Arachin polymorphism in groundnut ( arachis hypogaea l.)

Abstract

Arachin, the major seed storage protein of groundnut, showed polymorphism. The polymorphic forms were due to differences in molecular size, net charge and polypeptide composition of the native protein. Purified arachin at low ionic strength resolved into monomeric and dimeric forms both on sucrose density gradient centrifugation and cellulose acetate membrane (CAM) electrophoresis. The dimers had more net negative charge compared with the monomers. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) of arachin from the cultivar Spanish improved (SP) under non-reducing conditions showed three major components of M, 70.7, 63.8 and 60.9 k. Arachin from Trombay Groundnut 1 (TG-1) showed three components of M, 70.7, 63.8 and 59.5 k while in TG-18, a derivative of a cross between SP and TG-1, there were only two components of M, 70.7 and 63.8 k. Data from two dimensional gel electrophoresis in the presence and absence of 2-mercaptoethanol shows that each of the above components in turn consists of subunit pairs that are held together by disulphide linkages. The M,s of the major polypeptides for the three arachins is as follows: SP, 47.5, 45.1, 42.6 and 21.4 k; TG-1, 47.5, 45.1, 41.2 and 21.4 k; TG-18, 47.5,45.1 and 21.4 k. Two dimensional gel electrophoresis (IEF and SDS-PAGE) indicates that the arachin subunits consist of two major groups—the acidic polypeptides (three in SP, TG-1 and two in TG-18) and the basic polypeptides (three in SP, TG-1 and two in TG-18). The acidic polypeptides did not show charge variation while the basic polypeptides were charge heterogeneous. Absence of both an acidic polypeptide along with a basic polypeptide in TG-18 suggests that the acidic and basic polypeptides are probably products of the same gene and arise as a result of post-translational cleavage. Antibodies raised against purified arachin from SP reacted with arachin from TG-1 and TG-18 showing similar antigenic determinants. The acidic polypeptides show considerable homology in their structure as revealed by peptide mapping patterns.