PROCESSING OF THE 2S STORAGE PROTEIN PRONAPIN IN BRASSICA-NAPUS AND IN TRANSFORMED TOBACCO

The 2S protein napin in Brassica napus is synthesized as a proprotein from which an N-terminal and an internal propeptide are removed. In or der to investigate the mechanism of 2S storage-protein processing, N-t erminal sequences were determined for the light and heavy chains of al l major napin isoforms isolated. Mutants of a napin gene, with deletio ns of different portions of the propeptides, were transformed into tob acco and napin protein was isolated. Napin light and heavy chains were separated and their N-terminal amino acid sequences determined. Furth er, the C-terminal residues of one napin isoform isolated from B. napu s and one mutant napin isolated from tobacco were deduced from molecul ar-mass determinations of the constituent chains. Analyses suggested t hat the two propeptides are exposed at the surface of the proprotein. The light chain is processed to the correct length independent of the amino acid sequence in the N-terminal propeptide and the processing si te. The internal propeptide is attacked by endoproteases. Aminopeptida ses and carboxypeptidases then digest portions of the propeptide to th e extent allowed by the primary and the three-dimensional structures, often resulting in 2S protein chains with partly frayed ends.