Seed storage globulins: Origin and evolution of primary and higher order structures

Abstract

Legumin and vicilin are two-domain seed storage globulins similar in primary and higher order structures of their domains to single-domain plant germins as well as to the domains of two-domain and single-domain bacterial oxalate decarboxylases. Independent evolutionary pathways have been shown for the descent of the storage globulins and germins from two-domain and single-domain bacterial oxalate decarboxylases, respectively. As compared to vicilins, the primary and tertiary structures of legumins were found to most closely reflect the ancient features characteristic of a common precursor of storage globulins. During the evolution of the storage globulins, a mechanism specifically controlling their degradation has been formed. We found that limited proteolysis of soybean legumin and kidney bean vicilin in germinating seeds and in vitro leads to their regular changes, which initiate an extensive cleavage of storage globulin molecules by the one-by-one mechanism. As also shown, limited proteolysis of soybean legumin loosens the intersubunit interactions in its oligomeric molecule. Based on these data, we hypothesize that the deep one-by-one degradation of soybean legumin is triggered by its dissociation, which bares peptide bonds potentially susceptible to proteolytic attack but are masked in the oligomer.

Limited proteolysis initiates massive degradation of ginnacin, storage 11S globulin from Ginkgo seeds

“…To check this hypothesis, the amino acid sequences of a-chains from all 11S globulins of known tertiary structures and also from ginnacin were compared. As follows from Table 1, solvent accessibility of the region of b-helices h1-h3 and b-strand J' is 2.3-3.4 times higher than that of the b-barrel N-terminal half (b-strands B-E), which is not susceptible to limited proteolysis (2). This highlights the potential ability of this region for a deep splitting during proteolysis of 11S globulins.…”

“…It seems likely that these bonds in soybean 11S globulin (4) as well as in other 11S globulins are disrupted due to destruction of the region a-helix h1 -b-strand J'. As a result, the inter-subunit interactions become essentially loosened, and the 11S globulins acquire the ability for a reversible dissociation that exposes peptide bonds, which are potentially susceptible for proteolysis but are masked in the native molecules (2).…”

“…11S globulins and 7S globulins, the most characteristic seed storage proteins, are degraded during seed germination to nourish growing seedlings. During the evolution of seed storage globulins, extended disordered inserts of enhanced susceptibility to proteolytic attack (1) appeared in their sequences that have been inherited from spore plants (2). These inserts determine the susceptibility of the globulins to limited proteolysis that occurs at the very beginning of their degradation during seed germination and in vitro (1,2).…”

“…During the evolution of seed storage globulins, extended disordered inserts of enhanced susceptibility to proteolytic attack (1) appeared in their sequences that have been inherited from spore plants (2). These inserts determine the susceptibility of the globulins to limited proteolysis that occurs at the very beginning of their degradation during seed germination and in vitro (1,2). Further deep splitting of 11S and 7S globulins follows the one-by-one mechanism (3) that calls forth a massive degradation of their molecules (1).…”