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The gluten proteins in wheat are seed (grain) storage proteins that provide
a source of nitrogen and amino acids for the new plant upon germination of
the embryo.  The high levels of glutamine and proline in wheat gluten
proteins are present presumably because these amino acids can be efficiently
transformed by the plant into all the other amino acids (and then all the
many different proteins) needed by the new wheat plant for differentiation
and growth.
 
Don Kasarda, Albany, CA