alpha-Amylase biosynthesis: evidence for temporal sequence of NH2-terminal peptide cleavage and protein glycosylation.

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Abstract
During the biosynthesis of secretory proteins, an NH2-terminal peptide, referred to as a signal peptide, is cotranslationally cleaved off after the protein enters the cisternal space of the endoplasmic reticulum. The core glycosylation reaction of some secretory and viral membrane glycoproteins occurs as a cotranslational event. The temporal sequence of proteolytic cleavage of the signal peptide and glycosylation on the polysomes for any secretory or membrane glycoprotein is unknown. Proteolytically processed and already glycosylated chains of rice [Oryza sativa cv. Kimmaze] seed .alpha.-amylase exist on the polysomes; apparently, glycosylation is preceded by proteolytic processing during the biosynthesis of the .alpha.-amylase molecule.