alpha-Amylase Isoforms are Posttranslationally Modified in the Endomembrane System of the Barley Aleurone Layer

The subcellular site of the posttranslational modification of α-amylase was investigated in aleurone layers of barley (Hordeum vulgare L. cv Himalaya). Aleurone layers of Himalaya barley synthesize and secrete two groups of α-amylase isoforms, referred to as low-isoelectric point (low-pl) or HAMY1 and high-pl or HAMY2, when incubated in gibberellic acid and CaCl₂. Whereas homogenates of aleurone layers contain four isoforms of HAMY1 with pls 4.90, 4.72, 4.64, and 4.56, incubation media contain predominantly isoforms 4.72 and 4.56. Microsomal membranes isolated from aleurone layers contain all four isoforms of HAMY1. Microsomal membranes can be resolved into two peaks by isopycnic density gradient centrifugation: a peak of heavy membranes with endoplasmic reticulum and Golgi apparatus (GApp) marker enzyme activities and a peak of light membranes with characteristics of the GApp. The heavy membranes contain proportionally more HAMY1 pl 4.90 and 4.64 isoforms, whereas light membranes contain a higher proportion of pl 4.72 and 4.56 isoforms. Experiments with the ionophore monensin show that membranes of the GApp as well as the endoplasmic reticulum are involved in the posttranslational modification of HAMY1 isoforms. Monensin inhibits the secretion of α-amylase and causes the enzyme to accumulate within the cell. Precursor forms of HAMY1 accumulate in light membranes isolated from monensin-treated aleurone layers indicating that the GApp is involved in the conversion of the precursor to the secreted forms of the enzyme.