Several other proteinases occur in the endosperm, some secreted by the
aleurone tissue and some by the scutellum. Especially well known are the five
carboxypeptidases, one of which is specific to the scutellum. Two carboxypep-
tidases are synthesized de novo, which in wheat (but apparently not in barley)
are regulated by GA, while two others increase from a very low value during
endosperm mobilization. The endopepddases (e.g., cysteine proteinase) and
exopeptidases (e.g., carboxypeptidase) probably act in concert to degrade the
storage protein into amino acids and small peptides. Proteinases also hydrolyze
the reserve proteins in the aleurone layer cells (in the aleurone grains) to provide
amino acids for the synthesis of other enzymes