteins present in the mature seed cotyledons commences 30–35 d after pollination (DAP) and continues until 45 DAP. These proteins are mobilized rapidly beginning 1–2 d after seed imbibition and this coincides with an increase in specific activity, in the cotyledons, of two aminopeptidases and a carboxypeptidase. The soluble and insoluble proteins in the cotyledons of the mature seed probably function as storage proteins and support the growth of the germinated seed prior to the mobilization of the major protein storage reserves of the endosperm.

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