Properties, biosynthesis and processing of a sulfur‐rich protein in Brazil nut (Bertholletia excelsa H.B.K.)

Abstract

An abundant seed protein, which is exceptionally rich in the sulfur‐containing amino acids, methionine (18%) and cysteine (8%), is synthesized in Brazil nut embryos about 9 months after flowering. This sulfur‐rich protein consists of two low‐molecular‐mass polypeptide components, a 9‐kDa polypeptide and a 3‐kDa polypeptide. The two‐subunit polypeptides associate through disulfide linkage(s) to form a 12‐kDa protein molecule. We have demonstrated through in vitro translation studies, using RNA from 9‐month‐old embryos, that the sulfur‐rich protein is synthesized as a larger precursor polypeptide of 18 kDa. In addition, data from in vivo labelling studies of 9‐month‐old Brazil nuts suggest that there are two intermediate precursors of the sulfur‐rich protein, one of 15 kDa and another of 12 kDa. One of these precursors, the 12‐kDa polypeptide, accumulates for a 2‐month period in the developing embryos. From these data we infer that at least three stepwise cleavages are involved in the maturation of the sulfur‐rich protein from its 18‐kDa precursor. Copyright © 1987, Wiley Blackwell. All rights reserved