Signal resistance of a soluble protein to enzymic proteolysis. An unorthodox approach to the isolation and purification of germin, a rare growth-related protein
Onset of growth in germinating wheat embryos is marked by the conspicuous synthesis of germin, a soluble homopentameric protein. Germin is unusually stable in reducing environments containing sodium dodecyl sulfate, but the polymeric form is converted to a protomer (ca. 26 kdaltons) by brief heat treatment. In respect to these physical properties, germin is similar to nucleoplasmin, the putative nucleosome-assembly factor in Xenopus oocytes. To expand the comparison, we treated germin with gastric pepsin in the expectation that pepsin-catalyzed hydrolysis of germin might generate a series of fragments of the kind derived by pepsin digestion of nucleoplasmin. To our surprise, germin was refractory under conditions used to degrade nucleoplasmin. Further study has shown that germin exhibits a measure of stability toward the action of broad-specificity proteases which is unprecedented for a soluble protein. In this report, we document the remarkable resistance of this growth-related protein to enzymic proteolysis and project how this property may make it possible to isolate and purify an otherwise intractably rare, but "interesting" protein.