PLANT PHYSIOLOGY , Vol 104, Issue 1 127-133, Copyright © 1994 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Characterization of a Soybean [beta]-Conglycinin-Degrading Protease Cleavage Site
X. Qi, R. Chen, K. A. Wilson and A. L. Tan-Wilson
Department of Biological Sciences, State University of New York at Binghamton, Binghamton, New York 13902-6000
Protease C1, an enzyme from soybean (Glycine max [L.] Merrill cv Amsoy 71)
seedling cotyledons, was previously determined to be the enzyme responsible
for the initial degradation of the [alpha][prime] and [alpha] subunits, but
not the [beta] subunit, of [beta]-conglycinin storage protein. The sizes of
the proteolytic products generated by the action of protease C1 suggest
that the cleavage sites on the [alpha][prime] and [alpha] subunits of
[beta]-conglycinin may be located in their N-terminal domain, which is not
found in the [beta] subunit of [beta]-conglycinin. To check this
hypothesis, storage proteins from other plant species that are homologous
to either the [alpha][prime]/[alpha] or the [beta] subunit of
[beta]-conglycinin were tested as substrates. As expected, the convicilin
from pea (Pisum sativum), a protein homologous to the [alpha][prime] and
[alpha] subunits of [beta]-conglycinin, was digested by protease C1. The
vicilins from pea as well as vicilins from adzuki bean (Vigna angularis),
garden bean (Phaseolus vulgaris), black-eyed pea (Vigna unguiculata), and
mung bean (Vigna radiata), storage proteins that are homologous to the
[beta] subunit of soybean [beta]-conglycinin, were not degraded by protease
C1. Degradation of soybean [beta]-conglycinin involves a sequential attack
of the [alpha] subunit at multiple sites, culminating in the formation of a
stable intermediate of 53.5 kD and a final product of 48.0 kD. The cleavage
sites resulting in this formation of the intermediates and final product
were determined by N-terminal analysis. These were compared to the known
amino acid sequences of the three [beta]-conglycinin subunits. Results
showed these two polypeptides to be generated by proteolysis of the [alpha]
subunit at regions bearing long strings of acidic amino acid residues.
