Identification of glycinin in vivo as a polyamine-conjugated protein via a gamma-glutamyl linkage.
To identify a polyamine-conjugated protein by the action of transglutaminase in the absence of radiolabelled polyamine, extracts prepared from the leaves and developing soybean seeds were investigated for the specific activity of transglutaminase and the content of free polyamines. We identified the major storage protein, glycinin, as a polyamine-conjugated protein. This was established by the following procedures: (1) immunolocalization with antibody against putrescine prepared in rabbit against putrescine-BSA conjugate; (2) immunocross-reactivity on nitrocellulose transblot of the purified glycinin subunits by using antibody against putrescine; (3) identification of polyamines in acid hydrolysates of purified glycinin; (4) release of polyamines in proteolytic digests through the catalytic action of gamma-glutamylamine cyclotransferase, an enzyme specific for the disassembly of gamma-glutamylamines. The activity of gamma-glutamylamine cyclotransferase was also identified in soybean seeds.