Amino Acid Sequence of a New 2S Albumin fromRicinus communisWhich Is Part of a 29-kDa Precursor Protein

The isolation and sequence determination of a new 2S albumin storage protein fromRicinus communisseeds denoted 2S ASP-Ib are described. The fragment approach using selective enzymatic cleavage, Edman degradation, and mass spectrometry was used to demonstrate that the 11-kDa heterodimer protein linked by disulfide bridges has the following structure: short chain, GEREGSSSQQCRQEVQRKDLS-SCERYLRQSSS; long chain, <QQQESQQLQQCC-NQVKQVRDECQCEAIKYIAEDQIQQGQLHGEES-ERVAQRAGEIVSSCGVRCMR. The molecular weight of the intact protein, 11,140 ± 2, determined by matrix-assisted laser desorption mass spectrometry was consistent with the assigned structure. The S- and L-chains are identical to residues 18–49 and 66–130 of the precursor protein predicted by S. D. Irwin, J. N. Keen, J. B. C. Findlay, and J. M. Lord [(1990)Mol. Gen. Genet.222, 400–408], on the basis of the structure of a cDNA isolated using probes based on the sequence of another 2S albumin, described by F. S. Sharief and S. S. L. Li [(1982)J. Biol. Chem.257, 14753–14759], which we denote 2S ASP-Ia. Three of the four termini could have been produced by posttranslational processing by endopeptidase(s) and carboxypeptidase(s) which utilized basic residues as the cleavage sites. Mass spectrometric evidence suggested that the protein presented microheterogeneity at its termini, i.e., truncated forms presumably due to processing heterogeneity. The present characterization of the 2S ASP-Ib protein, the second 2S albumin fromRicinus communisseeds, demonstrates that the 237-residue precursor protein codes for two different heterodimer proteins containing 97 and 99 residues each. This system should be useful for studying the posttranslational processing of plant storage proteins.