The evolutionary relationships of the seed storage proteins : protein homologies

Abstract

The first eleven amino acids from the amino-terminal of the basic subunits of edestin from hemp and cocosin from coconut were sequenced using the manual DABITC/PITC double coupling procedure. Similarity scores generated by computer assisted sequence analyses for these sequences, in conjunction with those from the literature, suggest that the basic subunit sequences are highly conserved and that the legumin-like proteins are distributed throughout all the major groups of extant angiosperms, represented by bean, pea, Vicia faba, oat, pumpkin, hemp, and coconut. Further, computer analyses indicate that the basic subunit sequences are unique among the seed storage proteins. The acidic subunits of the legumins were also investigated. In soybean acidic subunit 2, the cystine at position #86 is known to disulfide pair with the basic subunit. Analyses of pea and soybean sequences from the literature indicate that the acidic subunits are conserved from their amino-terminal to approximately amino acid residue #98 suggesting the constraints and the importance of proper disulfide-bridging within the legumins. The high-molecular weight, vicilin-like glycoproteins from cotton and coconut were chemically and enzymatically digested. Digests included cyanogen bromide, Staphylococcus aureus V-8 protease, chymotrypsin, alkaline trypsin, and citraconylated trypsin. Resulting peptides were mapped by polyacrylamide gel electrophoresis and/or 2-dimensional chromatography/electrophoresis on silica gel. Where appropriate, maps were theoretically derived from vicilin sequence data existing in the literature. The maps indicate that the vicilins are homologues, but are more variable than the legumins. Sequence analysis of phaseolin and pea vicilin reinforce this interpretation. Analyses of prolamin sequences from maize, barley, wheat, and rye show little interspecies similarities. However, intraspecies similarities are present. In maize, the transcribed cDNA sequences of the 22,500 dalton proteins, pZP22.1 and B49, are significantly similar. The same is true for the cDNA sequencs of the 19,000 dalton proteins, A20 , A30, and (lamda)ZG99. Similarities between molecular weight classes are less pronounced, but indicate they are also homologues. Analysis of the limited sequence information for the albumins from castor bean and barley show them to be unrelated.